Optical frequency up-conversion by supercontinuum-free widely-tunable fiber-optic Cherenkov radiation

Optical analogue gravity physics: resonant radiation

The photonic crystal fibre (PCF) is a unique medium giving us the opportunity to perform experiments in carefully chosen regimes with precision and control. Using PCFs, we can perform analogue gravity experiments to study the physics of Hawking radiation and related processes such as resonant radiation. We discuss the similarities and differences between these processes and experimentally investigate the limits of effects of this type, dis- covering a new regime of record efficiency. We measure a 60% energy conversion efficiency from a pump to a visible femtosecond pulse by the process of resonant radiation, and demonstrate its extraordinary tunability in wavelength and bandwidth. Beyond analogue gravity, these femtosecond visible pulses provide a desirable laser source useful across a variety of modern scientific fields. This article is part of a discussion meeting issue 'The next generation of analogue gravity experiments'.

Four-wave mixing in Ar-filled hollow core bandgap photonic crystal fiber

To study the four-wave mixing (FWM) effect and wavelength conversion in a hollow core bandgap photonic crystal fiber filled with Ar, we conducted an experiment using a femtosecond laser with the pulse width of 120 fs, a repetition rate of 76 MHz, and tunable central wavelength from 760 to 980 nm. It is observed that new spectra are generated in both sides of the pump at a special wavelength, which can exactly satisfy the phase matching conditions of FWM. Combining experimental results with theoretical analysis, we find that the experimental phenomenon is mainly caused by FWM, and some other nonlinear phase effects, such as self-phase modulation, stimulated Raman scattering, and the soliton effect, have also occurred in this nonlinear process.

Nonlinearity-tailored fiber laser technology for low-noise, ultra-wideband tunable femtosecond light generation

The emission wavelength of a laser is physically predetermined by the gain medium used. Consequently, arbitrary wavelength generation is a fundamental challenge in the science of light. Present solutions include optical parametric generation, requiring complex optical setups and spectrally sliced supercontinuum, taking advantage of a simpler fiber technology: a fixed-wavelength pump laser pulse is converted into a spectrally very broadband output, from which the required resulting wavelength is then optically filtered. Unfortunately, this process is associated with an inherently poor noise figure, which often precludes many realistic applications of such super-continuum sources. Here, we show that by adding only one passive optical element-a tapered photonic crystal fiber-to a fixed-wavelength femtosecond laser, one can in a very simple manner resonantly convert the laser emission wavelength into an ultra-wide and continuous range of desired wavelengths, with very low inherent noise, and without mechanical realignment of the laser. This is achieved by exploiting the double interplay of nonlinearity and chirp in the laser source and chirp and phase matching in the tapered fiber. As a first demonstration of this simple and inexpensive technology, we present a femtosecond fiber laser continuously tunable across the entire red-green-blue spectral range.

Fiber-based 1150-nm femtosecond laser source for the minimally invasive harmonic generation microscopy

Harmonic generation microscopy (HGM) has become one unique tool of optical virtual biopsy for the diagnosis of cancer and the in vivo cytometry of leukocytes. Without labeling, HGM can reveal the submicron features of tissues and cells in vivo. For deep imaging depth and minimal invasiveness, people commonly adopt 1100- to 1300-nm femtosecond laser sources. However, those lasers are typically based on bulky oscillators whose performances are sensitive to environmental conditions. We demonstrate a fiber-based 1150-nm femtosecond laser source, with 6.5-nJ pulse energy, 86-fs pulse width, and 11.25-MHz pulse repetition rate. It was obtained by a bismuth borate or magnesium-doped periodically poled lithium niobate (MgO:PPLN) mediated frequency doubling of the 2300-nm solitons, generated from an excitation of 1550-nm femtosecond pulses on a large mode area photonic crystal fiber. Combined with a home-built laser scanned microscope and a tailor-made frame grabber, we achieve a pulse-per-pixel HGM imaging in vivo at a 30-Hz frame rate. This integrated solution has the potential to be developed as a stable HGM system for routine clinical use.

