Tailoring chromatic dispersion in chalcogenide–tellurite microstructured optical fiber

We report fabrication of a highly nonlinear hybrid microstructured optical fiber composed of chalcogenide glass core and tellurite glass cladding. The flattened chromatic dispersion can be achieved in such an optical fiber with near zero dispersion wavelength at telecommunication wavelengths λ = 1.35–1.7 μm, which cannot be achieved in chalcogenide glass optical fibers due to their high refractive index, i.e. n > 2.1. We demonstrate a hybrid 4-air hole chalcogenide–tellurite optical fiber (Δn = 0.25) with flattened chromatic dispersion around λ = 1.55 μm. In optimized 12-air hole optical fiber composed of the same glasses, the chromatic dispersion values were achieved between −20 and 32 ps/nm/km in a broad wavelength range of 1.5–3.8 μm providing the fiber with extremely high nonlinear coefficient 86,000 km⁻¹W⁻¹. Hybrid chalcogenide/tellurite fibers pumped with the near infrared lasers give good promise for broadband optical amplification, wavelength conversion, and supercontinuum generation in the near- to mid-infrared region.     

Invited paper: Short pulse generation in mid-IR fiber lasers

Mode-locked fiber lasers emitting short pulses of light at wavelengths of 2 μm and longer are reviewed. Rare-earth doped silica and fluoride fiber lasers operating in the mode-locked regime in the mid-IR (2–5 μm) have attracted attention due to their usefulness to spectroscopy, nonlinear optics, laser surgery, remote sensing and ranging to name a few. While silica fiber lasers are fundamentally limited to emission wavelengths below 2.2 μm, fluoride fiber lasers can reach to nearly 4 μm. The relative infancy of fluoride fibers as compared to silica fibers means the field has work to do to translate the mode-locking techniques to systems beyond 2 μm. However, with the recent demonstration of a stable, mode-locked 3 μm fiber laser, the possibility of achieving high performance 3 μm class mode-locked fiber lasers looks promising.     

Modulation instability initiated high power all-fiber supercontinuum lasers and their applications

High average power, all-fiber integrated, broadband supercontinuum (SC) sources are demonstrated. Architecture for SC generation using amplified picosecond/nanosecond laser diode (LD) pulses followed by modulation instability (MI) induced pulse breakup is presented and used to demonstrate SC sources from the mid-IR to the visible wavelengths. In addition to the simplicity in implementation, this architecture allows scaling up of the SC average power by increasing the pulse repetition rate and the corresponding pump power, while keeping the peak power, and, hence, the spectral extent approximately constant. Using this process, we demonstrate >10 W in a mid-IR SC extending from ～0.8 to 4 μm, >5 W in a near IR SC extending from ～0.8 to 2.8 μm, and >0.7 W in a visible SC extending from ～0.45 to 1.2 μm. SC modulation capability is also demonstrated in a mid-IR SC laser with ～3.9 W in an SC extending from ～0.8 to 4.3 μm. The entire system and SC output in this case is modulated by a 500 Hz square wave at 50% duty cycle without any external chopping or modulation. We also explore the use of thulium doped fiber amplifier (TDFA) stages for mid-IR SC generation. In addition to the higher pump to signal conversion efficiency demonstrated in TDFAs compared to erbium/ytterbium doped fiber amplifier (EYFA), the shifting of the SC pump from ～1.5 to ～2 μm is pursued with an attempt to generate a longer extending SC into the mid-IR. We demonstrate ～2.5 times higher optical conversion efficiency from pump to SC generation in wavelengths beyond 3.8 μm in the TDFA versus the EYFA based SC systems. The TDFA SC spectrum extends from ～1.9 to 4.5 μm with ～2.6 W at 50% modulation with a 250 Hz square wave. A variety of applications in defense, health care and metrology are also demonstrated using the SC laser systems presented in this paper.     

Design on a highly birefringent and highly nonlinear tellurite ellipse core photonic crystal fiber with two zero dispersion wavelengths

In this paper, a new photonic crystal fiber (PCF) with two zero dispersion wavelengths (ZDWs) based on the tellurite ellipse core is designed. The air holes in the cladding region have a V-shape distribution, which can increase the birefringence. By adjusting the size of tellurite ellipse core, different birefringence and nonlinearity coefficient can be obtained, and the dispersion can also be tailored. When the long axis of the tellurite ellipse core is 0.5 μm and the short axis is 0.25 μm, the birefringence of 7.66 × 10⁻² and nonlinearity of 3400 W⁻¹ km⁻¹ around 1550 nm are obtained. This PCF structure provides a way to get the high birefringence and nonlinearity at the same time, which can find extensive applications in the optical communication and sensor system.     

