Optical pulse source having variable repetition rates and wavelengths

0001The present invention relates to an optical pulse source and a method for producing variable repetition rate, wavelength varying pulse trains.

0002Supercontinuum lasers, delivering continuous spectra over an ultra broad bandwidth offer an ideal source for many applications within biomedical imaging, component characterisation, manufacturing control and defence research. The most common form of supercontinuum laser comprises an ultrashort pulse optical pump source operating at a pump wavelength in the Infra-Red region of the spectrum (typically around 800nm for a Ti:Sapphire laser or 1064nm for mode-locked fibre lasers and (diode pumped solid-state) lasers) and a highly nonlinear photonic crystal fibre, with specially designed dispersion properties. The interaction between the high intensity optical pump pulse and the nonlinear silica fibre causes extreme broadening into the visible and infra red regions of the spectrum to provide spectra spanning from approximately 450nm to beyond 2.5micrometer.

0003The phenomenon of supercontinuum was first proposed in the 1970's by Alfano et al [<nplcit id="ncit0001" npl-type="s"><text>R. R. Alfano and S. L. Shapiro, "Observation of self-phase modulation and small-scale filaments in crystals and glasses", Phys. Rev. Lett. 24, 592 (1970</text></nplcit>)] and in 2000 the first demonstration of a fibre-based supercontinuum made by Ranka and Windelar [<nplcit id="ncit0002" npl-type="s"><text>J. K. Ranka et al., "Visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm", Opt. Lett. 25 (1), 25 (2000</text></nplcit>)] (<patcit id="pcit0001" dnum="US4450427A"><text>US4,450,427</text></patcit>) using a Ti:Sapphire mode-locked femtosecond laser 10 to pump a photonic crystal fibre 11 with zero dispersion wavelength at approximately 760nm in free space through launch optics 12 as shown in <figref idref="f0001">Figure 1</figref>. More recently, researchers and companies have utilised nanosecond and picosecond based pump sources operable at around 1064nm wavelength to pump photonic crystal fibres with zero dispersion wavelength close to 1µm wavelength to generate high brightness supercontinuum products.

0004Commercial products such as Fianium's SC450 rely on mode-locked fibre oscillators and high power optical fibre amplifiers to generate highly intense pulses at the pump wavelength, which when injected into a nonlinear photonic crystal fibre (PCF), result in extreme spectral broadening into both the visible and infra red regions of the spectrum.

0005<figref idref="f0002">Figure 2</figref> shows an example of such a system, where a mode-locked fibre oscillator 21 produces low energy optical pulses of approximately 10ps duration and at a repetition rate of 20MHz to 100MHz. The pulses are amplified within a cascaded amplifier 22, comprising one or more stages of amplification, with optical isolation 23 between each stage. The output of the amplifier is a high energy pulse of up to 400nJ and tens of kilowatt peak power. The output of the amplifier is injected into length of highly nonlinear PCF 24 with anomalous dispersion at the pump wavelength (1064nm) and zero dispersion wavelength close to the pump wavelength. The spectral bandwidth of the pulse broadens within the PCF, creating a pulse of approximately 100nJ energy and a bandwidth from 400nm to 2.5um which exits this PCF.

0006The pulsed nature of these sources and high repetition rates (20MHz to 160/MHz), makes these sources attractive to both applications requiring quasi-continuous wave radiation as well as applications involving time resolved measurements such as FLIM (Fluorescent lifetime imaging) and TCSPC (time correlated single photon counting). However, often, the high repetition rates can be a limiting factor in lifetime imaging, where the pulse to pulse separation (50nsecs for a 20MHz source down to a few nanoseconds for 160MHz) can be much shorter than the lifetime of the sample under evaluation. In this event, one requires a lower pulse repetition rate. In general, most mode-locked laser systems operate at pulse repetition rates of several tens of MHz (DPSS typically 80MHz to 100MHz, and fibre lasers from 20MHz to several hundreds of MHz). Some applications require lower repetition rates - for example FLIM, where fluorescent labelled molecules are excited by an incident optical pulse and the decay of the fluorescence is monitored before the next optical pulse arrives. In FLIM, the lifetimes of interest can very often exceed tens or even hundreds of nanoseconds and therefore it is often required to have repetition rates of 40MHz and below. Repetition rates of less than 20MHz, while not impossible to achieve, are difficult to deliver from a mode-locked fibre oscillator due to high nonlinearity within a long cavity (10m for a 10MHz oscillator). From a DPSS source, such a cavity (5m long for 20MHz and 100m in length for 1MHz) is almost impossible to make due to the required complexity of the cavity design.

