Dispersion management in soft glass all-solid photonic crystal fibres

The development of all-solid photonic crystal fibres for nonlinear optics is an alternative approach to air-glass solid core photonic crystal fibres. The use of soft glasses ensures a high refractive index contrast (> 0.1) and a high nonlinear coefficient of the fibres. We report on the dispersion management capabilities in all-solid photonic crystal fibres taking into account four thermally matched glasses which can be jointly processed using the stack-and-draw fibre technique. We present structures with over 450 nm broadband flat normal dispersion and ultra-flat near zero anomalous dispersion below 5 ps/nm/km over 300 nm dedicated to supercontinuum generation with 1540 nm laser sources. The development of an all-solid photonic crystal fibre made of F2 and NC21 glasses is presented. The fibre is used to demonstrate supercontinuum generation in the range of 730?C870 nm (150 nm) with flatness below 5 dB.     

Non-linear applications of microstructured optical fibres

The enhancement of different non-linear processes in microstructured optical fibres can be achieved through manipulation of the dispersion characteristics of the fibre. This is demonstrated by extending the region of short wavelength operation of high power supercontinuum generation through four wave mixing in a cascaded fibre geometry where the dispersion of each fibre decreased on propagation. The technique is further refined in a demonstration utilizing long lengths of dispersion decreasing tapered microstructured fibres, where the supercontinuum extends to around 300 nm with spectral power densities in excess of 2 mW/nm in the uv. These long length tapers can also be utilized for adiabatic soliton pulse compression in new spectral regions, allowing the compression of 655 fs pulses to 45 fs.     

Design of high duty ratio nonlinear photonic crystal fiber with near-zero flattened dispersion after 1.205 μm

The paper presents a high nonlinear photonic crystal fiber (HN-PCF) with highly duty ratio for time-stretch analog-to-digital conversion (TSADC). The simulation results show that a nonlinear of 42.26 W⁻¹ km⁻¹ and the flattened dispersion of less than 1.0 ps/(nm km) are obtained in more than 495 nm waveband (1205–1700 nm). Owing to its high nonlinear coefficient and flattened dispersion, the high nonlinear PCF is expected to be suitable for supercontinuum (SC) generation. The numerical simulation results demonstrate that the proposed high duty ratio HN-PCF can generate wideband SC.     

35 W high power all fiber supercontinuum generation in PCF with picosecond MOPA laser

We demonstrate 35 W high power all fiber supercontinuum generation by pumping photonic crystal fiber (PCF) with a 57.7 W picosecond fiber MOPA. The picosecond fiber MOPA pumped supercontinuum source exhibits an optical-to-optical conversion efficiency of up to 61.7%, covering a spectral range from 600 nm to beyond 1700 nm. The compact and practical configuration of this supercontinuum source has potential to achieve higher power scale together with perfect continuum spectrum.     

Supercontinuum generation in single crystal sapphire fibers

In this paper, we report supercontinuum generation by launching ultra-short femtosecond laser pulses into single crystal sapphire fibers. The major advantages of using sapphire fiber for supercontinuum generation are: (1) high transparency up to 5 μm, (2) low material dispersion in the 0.8-5 μm spectral range, and (3) a higher laser damage threshold (500 times higher than that of silica). Thus, a very high power, super broadband [from visible to middle IR (up to 5 μm)], supercontinuum source can be realized by employing sapphire fiber for supercontinuum generation. Our experimental results also confirm that sapphire fiber can offer a broader supercontinuum spectrum than that of bulk sapphire counterpart under the same exciting conditions. This work opens the door to new opportunities in generating high power supercontinuum radiation (in particular, at the middle-IR regime), and will have a great impact on many applications, including sensing and broadband multi-spectrum free space communications.     

Adaptive femtosecond pulse shaping to control supercontinuum generation in a microstructure fiber

Efficient confinement of laser radiation in the core of a photonic crystal fiber increases the nonlinear processes resulting in supercontinuum generation. The technique of adaptive pulse shaping using an evolutionary algorithm provides a method to gain control over such highly nonlinear processes. Adaptive pulse shaping of the driving laser radiation passing through the photonic crystal fiber is employed to modify the shape and composition of the output supercontinuum. Amplitude and phase shaping are used to optimize the broadband emission between 500 and 700 nm, as well as a soliton centered at 935 nm. The intensities of the emission and of the soliton driven by a shaped laser pulse increase in comparison to an unshaped pulse by factors of 4 and 3, respectively. The spectral width in the range of 500-600 nm is increased by approximately 40%. In addition, the suppression of self-steepening effects in supercontinuum spectra is demonstrated.     