Spectrally-isolated violet to blue wavelength generation by cascaded degenerate four-wave mixing in a photonic crystal fiber

Generation of spectrally-isolated wavelengths in the violet to blue region based on cascaded degenerate four-wave mixing (FWM) is experimentally demonstrated for the first time in a tailor-made photonic crystal fiber, which has two adjacent zero dispersion wavelengths (ZDWs) at 696 and 852 nm in the fundamental mode. The influences of the wavelength λ_p and the input average power P_av of the femtosecond pump pulses on the phase-matched frequency conversion process are studied. When femtosecond pump pulses at λ_p of 880, 870, and 860 nm and P_av of 500 mW are coupled into the normal dispersion region close to the second ZDW, the first anti-Stokes waves generated near the first ZDW act as a secondary pump for the next FWM process. The conversion efficiency η_as2 of the second anti-Stokes waves, which are generated at the violet to blue wavelengths of 430, 456, and 472 nm, are 4.8, 6.48, and 9.66%, for λ_p equalling 880, 870, and 860 nm, respectively.

Progress in Cherenkov femtosecond fiber lasers

We review the recent developments in the field of ultrafast Cherenkov fiber lasers. Two essential properties of such laser systems - broad wavelength tunability and high efficiency of Cherenkov radiation wavelength conversion are discussed. The exceptional performance of the Cherenkov fiber laser systems are highlighted - dependent on the realization scheme, the Cherenkov lasers can generate the femtosecond output tunable across the entire visible and even the UV range, and for certain designs more than 40 % conversion efficiency from the pump to Cherenkov signal can be achieved. The femtosecond Cherenkov laser with all-fiber architecture is presented and discussed. Operating in the visible range, it delivers 100-200 fs wavelength-tunable pulses with multimilliwatt output power and exceptionally low noise figure an order of magnitude lower than the traditional wavelength tunable supercontinuum-based femtosecond sources. The applications for Cherenkov laser systems in practical biophotonics and biomedical applications, such as bio-imaging and microscopy, are discussed.

Experimental observation of multiple dispersive waves emitted by multiple mid-infrared solitons in a birefringence tellurite microstuctured optical fiber

We experimentally demonstrate multiple dispersive waves (DWs) emitted by multiple mid-infrared solitons in a birefringence tellurite microstuctured optical fiber (BTMOF). To the best of our knowledge, this is the first demonstration of multiple DWs in the non-silica fibers. By using a pulse of ~80 MHz and ~200 fs emitted from an optical parametric oscillator (OPO) as the pump source, DWs and solitons are investigated on the fast and slow axes of the BTMOF at the pump wavelength of ~1800 nm. With the average pump power increasing from ~200 to 450 mW, the center wavelength of the 1st DW decreases from ~956 to 890 nm, the 2nd DW from ~1039 to 997 nm, the 3rd DW from ~1101 to 1080 nm, and the 4th DW from ~1160 to 1150 nm. Meanwhile, obvious multiple soliton self-frequency shifts (SSFSs) are observed in the mid-infrared region. Furthermore, DWs and solitons at the pump wavelength of ~1400 and 2000 nm are investigated at the average pump power of ~350 mW.

Yb:fiber laser-based, spectrally coherent and efficient generation of femtosecond 1.3-μm pulses from a fiber with two zero-dispersion wavelengths

We report, to the best of our knowledge, the first experimental characterization of spectral coherence properties of wavelength conversion inside photonic crystal fibers with two zero-dispersion wavelengths (TZDWs) and demonstrate a low-noise femtosecond 1.3-μm source employing the TZDW fiber and a 1.3-W, 240-fs Yb:fiber amplifier as the seeding source. Theoretical investigation shows that pulse evolution in our TZDW fiber source is dominated by parametric amplification seeded by self-phase modulation broadening which efficiently converts the pump energy into two new wavelength bands in a deterministic manner, leading to low noise and coherent excitation of femtosecond pulses tunable in the 1.3-μm spectral region, with up to 3 nJ of pulse energy at 32% of conversion efficiency.

Noise characterization of broadband fiber Cherenkov radiation as a visible-wavelength source for optical coherence tomography and two-photon fluorescence microscopy

Optical sources in the visible region immediately adjacent to the near-infrared biological optical window are preferred in imaging techniques such as spectroscopic optical coherence tomography of endogenous absorptive molecules and two-photon fluorescence microscopy of intrinsic fluorophores. However, existing sources based on fiber supercontinuum generation are known to have high relative intensity noise and low spectral coherence, which may degrade imaging performance. Here we compare the optical noise and pulse compressibility of three high-power fiber Cherenkov radiation sources developed recently, and evaluate their potential to replace the existing supercontinuum sources in these imaging techniques.

