High Power, High Efficiency, Continuous-Wave Supercontinuum Generation using Standard Telecom Fibers

Introduction It is perhaps not surprising that using extremely high power and short pulse duration pump sources leads to dramatic nonlinear processes in optical fibres; in contrast, the generation of a supercontinuum from a continuous pump wave of relatively meagre power is at first sight, astounding. Yet supercontinua spanning over 1000 nm have been generated with pump powers of a few tens of watts – orders of magnitude lower than pulse pumped systems. The key to continuous wave (CW) supercontinuum generation is the utilisation of modulation instability (MI). This is inherent to any anomalously dispersive, nonlinear medium, and has been observed in a wide range of systems. This instability can enable the creation of the extremes of peak power and pulse duration necessary for dramatic nonlinear processes to occur, even from very low power CW pump lasers. But although MI from CW pump lasers was observed in the 1980s by Itoh et al. (1989), other factors required for efficient continuum generation were missing, causing another decade to pass before such results were obtained. A full review of experimental results will be presented later, but for reference, some examples of continuous wave supercontinua are shown in Fig. 8.1, which illustrate the high spectral power and the spectral smoothness and flatness which are characteristic of CW continuum generation. Although significantly different from the physical mechanisms involved in ultra-short – femtosecond based – supercontinuum generation, the basic physical processes underpinning CW continuum generation are the same as for longer pump pulses (greater than a few picoseconds) and the observations and conclusions developed in the 1980s surrounding these type of sources apply to the CW regime.

We have demonstrated a similar to 34 W continuous wave supercontinuum using the standard telecom fiber (SMF 28e). The supercontinuum spans over a bandwidth of similar to 1000 nm (>1 octave) from 880nm to 1900 nm with a substantial power spectral density of >1mW/nm from 880-1350 nm and similar to 50-100mW/nm in 1350-1900 nm. The distributed feedback Raman laser architecture was used for pumping the supercontinuum which ensured high efficiency Raman conversions and helped in achieving a very high efficiency of similar to 44% for supercontinuum generation. Using this architecture, Yb laser operating at any wavelength can be used for generating the supercontinuum and this was demonstrated by using two different Yb lasers operating at 1117nm and 1085 nm to pump the supercontinuum.

Supercontinuum (SC) light sources are nowadays a very common way to access a large span of wavelengths, usually ranging from the near ultraviolet (around 400 nm) to the infrared (around 2.4 μm). It corresponds to a range of interest for many applications in optics for measuring transmission, dispersion, or in biophotonics for achieving fluorescencemicroscopy, optical coherence tomography... Indeed these sources are really promising because they should allow to replace the N laser sources used in these experimental setups to access to all theses wavelengths by a single broad one and a spectral filtering apparatus. Most of these results have been obtained by using powerful pump lasers of several kilo-Watts peak power, operating from the femtosecond (Titane:Saphire) to the nanosecond regimes (Nd:YAG), launched in the low dispersion region of a microstructured optical fiber. Although these fibers are short enough (typically from 1 to 10 m) to neglect the linear absorption during the propagation of the pump, the spectral power density is relatively low (few hundreds of μw/nm)which could limit the implementation of SC sources inmany application devices. This is related to a technological limitation of the pump source because it is not easy to combine strong peak power and high average power. One of the simplest solutions to increase the the spectral power density of SC sources is to replace pulsed sources with continuous-wave (CW) light sources whose available average powers are much more important. Wewill see that the dynamics of SC formation is considerably different in this case, requiring to perform intensive numerical studies to optimize the fiber parameters. Indeed, longer fibers are required (from tens to hundreds of meters) which heightens sensitivity to fiber attenuation, namely of the OHpic absorption, that strongly impacts the soliton evolution. However extremely powerful SCs have been reported with more than 10 mW/nm of spectral power density. Furthermore, these pump sources are usually all-fiber that leads to a second advantage against most of pulsed SC because CW pump can be directly spliced on the PCF. It is also important to point out that these SC sources have different temporal properties than the ones of pulsed SCs. The first experimental demonstration of CW SC have been realized at the end of the nineties with a Raman laser launched in a standard telecommunication fiber. The spectral broadening was relatively restricted (around 200 nm) because it was mainly due to Raman effect Gonzalez-Herraez et al. (2003); Persephonis et al. (1996); Prabhu et al. (2000). A breakthrough was reached a few years later when stronger pump lasers (from more than one order of magnitude) based on Ytterbium doped fibers were combined with photonic crystal fibers 4