Supercontinuum generation in square photonic crystal fiber with nearly zero ultra-flattened chromatic dispersion and fabrication tolerance analysis

This paper presents a simple index-guiding square photonic crystal fiber (SPCF) where the core is surrounded by air holes with two different diameters. The proposed design is simulated through an efficient full-vector modal solver based on the finite difference method with anisotropic perfectly matched layers absorbing boundary condition. The nearly zero ultra-flattened dispersion SPCF with low confinement loss, small effective area as well as broadband supercontinuum (SC) spectra is targeted. Numerical results show that the designed SPCF has been achieved at a nearly zero ultra-flattened dispersion of 0 ± 0.25 ps/(nm·km) in a wavelength range of 1.38 μm to 1.89 μm (510 nm band) which covers E, S, C, L and U communication bands, a low confinement loss of less than 10⁻⁷ dB/m in a wavelength range of 1.3 μm to 2.0 μm and a wide SC spectrum (FWHM = 450 nm) by using picosecond pulses at a center wavelength of 1.55 μm. We then analyze the sensitivity of chromatic dispersion to small variations from the optimum value of specific structural parameters. The proposed index-guiding SPCF can be applicable in supercontinuum generation (SCG) covering such diverse fields as spectroscopy applications and telecommunication dense wavelength division multiplexing (DWDM) sources.     

Supercontinuum generation by nanosecond dual-pumping near the two zero-dispersion wavelengths of a photonic crystal fiber

Supercontinuum generation by dual-wavelength nanosecond pumping in the vicinity of both zero-dispersion wavelengths of a photonic crystal fiber (PCF) is experimentally demonstrated. It is shown in particular that two pumps at 1535 nm and 767 nm simultaneously pumping near the two zero-dispersion wavelengths of a specially designed PCF yields a combined visible and infrared supercontinuum spectrum spanning from 0.55 μm to 1.9 μm. We discuss the generation mechanisms underlying the continuum formation in terms of modulation instability and cascaded Raman generation.     

Experimental investigation of supercontinuum generation in highly nonlinear dispersion-shifted fiber pumped by spectrum-sliced amplified spontaneous emission

Spectral broadening of spectrum-sliced amplified spontaneous emission (SS-ASE) in highly nonlinear, dispersion-shifted fiber in different dispersion regimes is investigated experimentally. We find that, the spectral noise of the amplified SS-ASE pump from Er³⁺-doped fiber amplifier seeds the spectral broadening via four-wave mixing or modulation instability. Stimulated Raman scattering, red-shifted Raman solitons, and blue-shifted dispersion waves all enhance the broadening of the spectrum. The effect of the polarization state of pump on supercontinuum generation is also investigated, and it is found that, linear polarization is more efficient than random polarization for pumping supercontinuum. Supercontinuum with −10 dB bandwidth of 200 nm is generated by launching linearly polarized pump with 33.5 dB m power into anomalous dispersion regime near to zero dispersion wavelength of fiber.     

High power and high beam quality CW green beam generated by diode-side-pumped intracavity frequency doubled Nd:YAG laser

We report a stable high power and high beam quality diode-side-pumped CW green laser from intracavity frequency doubled Nd:YAG laser with LBO crystal. By using a advanced resonator, a large fundamental mode size in the laser crystal and a tight focus in the nonlinear crystal could be obtained simultaneously, which are favorable for high power and high beam quality CW green laser generation. The green laser delivered a maximum 532 nm output power of 40 W. The corresponding optical-to-optical conversion efficiency and electrical-to-optical conversion efficiency were 8.6% and 5.0%, respectively. Under 532 nm output power of 34 W, the beam quality factor was measured to be 1.6.     