0007<nplcit id="ncit0003" npl-type="s"><text>FRANK J.H. ET AL.: "A white light confocal microscope for spectrally resolved multidimensional imaging", J. MICROSC.,vol. 227,3 September 2007 (2007-09-03), pages 203-215, X P002516159</text></nplcit> describes spectrofluorometric imaging microscopy in a confocal microscope using a supercontinuum laser as an excitation source and a custom-built prism spectrometer for detection comprising the features of the preamble of claim 1.

0008<patcit id="pcit0002" dnum="US2005041702A1"><text>US2005/041702A1</text></patcit> describes a fibre-based source for high energy picosecond and nanosecond pulses.

0009According to a first aspect of the invention there is provided an optical pulse source as defined in claim 1.

0010According to a second aspect of the present invention there is provided a method producing variable repetition rate, wavelength varying pulse trains as defined in claim 11.

0011The optical pulse source enables optical supercontiuum pulses to be generated with flexible control of the output pulse repetition rate, and thus the pulse-to-pulse separation. Control of the inter pulse separation is of particular advantage where the source is used to provide supercontinuum illumination in applications where decay lifetimes are measured, such as FLIM, and the optical pulse source can be operated to generate supercontiuum pulses at pulse repetition rates of less than 20MHz for such applications.

0012The nonlinear optical element may comprise one or more nonlinear optical crystals such as lithium triborate (LBO), barium borate (BBO), cesium lithium borate (CLBO), periodically poled lithium niobate (PPLN) or periodically poled potassium titanyl phosphate (PPKTP) or a microstructured optical fibre, which is preferably one of highly non-linear photonic crystal fibre, side-hole fibre, hollow-core photonic bandgap fibre and solid-core photonic bandgap fibre.

0013The optical pump laser may comprise a mode-locked laser. The pump pulse repetition rate may be variable.

0014The optical pump laser may alternatively comprise a modelocked fibre oscillator operable to generate optical pump pulses. The optical pulse source preferably further comprises an optical fibre amplifier configured to receive pump pulses from the modelocked fibre oscillator and operable to amplify the pump pulses. The modelocked fibre oscillator and the amplifier together form a master oscillator power amplifier (MOPA). The gating means may be provided between the modelocked fibre oscillator and the optical fibre amplifier.

0015The gating means preferably comprises an optical modulator element such as an acousto-optic modulator, an electro-optic modulator or a semiconductor saturate absorber mirror (SESAM) modulator. The modulator element is preferably synchronised with the output of the optical pump laser, to selectively control the repetition rate of pulses launched into the fibre, to thereby selectively control the output pulse repetition rate. The gating means is preferably operable to control the output pulse repetition rate from a few Hertz up to one hundred MHz.

0016The optical pulse source further comprises a wavelength tunable optical bandpass filter configured to receive an optical supercontinuum pulse and to wavelength filter the pulse at a predetermined wavelength. The wavelength tunable optical bandpass filter is preferably configured to receive a plurality of optical supercontinuum pulses in a pulse set and to wavelength filter each pulse at a respective one of a corresponding plurality of predetermined wavelengths, to thereby form a wavelength interleaved optical pulse set. The wavelength tunable optical bandpass filter is preferably configured to filter a plurality of such pulse sets in series to thereby form a repeatingly wavelength interleaved optical pulse train.

0017The transmission spectrum of the wavelength tunable optical filter can be selected to pass any wavelength or set of wavelengths within the supercontinuum spectrum, to thereby control the spectral profile of each output optical pulse. The optical pulse source can thus be used as an optical source for Pulse interleave excitation Forster Resonance Energy Transfer (PIE-FRET).

0018The optical pulse source preferably further comprises second gating means provided after the nonlinear optical element configured to pulse-pick received optical pulses to a lower pulse repetition rate. The second gating means preferably comprises an acousto-optical modulator or a wavelength tunable optical bandpass filter. The wavelength tunable optical bandpass filter is preferably an acousto-optic tunable filter.

0019Preferably, a single wavelength tunable optical bandpass filter is configured to perform both wavelength filtering and pulse-picking.