Quantum cryptography systems can operate over relatively long distances in standard telecommunications fiber by taking advantage of the low transmission losses in these fibers at 1.3 or 1.5 microns. Although there has been much progress toward the development of highly efficient and low-noise detectors for these wavelengths, silicon avalanche photodiodes currently offer superior single photon counting performance, but only at visible and near IR wavelengths where the fiber transmission is poor. For ranges typical of local area networks, it is possible that a quantum key distribution (QKD) system operating below 850nm could be optimal, even though standard telecommunications fiber supports multiple optical modes at these wavelengths. We have recently developed an optical mode filter that allows efficient higher order mode rejection from standard telecommunications fiber near 830nm. We have used this type of filter to launch and recover QKD signals from a polarization-based system implementing the BB84 quantum cryptography protocol. Here we present results from testing and operation in installed fiber links ranging up to 3km. These results demonstrate that the filters can attenuate the higher order modes over 35dB while having a minimal (<1dB) impact on the fundamental mode carrying the QKD signal.

Currently, fiber Bragg gratings obtained on the basis of photoinduced optical fibers doped with a high concentration of germanium oxide are used as highly sensitive sensors. However, it is worth noting a significant drawback – the manufacturing technology of optical fibers doped with germanium is expensive. When recording Bragg gratings in a standard telecommunication fiber, where the molar concentration of germanium in the fiber core is from 3 % to 5 %, interference occurs due to very low and insufficient light sensitivity. Thus, an important role is played by solving the problem of low photosensitivity of standard telecommunication fibers for recording Bragg gratings. This paper presents the results of studies of the spectral characteristics of fiber Bragg gratings based on standard telecommunication fibers pre-saturated with hydrogen to increase photosensitivity. According to the results obtained, it was found that under the action of UV radiation in the presence of hydrogen, the photosensitivity of the fiber increases and the Bragg wavelength shift is associated with the saturation of the fiber with hydrogen, the effective modulation amplitude of the induced refractive index is equal to 1.2 with a refractive index of 1.438. This work proves that the VBR recorded in the S pre-saturated in hydrogen for 12 days is characterized by increased photosensitivity. The experimental results obtained make it possible to use a Bragg fiber array based on a standard telecommunications optical fiber saturated with hydrogen in the field of telecommunications, seismology, engineering geology as fiber-optic sensors of pressure, deformation, temperature, rotation and rotation, including in extreme environmental conditions

Simulation of splicing process of fibers with large effective area A_eff with standard telecommunication fibers is presented in the article. The aim of the simulation was to design the process of diffusion in spliced fibers in order to compensate radiuses of mode fields of spliced fibers. The simulation showed that refractive index profiles both in fibers with step and step (standard telecommunication fibers) index refractive profile reach, after appropriate design of diffusion process, single diffusion maximum, that may be approximated with Gaussian distribution. The results of estimation of loss of above mentioned splicings were presented. Adjusting of mode field radius and length of transit areas TEC (<i>Thermally Diffused Expanded Core Area</i>) was taken under consideration.

We report on energy-time and time-bin entangled photon-pair sources based on a periodically poled lithium niobate (PPLN) waveguide. Degenerate twin photons at 1314 nm wavelength are created by spontaneous parametric down-conversion and coupled into standard telecom fibers. Our PPLN waveguide features a very high conversion efficiency of about 10^(-6), roughly 4 orders of magnitude more than that obtained employing bulk crystals. Even if using low power laser diodes, this engenders a significant probability for creating two pairs at a time - an important advantage for some quantum communication protocols. We point out a simple means to characterize the pair creation probability in case of a pulsed pump. To investigate the quality of the entangled states, we perform photon-pair interference experiments, leading to visibilities of 97% for the case of energy-time entanglement and of 84% for the case of time-bin entanglement. Although the last figure must still be improved, these tests demonstrate the high potential of PPLN waveguide based sources to become a key element for future quantum communication schemes

A demonstration of the principles of quantum key distribution (QKD) is performed using a single-photon source in a proof of concept test-bed over a distance of 2 km in standard telecommunications optical fiber. The single-photon source was an optically-pumped quantum dot in a microcavity emitting at a wavelength of 895 nm. Characterization of the QKD parameters was performed at a range of different optical excitation powers. An investigation of the effect of varying the optical excitation power of the quantum dot microcavity on the quantum bit error rate and cryptographic key exchange rate of the system are presented.

