As the supercontinuum source reaches its fiftieth anniversary, in this short presentation I will highlight some of the steps in its development, from Alfano’s and Shapiro’s discovery and application of this phenomena using high energy, table-top, pulsed solid state lasers to the compact, all-fibre integrated configurations which have had a marked commercial impact. The principal scientific processes as well as some of the vital technological advances will be outlined which have led to extensive spectral versatility and allows the temporal format of the excitation pumps to range from the continuous wave to the femtosecond regimes.
PPLN based optical parametric oscillators pumped by high power lasers around 1 µm are well established sources for generating light in the 3-5 µm spectral region, of interest for a wide range of scientific, commercial and military applications. We have been investigating optical parametric amplification (OPA), or difference-frequency generation (DFG), single-pass alternatives to conventional resonant OPOs. This avoids the need for a cavity and the corresponding design constraints that this can impose; such as, fixed repetition rates, sensitive alignment and/or poor output beam qualities at high average power levels. In this paper, we review recent results on high average power ( > 6 W) nanosecond pulse generation in the 3.3-3.5 μm region at MHz repetition rates, employing Yb:fibre and Er:fibre master oscillator power amplifiers (MOPA) systems pumping PPLN OPAs. We use focused Gaussian beam theory to validate the experimental results. We will also discuss spectral extension further into the mid-infrared, using different nonlinear crystal and alternative rare-earth doped fibre MOPA and Raman shifted fibre laser combinations. Ongoing work aimed at the power scaling of the mid-infrared light in both the nanosecond pulsed and continuous wave regimes will be presented.
Fiber-based lasers and master oscillator power fiber amplifier configurations are described. These allow spectral versatility coupled with pulse width and pulse repetition rate selection in compact and efficient packages. This is enhanced through the use of nonlinear optical conversion in fibers and fiber-coupled nonlinear crystals, which can be integrated to provide all-fiber pump sources for diverse application. The advantages and disadvantages of sources based upon supercontinuum generation, stimulated Raman conversion, four-wave mixing, parametric generation and difference frequency generation, allowing spectral coverage from the UV to the mid-infrared, are considered.
Second harmonic generation (SHG) is a ubiquitous technique for extending the spectral coverage of laser sources into regions that would otherwise be technologically challenging to access. SHG schemes typically rely on the use of bulk optical components, resulting in systems with large footprints requiring precise optical alignment. Integration of the SHG components into a single unit facilitates the implementation of compact, robust and turn-key sources, suitable for applications in biophotonic imaging, amongst others. We report on the development of fiber-coupled frequency doubling modules and their application to novel fiberintegrated picosecond pulse sources in the visible and near-visible. The modules employ a simple, single-pass configuration using a periodically-poled lithium niobate (PPLN) crystal as the nonlinear conversion medium. They are readily adaptable for different fiber pump laser configurations and are configurable with either fiber-coupled or collimated free-space outputs. Two sources using the modules are presented, operating at 780 nm and 560 nm. The 780 nm source utilizes an erbium master oscillator power fiber amplifier (MOPFA) scheme. SHG was performed in a 35 mm long crystal, generating 3.5 W of 780 nm radiation with a pulse duration of 410 ps at 50 MHz and conversion efficiencies exceeding 20%. Results of this source being used for parametric wavelength conversion in photonic crystal fiber are discussed. The 560 nm source was based on SHG of a Raman amplified CW diode pumped by a pulsed ytterbium-fiber MOPFA. This source generated 450 mW of average power with conversion efficiencies greater than 20%.
Spectral broadening and the generation of new frequencies were initially observed in pulsed laser systems in the mid-1960s as an inherent feature of the uncontrollable nonlinear process such as self-focussing and self-phase modulation occurring primarily in the gain media and were looked upon as deleterious rather than a resource. With the advent of mode locked lasers to generate picosecond pulses new effects were observed. Developed by the Alfano group in bulk media external to the laser in the 1970s the supercontinuum or “white light” source has now evolved into a commercially successful and highly compact source that can readily extend over more than three octaves with spectral power densities exceeding 100mW/nm. In this presentation I will describe this remarkable evolution.
Experimental and numerical results on the interaction of the Raman-soliton continuum and dispersive waves through
inelastic collisions mediated by four wave mixing are presented. In particular the spectral broadening of the continuum to
longer wavelengths beyond a second zero dispersion point through four wave mixing of the dispersive wave produced by
soliton self-frequency shift cancelation and other solitons in the continuum is considered.