3 GHz, Yb-fiber laser-based, few-cycle ultrafast source at the Ti:sapphire laser wavelength

We demonstrate a compact ultrafast source centered at 850 nm with >200 nm bandwidth (full width at half-maximum) based on a 3 GHz Yb-fiber master-oscillator-power-amplifier system. The output pulses (with up to 13 W average power) from the laser system are coupled into short (<50 mm) pieces of photonic crystal fibers to excite broadband fiber-optic Cherenkov radiation; the resulting broad phase-matching bandwidth due to short fiber length produces an upconverted spectrum spanning in the wavelength range of 750-950 nm with average power of 94, 184, and 380 mW for fiber length of 28, 37, and 48 mm, respectively. The spectrum generated from the 37 mm fiber is then dechirped by eight double-chirped mirrors, leading to compressed pulses ~14 fs in duration. Such an ultrafast source is a promising substitute of multigigahertz mode-locked Ti:sapphire lasers for applications in optical frequency metrology and multiphoton coherent microscopy.

How long wavelengths can one extract from silica-core fibers?

The generation of wavelengths above 3 μm by nonlinear processes in short silica photonic crystal fibers is investigated numerically. It was found that wavelengths in the 3-3.5 μm range may be generated quite efficiently in centimeter-long fiber pieces when pumping with femtosecond pulses in the 1.55-2 μm range. Wavelengths in the range of 3.5-4 μm can in principle be generated, but these require shorter fiber lengths for efficient extraction. The results indicate that useful 3 μm sources may be fabricated with existing silica-based fiber technology.

Bright broadband coherent fiber sources emitting strongly blue-shifted resonant dispersive wave pulses

We predict and realize the targeted wavelength conversion from the 1550-nm band of a fs Er:fiber laser to an isolated band inside 370-850 nm, corresponding to a blue-shift of 700-1180 nm. The conversion utilizes resonant dispersive wave generation in widely available optical fibers with good efficiency (~7%). The converted band has a large pulse energy (~1 nJ), high spectral brightness (~1 mW/nm), and broad Gaussian-like spectrum compressible to clean transform-limited ~17 fs pulses. The corresponding coherent fiber sources open up portable applications of optical parametric oscillators and dual-output synchronized ultrafast lasers.

Coherent fiber supercontinuum for biophotonics

Biophotonics and nonlinear fiber optics have traditionally been two independent fields. Since the discovery of fiber-based supercontinuum generation in 1999, biophotonics applications employing incoherent light have experienced a large impact from nonlinear fiber optics, primarily because of the access to a wide range of wavelengths and a uniform spatial profile afforded by fiber supercontinuum. However, biophotonics applications employing coherent light have not benefited from the most well-known techniques of supercontinuum generation for reasons such as poor coherence (or high noise), insufficient controllability, and inadequate portability. Fortunately, a few key techniques involving nonlinear fiber optics and femtosecond laser development have emerged to overcome these critical limitations. Despite their relative independence, these techniques are the focus of this review, because they can be integrated into a low-cost portable biophotonics source platform. This platform can be shared across many different areas of research in biophotonics, enabling new applications such as point-of-care coherent optical biomedical imaging.

Tunable vacuum-UV to visible ultrafast pulse source based on gas-filled Kagome-PCF

An efficient and tunable 176-550 nm source based on the emission of resonant dispersive radiation from ultrafast solitons at 800 nm is demonstrated in a gas-filled hollow-core photonic crystal fiber (PCF). By careful optimization and appropriate choice of gas, informed by detailed numerical simulations, we show that bright, high quality, localized bands of UV light (relative widths of a few percent) can be generated at all wavelengths across this range. Pulse energies of more than 75 nJ in the deep-UV, with relative bandwidths of ~3%, are generated from pump pulses of a few μJ. Excellent agreement is obtained between numerical and experimental results. The effects of positive and negative axial pressure gradients are also experimentally studied, and the coherence of the deep-UV dispersive wave radiation numerically investigated.