Flat Supercontinuum Generation at 1550 nm in a Dispersion-Flattened Microstructure Fibre Using Picosecond Pulse

The generation of a flat supercontinuum of over 80nm in the 1550nm region by injecting 1.6ps 10 GHz repetition rate optical pulses into an 80-m-long dispersion-flattened microstructure fibre is demonstrated. The fibre has small normal dispersion with a variation smaller than 1.5 （ps·nm^-1·km^-1） between 1500 and 1650nm. The generated supercontinuum ranging from 1513 to 1591 nm has the flatness of ±1.5 dB and it is not so flat in the range of several nanometres around the pump wavelength 1552nm. Numerical simulation is also used to study the effect of optical loss, fibre parameters and pumping conditions on supercontinuum generation in the dispersion-flattened microstructure fibre, and can be used for further optimization to generate flat broad spectra.     

Supercontinuum generation by combining clad-pumped Er/Yb co-doped fiber amplifier and highly nonlinear photonic crystal fiber

We present an experimental study on supercontinuum generation by combining a clad-pumped Er/Yb co-doped fiber amplifier (EYDFA) and a highly nonlinear photonic crystal fiber (HNL-PCF). By using the nonlinear polarization rotation technique, a stable femtosecond optical pulse seed signal with a central wavelength of 1556.36 nm and a spectral line width of ∼5.6 nm has been obtained. Then, this pulsed seed signal is amplified by the EYDFA, the amplified pulse, which, with the broaden spectrum, propagates in the HNL-PCF. The 20 dB bandwidth of ∼520 nm from 1230 to 1750 nm is obtained.     

Design of highly nonlinear octagonal photonic crystal fiber with near-zero flattened dispersion at 1.31μm waveband

This paper presents a highly nonlinear octagonal photonic crystal fiber (HN-OPCF) with a Ge-doped core for dental optical coherence tomography (OCT) applications. The simulation results show that a nonlinear coefficient of 56.3W⁻¹ km⁻¹ and the flattened dispersion of less than −1:0 ps/(nm·km) are obtained in 75 nm waveband (1265–1340 nm). Owing to its high nonlinear coefficient and flattened dispersion, the HN-OPCF is expected to be suitable for supercontinuum generation, which is very important in OCT applications. The numerical simulation results demonstrate that the proposed HN-OPCF can generate wideband supercontinuum.     

Build-up of supercontinuum in heated and unheated photonic crystal fibers using a chirped femtosecond laser

The build-up of supercontinuum in a photonic crystal fiber (PCF) has been investigated experimentally as a function of pump power using chirped 100-fs pulses from a Ti:sapphire laser. As compared with the PCF at room temperature, a new blue-shifted spectral component is observed in the initial steps of supercontinuum (SC) generation when the central part of PCF is heated to 120 °C by a hot plate. In addition, the slope efficiency of SG is slightly improved with the slightly extension of supercontinuum spectrum in blue edge at high pump powers. The change in dispersion property as well as the effective cascading of nonlinear photonic crystal fibers for heated PCF would be the main attributions.     

石英光子晶体光纤中高功率中红外超连续谱的产生

非石英光纤在产生大功率超连续谱方面存在难以克服的局限性.本文首次报道了采用石英光纤产生大功率中红外超连续谱.精心设计光纤结构使色散有利于超连续谱向中红外波段展宽,同时保证相对较大的芯径以承受较高的泵浦功率.合理选择光纤长度,在保证光谱展宽到3.4 μm的情况下使光纤损耗的影响降低到最小限度.研究表明,在1.95 μm皮秒脉冲泵浦下,采用色散适宜的石英光子晶体光纤可以产生20 dB带宽覆盖1 550~3 420 nm的超连续谱.超连续谱的平均功率可达56.6 W.     

Temperature effects on supercontinuum generation using a continuous-wave Raman fiber laser

We describe the effect of temperature variations on supercontinuum (SC) generation in optical fibers using a continuous-wave (CW) Raman fiber laser as a pump. We achieve supercontinuum generation by pumping only ∼2 W of power into a 7 km-long nonzero dispersion-shifted fiber (NZDSF) in the region of small anomalous dispersion. In these conditions, the supercontinuum builds up basically on modulational instability and Raman. At room temperature, the supercontinuum covers effectively the S, C and L transmission bands defined by the International Telecommunication Union (ITU). Temperature tuning of the fiber environment provides a means of tuning the fiber dispersion, and thus a means of changing the width and shape of the supercontinuum spectrum. We demonstrate a 27% increase in the 10-dB SC width. We believe that the application of this new tuning mechanism to other experimental configurations using pulsed sources might be used to produce extremely broad supercontinuums.     