A highly nonlinear photonic quasi-crystal fiber with low confinement loss and flattened dispersion

We proposed a novel photonic quasi-crystal fiber with near-zero flattened dispersion, highly nonlinear coefficient, and low confinement loss by using the dual concentric core structure. By optimizing the structure parameter, the proposed photonic quasi-crystal fiber can achieve a nonlinear coefficient larger than 33 W⁻¹ km⁻¹ and near-zero flatten dispersion of 0 ± 3.4 ps/nm/km with a near-zero dispersion slope of 8.5 × 10⁻³ ps/nm²/km at the wavelength of 1550 nm. Near-zero flattened dispersion and low confinement loss in the ultralow order of 10⁻⁷ dB/m are simultaneously obtained in the wavelength range from 1373 to 1627 nm. Furthermore, two zero dispersion wavelengths can be achieved in a wide wavelength ranger from 1373 to 1725 nm. From the point of view of practical fabrication, the influence of deviation of each air hole diameter within 3% of imperfections on dispersion, nonlinearity, and is discussed to verify the robustness of our design.     

Flatly broadened mid-infrared supercontinuum generation in a cascade of thulium-doped silica fiber amplifiers

Single-mode, flatly broadened supercontinuum (SC) generated in a thulium two-stage fiber amplifier spanning nearly the mid-infrared band is reported. The output average power and 10 dB bandwidth of the obtained SC are over 2.3 W and 570 nm (from 1.95 μm to 2.52 μm), respectively. For wavelengths beyond 2.4 μm the output power was 495 mW constituting almost 21% of the total output power. Applying Tm-doped single-mode silica fibers as nonlinear and amplification media it was possible to extend the long wavelength cut-off to 2.7 μm.     

Advances in 2-μm Tm-doped mode-locked fiber lasers

Over the last five years, the number of demonstrations of mode-locked thulium-doped fiber lasers with output wavelengths around 2 μm has increased rapidly. Mode-locked Tm-doped fiber lasers now provide pulse energies above 150 μJ and durations less than 30 fs (although not simultaneously). Applications for these sources are continuously being developed as they become commercially available and currently include medicine, environmental sensing, materials processing, and defense. A review of previously demonstrated mode-locked thulium-doped fiber lasers up to the state-of-the-art will be presented along with the aforementioned applications of these sources.     

High-power mid-infrared supercontinuum sources: Current status and future perspectives

Mid-infrared (mid-IR) supercontinuum (SC) sources have recently gained much interest, as a key technology for such applications as spectral molecular fingerprinting, laser surgery, and infrared counter measures. However, one of the challenges facing this technology is how to obtain high power and broadband light covering a spectral band of at least 2–5 µm, especially with a very efficient output power distribution towards the mid-IR region. This directly affects their usage in the practical applications mentioned above. Typically, an SC is generated by pumping a piece of nonlinear fibre with high-intensity femtosecond pulses provided by mode-locked lasers. Although this approach can lead to wide continuum generation, the output power is limited only to the milliWatt level. Therefore, to achieve high-power SC light, other laser systems need to be employed as pump sources.This paper briefly reviews SC sources, restricted to those with an average output power of over 0.4 W and simultaneously with a long-wavelength edge of the continuum spectrum of over 2.4 µm. Firstly, the concepts of SC generation, including the nonlinear phenomena governing this process and the most relevant mid-IR fibre materials, are presented. Following this study, a review of the main results on SC generation in silica and soft-glass fibres, also including my experimental results, is presented. Emphasis is given to high-power SC generation with the use of different pump schemes, providing an efficient power distribution towards longer wavelengths. Some discussion and prospective predictions are proposed at the end of the paper.     