We report a 29 W CW supercontinuum spanning from 1.07 μm to 1.67 μm with a spectral power density of 50 mW/nm up to 1.4 μm. The continuum is produced in a short length of photonic crystal fiber (PCF) with two zero dispersion wavelengths. Ultimately the second zero limits the long wavelength edge of the continuum. We also find that despite using much shorter lengths of PCF the affects of OH- absorption are still visible in the supercontinuum produced.
Various techniques are described for the efficient non linear conversion of high power fibre laser sources. These include
the frequency doubling in periodically poled crystals of polarization preserving high power fibre Raman masteroscillator
power fibre amplifier schemes, or the frequency doubling of a high power, narrow line bismuth-doped fibre
laser, primarily for visible generation in the region of 589 nm. Above 2 microns where loss in silica based fibres
prohibits efficient Raman generation, the use of heavily doped (~75%) GeO2 fibres has been demonstrated as an efficient
and effective all-fibre configuration to extend high power cw operation into the near infra red spectral region. For many
applications, high peak power multiple wavelengths are required simultaneously. The integration of pulsed fibre lasers
operating around 1 &mgr;m and photonic crystal fibre provides a simple mechanism to achieve this goal. Generally, the upper
wavelength limit of supercontinuum generation is restricted by propagation loss and this has a consequential effect of
inhibiting short wavelength generation through four wave mixing in the fibre. We have developed a technique employing
long lengths of tapered PCFs that allow efficient phase matched four wave mixing to the short wavelength region and
permits the generation of spectral power densities as high as 5mW/nmin the uv.
A functional extension of ultrahigh resolution OCT (UHR OCT) has been developed, that has the potential to establish this technique as an optical analogue to electrophysiology, by detecting depth resolved variations in optical backscattering caused by physiological tissue changes. This technique has been used to perform in vitro studies on excised, but physiologically intact, rabbit retinas and in vivo experiments on human retinas. UHR OCT has been synchronized with the white light stimulus to properly detected spatially resolved alterations in optical backscattering over time caused by lightinduced
intraretinal, physiological changes and has been correlated with simultaneous ERG recordings. Preliminary results demonstrate the potential of this novel extension of UHR OCT to detect time-dependent optical backscattering changes after application of a white light stimulus in specific retinal layers, especially in the inner and outer segments of the photoreceptor layer. Control experiments, including no light stimulus or application of drugs (in in vitro studies only) that inhibit the physiological responses of certain type of retinal cells confirm the physiological origin of the
detected backscattering changes. Detection of cell activity and cell physiology by UHR OCT would enable a better understanding of basic physiological phenomena and may also contribute to better understanding of retinal pathogenesis.
High performance, short coherence length light sources with broad bandwidths and high output powers are critical for high speed, ultrahigh resolution OCT imaging. We demonstrate an all-fiber continuous-wave Raman light source based on a photonic crystal fiber, pumped by a continuous-wave Yb-fiber laser, which generates 330 mW output power and 140 nm bandwidths. The light source is compact, robust, turnkey and requires no optical alignment. In vivo high speed, ultrahigh resolution OCT imaging of tissues with < 5 μm axial resolution at 1.3 μm center wavelength is demonstrated.
Advances in high power fibre lasers and amplifiers and in novel non-linear fibres that can be readily integrated with such pumps have led to a family of high power super-continuum sources that extend throughout the complete window of transparency of silica based fibres. The systems have been operated femtosecond, picosecond and nanosecond as well as cw. Average powers of 10’s Watts can be easily achieved, giving flat spectral power densities in excess of 10’s mW per nm from 400 nm to beyond 2.2 μm.
This paper reviews ultrafast Kerr Lens Mode-locked solid- state lasers with particular emphasis on all-solid-state diode-pumped laser technology which has the potential to provide low-cost compact devices for ultrafast instrumentation, particularly for biomedical applications.We have demonstrated the use of ultrafast solid-state lasers for 3D imaging through turbid media using time-gated photorefractive holography, and for fluorescence lifetime imaging.
The development of low-cost and potentially compact femtosecond laser oscillators and amplifiers is discussed. The first all-solid-state femtosecond Cr:LiSAF laser is described. A tunable femtosecond solid-state amplifier system pumped by only 3 W of 488 nm argon ion radiation has been demonstrated to deliver microjoule pulses at repetition rates up to approximately 20 kHz, with a maximum pulse energy of 14 (mu) J obtained at 5 kHz. The first all-solid-state, tunable, diode-pumped Cr:LiSAF regenerative amplifier, which amplifies femtosecond pulses to energies exceeding 1 (mu) J at up to 16 kHz repetition rate, is also reported.