Low-Noise Operation of All-Fiber Femtosecond Cherenkov Laser

We investigate the noise properties of a femtosecond all-fiber Cherenkov radiation source with emission wavelength 600 nm, based on an Yb-fiber laser and a highly nonlinear photonic crystal fiber. A relative intensity noise as low as 103 dBc/Hz, corresponding to 2.48% pulse-to-pulse fluctuation in energy, is observed at the Cherenkov radiation output power of 4.3 mW, or 150 pJ-pulse energy. This pulse-to-pulse fluctuation is at least 10.6-dB lower compared to spectrally sliced supercontinuum sources traditionally used for ultrafast fiber-based generation at visible wavelengths. Low noise makes all-fiber Cherenkov sources promising for biophotonics applications such as multiphoton microscopy, where minimum pulse-to-pulse energy fluctuation is required. We present the dependency of the noise figure on both the Cherenkov radiation output power and its spectrum.

Optical Cherenkov radiation in an As2S3 slot waveguide with four zero-dispersion wavelengths

We propose an approach for an efficient generation of optical Cherenkov radiation (OCR) in the near-infrared by tailoring the waveguide dispersion for a zero group-velocity mismatching between the radiation and the pump soliton. Based on an As(2)S(3) slot waveguide with subwavelength dimensions, dispersion profiles with four zero dispersion wavelengths are found to produce a phase-matching nonlinear process leading to a broadband resonant radiation. The broadband OCR investigated in the chalcogenide waveguide may find applications in on-chip wavelength conversion and near-infrared pulse generation.

All-fiber femtosecond Cherenkov radiation source

An all-fiber femtosecond source of spectrally isolated Cherenkov radiation is reported, to the best of our knowledge, for the first time. Using a monolithic, self-starting femtosecond Yb-doped fiber laser as the pump source and the combination of photonic crystal fibers as the wave-conversion medium, we demonstrate milliwatt-level, stable, and tunable Cherenkov radiation at visible wavelengths 580-630 nm, with pulse duration of sub-160-fs, and the 3 dB spectral bandwidth not exceeding 36 nm. Such an all-fiber Cherenkov radiation source is promising for practical applications in biophotonics such as bioimaging and microscopy.

Efficient frequency shifting of dispersive waves at solitons

We demonstrate frequency redshifting and blueshifting of dispersive waves at group velocity horizons of solitons in fibers. The tunnelling probability of waves that cannot propagate through the fiber-optical solitons (horizons) is measured and described analytically. For shifts up to two times the soliton spectral width, the waves frequency shift with probability exceeding 90% rather than tunnelling through the soliton in our experiment. We also discuss key features of fiber optical Cherenkov radiation such as high efficiency and large bandwidth within this framework.

Nonlinear polarization dynamics in a weakly birefringent all-normal dispersion photonic crystal fiber: toward a practical coherent fiber supercontinuum laser

Dispersion-flattened dispersion-decreased all-normal dispersion (DFDD-ANDi) photonic crystal fibers have been identified as promising candidates for high-spectral-power coherent supercontinuum (SC) generation. However, the effects of the unintentional birefringence of the fibers on the SC generation have been ignored. This birefringence is widely present in nonlinear non-polarization maintaining fibers with a typical core size of 2 µm, presumably due to the structural symmetry breaks introduced in the fiber drawing process. We find that an intrinsic form-birefringence on the order of 10(-5) profoundly affects the SC generation in a DFDD-ANDi photonic crystal fiber. Conventional simulations based on the scalar generalized nonlinear Schrödinger equation (GNLSE) fail to reproduce the prominent observed features of the SC generation in a short piece (9-cm) of this fiber. However, these features can be qualitatively or semi-quantitatively understood by the coupled GNLSE that takes into account the form-birefringence. The nonlinear polarization effects induced by the birefringence significantly distort the otherwise simple spectrotemporal field of the SC pulses. We therefore propose the fabrication of polarization-maintaining DFDD-ANDi fibers to avoid these adverse effects in pursuing a practical coherent fiber SC laser.

Femtosecond Fiber Lasers Based on Dissipative Processes for Nonlinear Microscopy

Recent progress in the development of femtosecond-pulse fiber lasers with parameters appropriate for nonlinear microscopy is reviewed. Pulse-shaping in lasers with only normal-dispersion components is briefly described, and the performance of the resulting lasers is summarized. Fiber lasers based on the formation of dissipative solitons now offer performance competitive with that of solid-state lasers, but with the benefits of the fiber medium. Lasers based on self-similar pulse evolution in the gain section of a laser also offer a combination of short pulse duration and high pulse energy that will be attractive for applications in nonlinear bioimaging.