Ultra-flattened chromatic dispersion and highly nonlinear photonic crystal fibers with ultralow confinement loss employing hybrid cladding

In this paper, two new types of dispersion-flattened photonic crystal fibers (DF-PCFs) with highly nonlinear and ultralow confinement loss are proposed. These new PCF structures adopt hybrid cladding with different air-holes diameters, pitches and air-holes arranged fashions. In order to analyze the proposed PCFs, the full-vector finite element method with anisotropic perfectly matched layers has been used. Results show that the ultra-flattened dispersion of 0.931 ps/(nm km) (DF-PCF1) and 1.533 ps/(nm km) (DF-PCF2) can be achieved in the wavelength range from 1.3 to 1.6 μm with confinement losses lower than 0.001 dB/km in the same wavelength range. Meanwhile, the nonlinear coefficients of our proposed PCFs are greater than 23.83 W⁻¹ km⁻¹ (DF-PCF1) and 29.65 W⁻¹ km⁻¹ (DF-PCF2) at the wavelength of 1.55 μm, and two near-zero dispersion values of 0.328 ps/(nm km) (DF-PCF1) and −0.015 ps/(nm km) (DF-PCF2) can also be obtained at the same wavelength. Furthermore, the influence of manufacturing imperfections of parameters on dispersion and nonlinearity is discussed to verify the robustness of our design.     

Proposal for highly birefringent broadband dispersion compensating octagonal photonic crystal fiber

In this paper, we propose and demonstrate a highly birefringent photonic crystal fiber based on a modified octagonal structure for broadband dispersion compensation covering the S, C, and L-communication bands i.e. wavelength ranging from 1460 to 1625 nm. It is shown theoretically that it is possible to obtain negative dispersion coefficient of about −400 to −725 ps/(nm km) over S and L-bands and a relative dispersion slope (RDS) close to that of single mode fiber (SMF) of about 0.0036 nm⁻¹. According to simulation, birefringence of the order 1.81 × 10⁻² is obtained at 1.55 μm wavelength. Moreover, effective area, residual dispersion, effective dispersion, confinement loss, and nonlinear coefficient of the proposed modified octagonal photonic crystal fiber (M-OPCF) are also reported and discussed.     

A self-seeded tunable multiwavelength erbium-doped fiber ring laser with fiber bending scheme

A new tunable multiwavelength fiber laser is proposed. Such a laser contains a homogeneous and inhomogeneous broadening media, i.e., a Fabry–Perót laser diode and an erbium-doped fiber amplifier, in the laser cavity. The Fabry–Perót laser diode is used to obtain tunable multiwavelength lasing. By adjusting the injection current of the Fabry–Perót laser diode, emission at a single wavelength, dual wavelengths, triple wavelengths or quadruple wavelengths are obtained. The lasing wavelength is tuned by bending a section of fiber in the laser cavity. The tuning ranges for single wavelength lasing and dual wavelength lasing are 20 nm and 10 nm, respectively, while those for triple wavelength lasing and quadruple wavelength lasing are 7 nm and 3 nm, respectively.     

A new method for fiber chromatic dispersion measurement with microwave interference effect

The chromatic dispersion (CD) is a key parameter for optical fibers. Based on the microwave interference effect, a new method for CD measurement of optical fibers is proposed. The radio frequency (RF) signals carried by two light-waves with different wavelengths transmit through the dispersive optical fiber under test. After photo-detector they interfere with each other due to the different phase shifts induced by the CD of fiber. The CD can be obtained by monitoring the changing interference RF power with scanning the wavelength of tunable laser source. The CD values of single mode fiber and dispersion compensating fiber are measured within the wavelength range from 1525 to 1605 nm. The common phase shift method is used to measure the CDs of the two types of fiber, which demonstrates the feasibility and veracity of the proposed method.     

A single mode ultra flat high negative residual dispersion compensating photonic crystal fiber

In this paper, a single mode photonic crystal fiber based on hexagonal architecture is numerically demonstrated for the purpose of residual dispersion compensation in the wavelength range of 980–1580 nm. The designed fiber offers ultraflattened negative dispersion in the near-infrared to most widely used S to L wavelength bands and average dispersion of about −138 ps/(nm km) with an absolute dispersion variation of 12 ps/(nm km). Besides, the proposed fiber successfully operates as a single mode in the entire band of interest. Moreover, to check the dispersion accuracy, sensitivity of the fiber dispersion properties to a ±1–5% variation in the optimum parameters is studied for practical conditions.     