0020The second gating means is preferably further operable to reduce the optical intensity of one or more optical pulses in the pulse set or train. The second gating means can thereby be operated to control the optical intensity of the pulses in a train of optical pulses of different wavelengths, so that the optical intensity can be changed for pulses of different wavelengths.

0021The optical pulse source may alternatively further comprise an optical beam splitter coupled to the output of the optical supercontinuum pulse source, to thereby split the optical supercontinuum pulses into first and second split pulses in first and second optical paths, first and second optical filters having different wavelength transmission spectra respectively provided in the first and second optical paths, an optical delay element provided in the first optical path, and an optical beam combiner arranged to recombine the first and second split pulses from the first and second optical paths into pulse train in a common optical path. A pulse train comprising interleaved pulses of two different wavelengths is thereby produced.

0022The present application describes a method of scaling the energy of optical supercontinuum pulses generated using the optical pulse source as described above, the method comprising the steps of: <ul id="ul0001" list-style="none" compact="compact"><li>reducing the pump pulse repetition rate of the optical pump laser; and</li><li>increasing the pump pulse duration.</li></ul>

0023This invention therefore enables the development of higher energy supercontinuum pulse sources for certain applications where higher pulse intensity is required (stimulated emission depletion (STED) microscopy is one example).

0024The method may comprise reducing the pump pulse repetition rate to less than 20MHz and increasing the pump pulse duration to greater than 10 picoseconds, and most preferably to at least 200 picoseconds.

0025The light source of the present invention is suitable as an illumination source for stimulated emission depletion microscopy, the illumination source comprising an optical pulse source as described above configured to generate output optical pulses having a repetition rate in the range 1MHz to 10MHz and an energy spectral density of at least 100 picojoules per nanometer, the optical pulse source being operated to scale the energy of the optical supercontinuum pulses by reducing the pump pulse repetition rate of the optical pump laser and increasing the pump pulse duration.

0026The optical pulse source may be operated to reduce the pump pulse repetition rate to less than 20MHz. The optical pump laser is preferably operable to generate optical pump pulses having a pump pulse duration of greater than 10 picoseconds, and most preferably at least 200 picoseconds.

0027The light source of the present invention is suitable for an optical fluorescence microscope based on Stimulated emission and depletion, the microscope comprising an illumination source as described in paragraphs 22 or 23 above, the illumination source being configured to provide both excitation illumination and stimulated emission illumination.

0028In conventional Stimulated emission and depletion (STED) microscopes (<nplcit id="ncit0004" npl-type="s"><text>G.Donnert et al, Proceedings of the Natural Academic Society U.S.A 103,11440-11445 (2006</text></nplcit>)) two lasers are used simultaneously, one (typically a Ti: Sapphire laser) with high optical intensity to form the STED beam and a second tunable visible laser to excite the fluorescence. Conventional supercontinuum lasers, based on modelocked fibre laser-pumped PCF's or Ti:Sapphire laser-pumped PCF's are too low in pulse energy for STED. Using a reduced repetition rate and longer pulse, according to the present invention, an optical pulse source is provided which is operable to generate supercontinuum pulses having a pulse energy spectral density approaching 1nJ/nm, making this a suitable single-source solution for both fluorescence excitation and STED within a STED microscope.

0029Another benefit of this invention is it provides the ability to be flexible in pump pulse duration and the length of microstructure optical fibre required to generate the supercontinuum output pulses.

0030In all cases described within this invention, we relate specifically, but not exclusively to, the wavelengths falling within the gain bandwidth of Ytterbium doped silica fibres - between 1020nm and 1080nm. Equally, this invention can relate to supercontinuum pulses generated using a Nd-doped fibre laser or Er-doped fibre laser or DPSS laser source as the optical pump laser.