We demonstrate a novel multiwavelength light source around the 1.7-μm region based on continuous-wave (CW) supercontinuum (SC) generation for the deeper imaging of low-water-absorption sample with a spectrally sampled optical coherence tomography (OCT) system. This self-regulated Gaussian spectrum of CW SC source has an extremely smooth flat width with more than 210-nm full-width at half-maximum bandwidth centered at the 1.7-μm region, a high output power of 500 mW, and a low temporal instability with less than 0.1 dB for an hour. In order to improve the OCT signal sensitivity without increasing the optical illumination power, a multiwavelength light source based on this CW SC was implemented by using a tunable fiber Sagnac comb filter. When it was applied to the spectrally sampled OCT imaging, the multiwavelength light source enabled a dynamic range improvement, compared to conventional continuous spectral light source with the same average optical power.

We experimentally investigate continuous-wave (CW) laser-pumped cascaded random Raman fiber-laser (CRRFL) architectures based on standard telecom fiber for low intensity noise supercontinuum (SC) generation. Our objective is to identify conventional pump laser designs that minimize wavelength and radio frequency (RF) resolved relative intensity noise (RIN) - a key determinant of sensitivity of low coherence interferometry application like ultrahigh-resolution optical coherence tomography (UHR-OCT). Three CW Yb-fiber pump configurations were evaluated: a single-stage fiber Bragg grating (FBG)-based bidirectional linear cavity, an FBG and a unidirectional ring-cavity seeded two-stage master-oscillator power-amplifier (MOPA) systems. Each laser delivered ∼75 W at 1064 nm into 2 km of SMF-28 Ultra, generating up to ∼34 W of SC spanning 1000-1700nm (measurement-limited) with power spectral densities (PSDs) exceeding 50 mW/nm. Spectrally resolved RIN was measured from 1200-1700nm across two RF bands: the quantum-noise-dominated regime (1-40 MHz) and the UHR-OCT relevant regime (10-160 kHz). Ring-based CRRFL SC systems consistently exhibited the lowest integrated rms RIN (∼2.5% in the 1-40 MHz band and ∼0.35% in the 10-160 kHz band), outperforming FBG-based implementations (5% and 0.4%, respectively). Importantly, this improvement persists even when the ring-laser free spectral range (FSR) is matched to that of the FBG laser, confirming that the RIN reduction arises from the intrinsic unidirectional lasing dynamics of the ring cavity rather than cavity FSR differences. To assess application-level relevance, we benchmark our best CRRFL-SMF SC sources against a commercial picosecond photonic crystal fiber (PCF)-based SC system. Across 1200-1700nm, the ring-MOPA-pumped SC exhibits at least a four-fold reduction in spectrally resolved RIN while providing comparable spectral flatness and nearly an order-of-magnitude higher PSD (>50 mW/nm vs ∼5 mW/nm). To our knowledge, this is the first systematic wavelength and RF frequency resolved noise characterization, including an explicit assessment of cavity FSR effects of conventional CW pumped CRRFL based SC sources, establishing their strong potential as low-cost, low-noise alternatives to commercial PCF-SCs for UHR-OCT and other low-coherence interferometry applications.