Experimental Study on the Role of Chromatic Dispersion in Continuous-Wave Supercontinuum Generation

The influence of chromatic dispersion on continuous-wave (CW)-pumped supercontinuum (SC) generation in kilometer-long standard fibers is experimentally investigated. We perform our study by means of a tunable, high-power fiber ring laser pumping a dispersion-shifted fiber in the wavelength range of small and medium anomalous dispersion. Our results show that, at low input powers, chromatic dispersion plays a dominant role on nonlinear pump spectral broadening, giving rise to a broader spectrum when pumping just above the zero-dispersion wavelength of the fiber. At higher input powers, however, the width of the generated SC spectrum is mostly due to the Raman effect, hence more independent of the value of the chromatic dispersion coefficient. We show that, in this case, the optimum pumping wavelengths for SC generation are not so close to the zero-dispersion wavelength of the fiber as in the previous case. In these conditions, as the chromatic dispersion grows, we can obtain square-shaped and high-power density spectra, which seem extremely promising for applications in optical coherence tomography.      

Polarization Mode Dispersion in Distributed Erbium-Doped Fibers

In the design of distributed erbium-doped fibers (DEDFs) for ultra-high bit-rate ( > 40 Gbit/s) soliton transmission, polarization mode dispersion (PMD) is an important design parameter. We have measured the PMD values of DEDFs from two different sources and found them to be an order of magnitude higher than those suitable for ≥ 40 Gbit/s soliton transmission. Theoretical modeling and microscopic inspection of fiber core ellipticity have been carried out to understand the high PMD values in these DEDFs. It is believed that core ellipticity and erbium doping process-induced stress are the main reasons for the high PMD values in these fibers. This highlights an important design and fabrication problem which must be solved before DEDFs can fulfil their promise as a channel for long-haul ultra-high bit-rate soliton transmission.     

微结构光纤中的相位匹配分析

通过数值计算分析,讨论了微结构光纤（MF）中四波混频（FWM）的相位匹配条件与色散分布的关系.首先,介绍了光纤中四波混频基本理论,并且推导出了符合MF的相位匹配公式.其次,对零色散点分别为一个和两个的MF相位匹配进行了数值分析.最后,通过数值计算对一些实验中出现的四波混频现象进行了解释.     

Design, performance, and limitations of fibers for cladding-pumped Raman lasers

In this paper, we present basic design rules for double-clad fibers that enable efficient 1st-Stokes operation of cladding-pumped fiber Raman devices. Limiting factors that we treat include unwanted 2nd-Stokes generation, material damage in the core, pump pulse dispersion, inner-cladding NA, background loss, pulse shape and pump noise. With a well-designed fiber, we calculate that a pump-to-signal brightness enhancement of over 2000 is possible, with certain pump parameters. On experimental work, we report a 100 W cladding-pumped fiber Raman laser at 1120 nm. Furthermore, a double-clad Raman fiber with large-mode area, ～40 μm core diameter, pumped by a Q-switched Nd:YAG laser is used to generate Stokes pulses with 1 mJ energy.     

Grating compensation of third-order fiber dispersion

Subpicosecond optical pulses propagating in single-mode fibers are severely distorted by third-order dispersion even at the fiber's zero-dispersion wavelength (λ₀). Using cross-correlation techniques, the authors measured the broadening of a 100-fs pulse to more than 5 ps after passing through 400 m of fiber near λ₀. The measured asymmetric and oscillatory pulse shape is in agreement with calculations. A grating and telescope apparatus was configured to simultaneously equalize both third- and second-order dispersion for wavelengths slightly longer than λ ₀. Nearly complete compensation has been demonstrated for fiber lengths of 400 m and 3 km of dispersion-shifted fiber at wavelengths of 1560-1580 nm. For the longer fibers, fourth-order dispersion due to the grating becomes important     

Optical hydrogen sensor based on etched fiber Bragg grating sputtered with Pd/Ag composite film

A novel fiber optical fiber hydrogen sensor based on etched fiber Bragg grating coated with Pd/Ag composite film is proposed in this paper. Pd/Ag composite films were deposited on the side-face of etched fiber Bragg grating (FBG) as sensing elements by magnetron sputtering process. The atomic ratio of the two metals in Pd/Ag composite film is controlled at Pd:Ag = 76:24. Compared to standard FBG coated with same hydrogen sensitive film, etched FBG can significantly increase the sensor’s sensitivity. When hydrogen concentrations are 4% in volume percentage, the wavelength shifts of FBG-125 μm, FBG-38 μm and FBG-20.6 μm are 8, 23 and 40 pm respectively. The experimental results show the sensor’s hydrogen response is reversible, and the hydrogen sensor has great potential in hydrogen’s measurement.