0031The preferred embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings in which: <ul id="ul0002" list-style="none" compact="compact"><li><figref idref="f0001">Figure 1</figref> is a schematic illustration of a prior art arrangement for the generation of optical supercontinuum;</li><li><figref idref="f0002">Figure 2</figref> is schematic illustration of a prior art supercontinuum fibre laser;</li><li><figref idref="f0003">Figure 3</figref> is a schematic illustration of an optical pulse source suitable for the present invention generated from a free-space DPSS modelocked laser source;</li><li><figref idref="f0004">Figure 4</figref> is a schematic illustration of an optical pulse source suitable for the present invention;</li><li><figref idref="f0005">Figure 5</figref> is a schematic illustration of an optical pulse source according to a first embodiment of the present invention;</li><li><figref idref="f0006">Figure 6</figref> is a schematic illustration of an optical pulse source according to a second embodiment of the present invention;</li><li><figref idref="f0007">Figure 7</figref> is a representation of a first set of output optical pulse trains a) exiting the microstructured optical fibre and b) exiting the AOTF, which may be generated by the optical pulse source of <figref idref="f0006">Figure 6</figref>;</li><li><figref idref="f0007">Figure 8</figref> is a representation of a second set of output optical pulse trains a) exiting the microstructured optical fibre and b) exiting the AOTF, which may be generated by the optical pulse source of <figref idref="f0006">Figure 6</figref>; and</li><li><figref idref="f0008">Figure 9</figref> is a representation of a third set of output optical pulse trains a) exiting the microstructured optical fibre and b) exiting the AOTF, which may be generated by the optical pulse source of <figref idref="f0006">Figure 6</figref>.</li></ul>

0032Referring to <figref idref="f0003">Figure 3</figref> an optical pulse suitable for the invention provides source 30 comprising an optical pump laser source 31, a microstructured optical fibre 35 and gating means 33.

0033The optical pump laser in this example takes the form of a diode pumped solid state (DPSS) laser source 31, (for example a mode-locked 1064nm laser operating at 76MHz) which is operable to produce pulses of approximately 200nJ energy and duration 10ps. The output pulses from the pump laser 31 pass through a set of lenses 32 to the gating means 33, which here comprises an optical modulator 33, synchronised to the pump laser 31 and operable so that it can gate the pump pulses and produce a pulse train having a reduced repetition rate (Rr) where Rr = Rf/N, where Rf is the pump laser fundamental frequency (in this example 76MHz) and N is a positive integer. The pulses output from the modulator 33 are injected via optics 34 into the microstructured fibre 35, which here comprises a nonlinear element such as a photonic crystal fibre (PCF) 35 Propagation through the PCF 35 causes the pulses to broaden spectrally to produce supercontinuum pulses at the reduced, output pulse, repetition rate Rr. <figref idref="f0003">Figure 3</figref> thus illustrates a low repetition rate picosecond or femtosecond supercontinuum pulse source, in this case pumped from a free-space DPSS mode-locked laser source.

0034One of the major drawbacks with the use of free-space optics, as shown in <figref idref="f0003">Figure 3</figref>, is that launch stability into the small structures of nonlinear PCF's is poor and the end face of these fibres can easily be damaged.

0035<figref idref="f0004">Figure 4</figref> shows an all fibre optical supercontinuum pulse source 40 suitable for the invention, similar to the arrangement shown in <figref idref="f0002">Figure 2</figref>. The optical pump laser comprises a mode-locked fibre oscillator 41, operable at a repetition rate Rf of 40MHz, to produce low energy pulses at 1064nm and having a pulse duration of approximately 10ps,. The pump pulses are amplified in an optical fibre amplifier 42. The pump pulses are then injected into gating means, in the form of a fibre-coupled modulator 43, which in this example comprises an acousto-optic modulator (AOM). It will be appreciated that an Electro-Optic Modulator (EOM), SESAM modulator or any other type of fast modulation device, capable of switching with rise times of less than one microsecond may alternatively be used. The modulator 43 is synchronised to the oscillator frequency (Rf) and is operable to gate the oscillator output, i.e. the pump pulses, down to a repetition rate of a few tens of KHz, and hence pulse-pick the pump pulses, providing a selectable pump pulse frequency Rr = Rf/N (N is a positive integer value).

0036The reduced repetition rate pump pulses output from the modulator 43 are amplified in a power amplifier 45. The pump laser 41 and the power amplifier 45 together form a master oscillator power amplifier (MOPA) 46. The pump pulses output from the MOPA 46 have a maximum pulse peak power, governed by the nonlinear limitations of the power amplifier 45 of, for example, approximately 40KW, corresponding to a pump pulse energy of approximately 400nJ.

0037Following amplification, the pump pulses are input into a length of highly nonlinear microstructured optical fibre, in this example PCF 44, with a zero dispersion wavelength at approximately 1040nm. Propagation through the PCF 44 produces extensive spectral broadening of the pump pulses into the visible and further out into the IR region of the spectrum, thereby forming supercontinuum pulses. The supercontinuum pulses have a supercontinuum spanning from below 450nm to greater than 2000nm, a total supercontinuum pulse energy of approximately 100nJ, and energy spectral density of approximately 50picoJoules/nm.