General techniques for the quality control of long lengths of optical fiber for telecommunications applications have been developed during the past 10 years in concert with ongoing handling scenarios that involved cabled versions of the tested fiber. In the present work, we discuss the same quality control techniques, but apply the results to stand-alone fiber (primary or secondary buffer only). Furthermore, by modifying the normal quality control techniques, we have produced a stand-alone fiber that exhibits superior mechanical performance for missile payout applications when compared with standard telecommunications fiber. High quality optical fibers, which were produced to more exacting standards than long-haul telecommunications fibers, were subsequently exposed to high-stress bend tests along multikilometer lengths of the fiber. Mechanical failures during these tests could occur at stresses lower than the required 200 ksi proof test. The fracture surfaces for these failures were examined by normal fractographic techniques. These surfaces clearly showed that mechanical failure occurred at stresses below the proof stress. Furthermore, an appreciable number of these failures were accompanied by damage to the buffer, presumably preceding the mechanical failure. To correct the situation, we introduced changes in both the process and the testing for these fibers. During processing, the fibers were prepared with high quality silica substrate tubing, and all processing was carried out in a clean room environment. Proof testing subsequent to production was changed from a single test at 200 ksi to multiple testing at 100 ksi increments. Testing was further modified to include preinspection of the entire buffer surface for flaws of a serious enough nature to lead to mechanical damage to the underlying glass. The interaction of all of the previously noted changes helped to increase the reliability of the optical fiber for missile payout applications. Fractographs of the failures, proof test data, and identification of the buffer flaws, independent of fiber manufacturer or manufacturing process, will be discussed.

Bismuth-doped optical fibres fully compatible with standard telecommunication fibres are developed. Lasers based on such fibres emitting in the spectral range 1430–1500 nm with an efficiency of up to 7.6% at room temperature and up to 10.5% at a temperature of -65°C are fabricated.

We demonstrate a 143.4 W high power combined white supercontinuum (SC) source using a (7 × 1) SC fiber combiner. Based on the criteria of adiabatic tapering and brightness conservation, a (7 × 1) SC fiber combiner is designed and theoretically investigated, and the simulated results of the transmission efficiencies of laser at different wavelengths of the combiner verify that the combiner has the ability of combining white SC sources efficiently. Then, the (7 × 1) SC fiber combiner is fabricated, and three white SC fiber laser sources with average power of about 50 W are set up for beam combining experiment. Finally, a 143.4 W high power combined white SC source is achieved with high combining efficiency of 97.4%. and the corresponding combined SC spectrum ranges from 450 nm to 1700 nm, which remains consistent with the overall spectral range of the three SC sources. The experiment result indicates that the (7 × 1) SC fiber combiner is greatly capable of combining high power white SC sources efficiently, thereby increasing the average power of the white SC source significantly. To the best of our knowledge, this is the first demonstration of a fiber combiner for high power white SC sources with such a high combining efficiency.

Towards tuning of thermal sensitivity of the long period fiber gratings using a liquid crystal layerA high-efficiency thermal tuning filter based on a long-period fiber grating (LPFG) combined with a low-birefringence liquid crystal (LB LC) cladding layer is presented. Two types of LPFGs were studied and compared: the LPFGs based on a standard telecommunication fiber produced by an electric arc technique, and the LPFGs based on a boron co-doped fiber written by a UV technique. Both types of LPFGs when enhanced with an external LB LC layer exhibit two different temperature sensitivities, which depend on the temperature range of operation. For the LPFGs based on standard telecommunication fiber we can conclude that the presence of the LB LC cladding increases the thermal tuning efficiency by more than one order of magnitude over the value for the LPFGs in air. In the case of the LPFGs based on the boron co-doped fiber we discovered it is possible to obtain either a temperature-independent attenuation band or the attenuation bands with high temperature sensitivities, just by careful choice of the order of the cladding mode and the operating wavelength.

Supercontinuum generation in a highly nonlinear, dispersion-shifted fiber at 1550 nm is discussed. Spectrum generation under both pulsed and continuous-wave conditions is considered. With a few meters of highly nonlinear, dispersion-shifted fiber and a femtosecond erbium fiber laser, an octave-spanning supercontinuum is demonstrated. Kilometer lengths of nonlinear fiber pumped by a continuous-wave Raman fiber laser are shown to generate a continuum with a bandwidth greater than 247 nm. A nonlinear Schrodinger-equation model is used to investigate the effect of varying the dispersion on the pulsed continuum and noise effects in the continuous-wave continuum.