High energy femtosecond supercontinuum light generation in large mode area photonic crystal fiber

A large mode area photonic crystal fiber (LMA PCF) with an effective area of 180 μm² is used to generate a high energy, micro-joule range, flat, octave spanning supercontinuum (SC) extending from ~ 600 nm to ~ 1720 nm. A train of femtosecond pulses from a widely-tunable parametric amplifier pumped by a Ti:Sapphire regenerative amplifier system are coupled into a 20 cm length of LMA PCF generating a SC of 1.4 μJ energy. We present an experimental study of the high energy SC as a function of the input power and the pumping wavelength. The spectrum obtained at a pump wavelength of 1260 nm presents spectral flatness variation less than 12 dB over more than 1.1 octave bandwidth. The physical processes behind the SC formation are described in the normal and the anomalous dispersion regions. Our experimental results are successfully compared with the numerical solution of the nonlinear Schrödinger equation.     

Different supercontinuum generation processes in photonic crystal fibers pumped with a 1064-nm picosecond pulse

Picosecond pulse pumped supercontinuum generation in photonic crystal fiber is investigated by performing a series of comparative experiments. The main purpose is to investigate the supercontinuum generation processes excited by a given pump source through the experimental study of some specific fibers. A 20-W all-fiber picosecond master oscillator-power amplifier （MOPA） laser is used to pump three different kinds of photonic crystal fibers for supercontinuum generation. Three diverse supercontinuum formation processes are observed to correspond to photonie crystal fibers with distinct dis- persion properties. The experimental results are consistent with the relevant theoretical results. Based on the above analyses, a watt-level broadband white light supercontinuum source spanning from 500 nm to beyond 1700 nm is demonstrated by using a picosecond fiber laser in combination with the matched photonic crystal fiber. The limitation of the group velocity matching curve of the photonic crystal fiber is also discussed in the paper.     

Optimization of optical parametric chirped pulse amplification with different pump wavelengths

Optical parametric chirped pulse amplification with different pump wavelengths was investigated using LBO crystal, at signal central wavelength of 800 nm. According to our theoretical simulation, when pump wavelength is 492.5 nm, there is a maximal gain bandwidth of 190 nm centered at 805 nm in optimal noncollinear angle using LBO. Presently, pump wavelength of 492.5 nm can be obtained from second harmonic generation of a Yb:Sr₅(PO₄)₃F laser. The broad gain bandwidth can completely support ∼6 fs with a spectral centre of seed pulse at 800 nm. The deviation from optimal noncollinear angle can be compensated by accurately tuning crystal angle for phase matching. The gain spectrum with pump wavelength of 492.5 nm is much better than those with pump wavelengths of 400, 526.5 and 532 nm, at signal centre of 800 nm.     

Multi-wavelength operation of LD side-pumped Nd:YAG laser

A dual-wavelength laser at 1064 nm and 1319 nm is obtained by a single Nd:YAG crystal rod. On the basis of 1064 nm and 1319 nm dual-wavelength laser installation, the second harmonic waves at 532 nm and 660 nm can be achieved by using non-linear frequency conversion technology. When 1064 nm and 1319 nm lasers oscillate simultaneously, the maximum output power is 30.5 W and 8.78 W, respectively. When the 1319 nm laser is restrained, we obtain a 35.6 W maximum output power at 1064 nm and by contrary 11.2 W at 1319 nm. The maximum output powers of 532 nm and 660 nm lasers are 5.34 W and 1.353 W when oscillating simultaneously. With one of them restrained, the maximum output power is 6.72 W at 532 nm and 1.90 W at 660 nm. The optimum repetition rate of the acousto-optic Q-switch is 10.5 KHz and 20.5 KHz for 532 nm and 660 nm lasers, respectively. The optical-to-optical conversion efficiency from the fundamental waves to the harmonic waves is 17.5% and 15.4%. The instability is less than 2%.