0038In this example, since the oscillator remains the same, for a given output pulse energy one achieves nominally identical spectra for all repetition rates. The pulse energy is fixed and clamped, but the pulse to pulse separation increased with reducing repetition rate.

0039In an alternative arrangement, the optical pulse source 40 may be operated with the low-power mode-locked fibre oscillator (pump laser) 41 generating pulses having a pulse duration of 200ps. Amplification within a fibre amplifier with nonlinear clamp on the peak power of approximately 40KW, results in a corresponding maximum pump pulse energy of 8µJ (for example, 1MHz, 8W average power and 200ps pulse duration). Supercontinuum pulses having a total supercontinuum pulse energy of approximately 2µJ and energy spectral density of approximately inJ/nm are produced.

0040Referring to <figref idref="f0005">Figure 5</figref>, a first embodiment of the invention provides an optical pulse source 50 which adds to the optical pulse source 40 of <figref idref="f0004">Fig. 4</figref>.

0041In this embodiment, the optical pulse source 50 further comprises a polarising beam splitter 52 (a non-polarising beam splitter or other similar splitting mechanism may alternatively be used), optical filters 53 and optical delay element 54, and a beam combiner 56. The output 51 of the optical pulse source 40 is split into two beam paths (A, B) by the beam splitter 52. The optical filters 53 are provided in each of the split beam paths; the optical filters may be acousto-optic tunable filters (AOTF), each being operable to transmit a different wavelength, or standard filters of different transmission wavelengths.

0042The optical delay element 54, which takes the form of an optical fibre delay line, is provided in one beam path (A). For a 20MHz optical supercontinuum pulse repetition rate output from the optical pulse source 40, a delay of 25nsecs is required for the pulses travelling along path A. The optical fibre delay line 54 thus comprises a delay path of between 5 and 7.5m in length. A coupling lens 55 is provided for launching the split pulses into the delay line 54. Propagation along path A causes the split optical pulses in that path to be delayed with respect to the optical pulses in the other split beam path (B). It will be appreciated that this can alternatively be achieved in free-space.

0043The beam combiner, which comprises a dichroic mirror 56 but may alternatively comprise a fibre-coupler, is provided at the ends of the split beam paths (A, B) to recombine the two split pulse trains, to thereby form a pulse train comprising interleaved pulses of two different wavelengths.

0044Although an optical fibre delay line 54 is the preferred choice, this requires launching of the light into an optical fibre (54) by a suitable lens (55), incurring additional loss and instability. This approach is attractive as an experiment, but is less practical for a commercial instrument. In addition, this approach cannot use a variable supercontiuum pulse repetition rate, since the required path imbalance, introduced by the delay line, changes with repetition rate.

0045A second embodiment of the invention provides an optical pulse source 60, as shown in <figref idref="f0006">Figure 6</figref>, comprising an optical pulse source 40 according to <figref idref="f0004">Fig. 4</figref> and a wavelength tunable optical bandpass filter 63.

0046The optical pulse source 40 is operable to generate optical supercontinuum pulses having wavelength spectra from 400nm to 2000nm, at pulse repetition rates, variable between 40MHz and less than 100kHz. The optical pulse source 40 is provided with an on-board micro-processor control, which determines when the supercontinuum pulses are delivered, and provides a TTL output trigger signal. The optical output of the optical pulse source 40 is in free space, and the beam is collimated or waisted by optics 62.

0047The wavelength tunable optical bandpass filter 63 comprises an acousto-optic tunable filter (AOTF) driven by an RF driver or Direct Digital Synthesiser (DDS) 64. The AOTF 63 is operable over a spectral range (for example from 400nm to 700nm) covered by the optical supercontinuum pulse spectrum. The DDS 63 is synchronised with the optical pulse source 40 either through an external trigger 61, or from a microprocessor, such that the AOTF can be modulated at the operating frequency of the optical pulse source 40 or at any lower frequency. The AOTF 63 can also be operated in burst mode, to transmit a burst of optical pulses.

0048By changing the RF frequency of the AOTF driver 64, one can tune the filter output to any wavelength lying within the AOTF 63 and supercontinuum spectral ranges. Typical AOTF's have spectral ranges of, for example, from 400nm to 700nm in the visible region, from 700nm to 1100nm in the near infra-red (NIR) and from 1100nm to 2000nm in the IR. All of these wavelengths are available within the spectrum of the supercontinuum optical pulses generated by the optical pulse source 40.