Generation of Cerenkov radiation at 850 nm in higher-order-mode fiber

We demonstrate generation of Cerenkov radiation at 850 nm in a higher-order-mode (HOM) fiber. The LP02 mode in this solid, silica-based fiber has anomalous dispersion from 690 nm to 810 nm. Cerenkov radiation with 3 nJ pulse energy is generated in this module, exhibiting 60% energy conversion efficiency from the input. The HOM fiber provides a valuable fiber platform for nonlinear wavelength conversion with pulse energies in-between index-guided silica-core photonic crystal fibers and air-core photonic bandgap fibers.

Fiber-optic Cherenkov radiation in the few-cycle regime

Fiber-optic Cherenkov radiation has emerged as a wavelength conversion technique to achieve isolated spectrum in the visible wavelength range. Most published results have reinforced the impression that CR forms a narrowband spectrum with poor efficiency. We both theoretically and experimentally investigate fiber-optic Cherenkov radiation excited by few-cycle pulses. We introduce the coherence length to quantify the Cherenkov-radiation bandwidth and its dependence on propagation distance. Detailed numerical simulations verified by experimental results reveal three unique features that are absent when pumped with often-used, long pulses; that is, continuum generation (may span one octave in connection with the pump spectrum), high conversion efficiency (up to 40%), and broad bandwidth (70 nm experimentally obtained) for the isolated Cherenkov radiation spectrum. These merits allow achieving broadband visible-wavelength spectra from low-energy ultrafast sources which opens up new applications (e.g. precision calibration of astronomical spectrographs).

Molecular histopathology by spectrally reconstructed nonlinear interferometric vibrational imaging

Sensitive assays for rapid quantitative analysis of histologic sections, resected tissue specimens, or in situ tissue are highly desired for early disease diagnosis. Stained histopathology is the gold standard but remains a subjective practice on processed tissue taking from hours to days. We describe a microscopy technique that obtains a sensitive and accurate color-coded image from intrinsic molecular markers. Spectrally reconstructed nonlinear interferometric vibrational imaging can differentiate cancer versus normal tissue sections with greater than 99% confidence interval in a preclinical rat breast cancer model and define cancer boundaries to ± 100 μm with greater than 99% confidence interval, using fresh unstained tissue sections imaged in less than 5 minutes. By optimizing optical sources and beam delivery, this technique can potentially enable real-time point-of-care optical molecular imaging and diagnosis.

All-fiber design of hybrid Er-doped laser/Yb-doped amplifier system for high-power ultrashort pulse generation

We propose a design of an all-fiber laser system that combines the most advanced Er:fiber laser in the telecommunication range and an efficient Yb-doped amplifier for generation of high-power ultrashort pulses. The system is based on nonlinear wavelength conversion of 1.56 μm ultrashort Er:fiber laser pulses to the 1 μm range in a short pigtail of dispersion-shifted silica fiber with subsequent amplification in the Yb-doped fiber amplifier. Pulses with a duration as short as 85 fs and averaged power of 200 mW are demonstrated.

Highly efficient Cherenkov radiation in photonic crystal fibers for broadband visible wavelength generation

We investigate the dependence of Cherenkov radiation (CR) on pump pulse parameters and its evolution along the propagation distance. Using a Ti:sapphire laser emitting 10 fs pulses as the pump source, we demonstrate highly efficient (>40%), broadband (>50 nm) CR in the visible-wavelength range with a threshold energy less than 100 pJ and a tuning range over 100 nm.

Ultraviolet-visible non-supercontinuum ultrafast source enabled by switching single silicon strand-like photonic crystal fibers

Cherenkov radiation from short photonic crystal fiber with a high air-fill fraction can selectively convert the 1020 nm fs pump pulses from a laser oscillator to the fundamental-mode signal pulses at a significantly shorter wavelength. Across the ultraviolet-visible spectral region, the typical fiber output is characterized by a single isolated Cherenkov band having a multimilliwatt-level average power, a Gaussian-shaped spectrum, and a 3-dB bandwidth of 15 nm. By selecting photonic crystal fibers with smaller cores, the central wavelength of the Cherenkov band can be easily extended to 347 nm in the ultraviolet, in sharp contrast to various supercontinuum or non-supercontinuum fiber sources that have difficulty extending their emission spectra below 400 nm. The supercontinuum generation often associated with fs pulse-pumped fibers is efficiently suppressed by detuning the zero-dispersion wavelength of the photonic crystal fiber far shorter than the pump wavelength, a condition termed as the short nonlinear-interaction condition.