0049The first order diffracted output of the AOTF 63 forms the output of the optical pulse source 60, and may be launched into an optical fibre or used in free space. The zero-order of the AOTF, is either absorbed by a suitable beam dump 65 or can form a second output of the optical pulse source 60.

0050In this example, the AOTF 63 is triggered to operate at one user-defined wavelength for one pulse and a second user defined wavelength for the next pulse, thus generating a wavelength interleaved output pulse train of alternating wavelength pulses. The optical pulse source 60 is therefore suitable for use as an illumination source for PIE-FRET.

0051Dependent on the speed of the AOTF 63 (often limited by the spot size of the beam through the crystal), the AOTF can be configured to deliver pulses at the optical supercontinuum pulse repetition rate, as shown by <figref idref="f0007">figure 7</figref>, where <figref idref="f0007">Figure 7a</figref> shows the pulses from the optical pulse source 40 and 7b shows the AOTF output, i.e. the output of the optical pulse source 60, which comprises a wavelength interleaved train of filtered pulses at two different wavelengths (represented by the solid and dashed lines).

0052The AOTF 63 may additionally be operable as a pulse-picker, to reduce the pulse repetition rate for the output pulses. As shown by <figref idref="f0007">figure 8</figref>, the supercontinuum pulse train is shown in <figref idref="f0007">figure 8a</figref>, and the AOTF output pulse train of wavelength filtered and reduced repetition rate pulses at two different wavelengths (represented by the solid and dashed lines) is shown in <figref idref="f0007">Figure 8b</figref>.

0053The AOTF 63 is operable to filter the supercontinuum optical pulses at more than 2 wavelengths, to produce a very complex pulse train which might have numerous applications within advanced imaging applications. <figref idref="f0008">Figure 9</figref> for example, shows the supercontinuum optical pulses (<figref idref="f0008">Figure 9a</figref>) at a given repetition rate, and an output optical pulse train from the optical pulse source 60 (<figref idref="f0008">Figure 9b</figref>) with the AOTF 63 operating to filter each set of four supercontinuum pulses at four different wavelengths, to thereby generate an output optical pulse train comprising interleaved optical pulses of four different wavelengths.

0054The AOTF 63 is also operable to amplitude control the output pulse intensities, which provides another degree of flexibility in optimising system performance. This could, for example, balance the illumination intensities for optical pulses at different wavelengths to pre-compensate for different fluorescence levels from donor and acceptor fluorophores in FRET.

0055The combination of the variable optical supercontinuum pulse repetition rate and AOTF to form a variable repetition rate and wavelength optical pulse source, means that one can produce a series of repetition rates and with arbitrary, wavelength varying pulse trains.

0056It will be appreciated that the described variable repetition rate optical pulse source may be replaced by a fixed repetition rate optical pulse source generating supercontinuum pulses, provided that the AOTF can operate sufficiently fast. For example, it is difficult to produce modelocked lasers at repetition rates much below 20MHz, and this would require a switching speed of the AOTF to be of the order of 10nsecs, to reliably switch between colours with good extinction ratio between adjacent pulses - essential for FRET applications where very sensitive electronics and detectors are used.

0057Typically, switching speeds of the 100nsec to microsecond level are achievable with AOTF's, meaning that an optical pulse repetition rate of 5MHz and below becomes a realistic solution. The variable repetition rate optical pulse source presents a means to achieving this speed, and also provides the flexibility to further vary the repetition frequency.

0058An example in which the optical pulse source 60 may be used as an illumination source, is in an imaging application known as pulsed-interleaved-excitation, most commonly used within FRET (Forster Resonance Energy Transfer). FRET is the radiation-less energy transfer from an optically excited donor fluorophore to a nearby acceptor molecule through dipole-dipole interaction, and enables quantitative length measurement due to the strong distance dependence of FRET. In conventional FRET, the quality of measurement is hampered by incomplete FRET pairs (having a missing or non-functional acceptor molecule), which look very similar to FRET pairs with large donor-acceptor distances. In addition, determination of the FRET efficiency requires quantitative information on the detection efficiency for donor and acceptor fluorescence - including spectral crosstalk.

0059Pulse interleave excitation FRET (PIE-FRET) overcomes these issues by having consecutive pulses operating at different wavelengths. The first pulse operates at a wavelength to excite a donor molecule and initiate FRET. This leads to donor-to-acceptor fluorescence. The second pulse operates at a wavelength to excite directly the acceptor fluorophore but not the donor. This process determines whether or not the FRET pair has a properly functioning acceptor. PIE-FRET has been demonstrated using two different colour laser diodes, modulated at the same frequency but with one diode delayed in time with respect to the other, producing a pulse train of alternating colour pulses. Typically, this process is carried out at fairly high repetition rates (20MHz, to 40MHz) with pulse-to-pulse time separations of the order of 50ns to 25ns respectively, which is usually acceptable for measurement of most biological lifetimes, and provides high throughput for building up large images. The restriction of the two diode approach, is that the wavelengths are limited to those available from suitable laser diodes. The optical pulse source 60 of the present invention allows the production of pulse trains having a combination of any sets of wavelengths within the supercontinuum spectral range.

0060In a third embodiment of the invention, the use of a fibre laser incorporating a pulse-picker element allows for the delivery of higher energy supercontinuum lasers within the visible region of the spectrum.

0061In some examples, it is important to be able to scale the energy from the supercontinuum optical pulses - particularly within the visible region of the spectrum. Using the same system configuration as <figref idref="f0004">Figure 4</figref>, it is possible to scale the average supercontinuum energy by reducing the pump pulse repetition rate but increasing the pump pulse duration to hundreds of picoseconds. This effectively results in a higher energy 1064nm fundamental input pulse (the peak power of the pulse is limited within the fibre amplifier by nonlinear interactions dominated by stimulated Raman scattering (SRS)) and a corresponding increase in the average output supercontinuum pulse energy across the supercontinuum spectra exiting the PCF.

0062By increasing the pulse duration, one therefore increases the average pulse energy attainable within the SRS limit of the fibre amplifier. The result, after injection into the PCF, is an increase in the resulting spectral energy density of the supercontinuum pulses.

0063For example, a standard supercontinuum laser operating at 20MHz repetition rate and delivering 2W average power in the supercontinuum, produces a supercontinuum pulse energy of approximately 100nJ. When the visible region of this source output is filtered, the average spectral energy density is in the region of 50pJ/nm (400mW over a 400nm bandwidth).

0064In known commercial supercontinuum fibre lasers, such as the SC450 range from Fianium Ltd or the SuperK range from Koheras A/S, the average energy spectral density of pulses within the visible region of the spectrum ranges from approximately 35pJ/nm (SuperK Rapid, 3mW/nm at 80MHz) to 50pJ/nm (SC450, 1mW/nm at 20MHz). For these sources, the total supercontinuum power ranges from 2W to 6W and is generated using pulsed optical pump sources of between 8W and 20W average power. The spectral broadening in the photonic crystal fibre is generated by the high peak power of the pump source optical pulses which can be as high as 40KW (8W, 20MHz, 1opicoseconds). As an example, a typical SC450-2 supercontinuum laser with a power spectral density in the visible of 1mW/nm, has an energy spectral density of approximately 50pJ/nm (50*10^-12 J/nm), and is pumped by a pulsed optical pump source operating at 20MHz with a pulse duration of approximately 10picoseconds and an output power of approximately 8W.

0065In order to scale the energy spectral density of these know supercontinuum sources at these repetition rates to 500pJ/nm, pump laser source having between 10 and 15 times the output power would be required, equating to a pulse optical pump source with average power of between 80W and 285W.

0066In this embodiment, a pulse-picked optical pulse source 40, as shown in <figref idref="f0006">Figure 6</figref>, operating at a 5MHz pulse repetition rate and a 10m length of PCF 44, achieves the same visible power and spectral bandwidth but this results in a 5-Fold increase in spectral energy to 200pJ/nm. Starting with a 200ps optical pulse and a standard length of nonlinear fibre, it is possible to scale the spectral energy density to in excess of 1 nJ/nm at 1MHz, with an average visible power of approximately 400mW (over a 400nm band) at 1MHz.

0067A fourth embodiment of the invention comprises a pulse-picked optical pulse source 40, as shown in <figref idref="f0006">Figure 6</figref>, in which the pump pulse repetition rate is reduced to 1MHz, and the pulse duration increased to approximately 200picoseconds. In order to generate the required spectral broadening within the PCF 44, 40KW of pulse peak power is attained from the pulsed pump source 41 at an average output power of 8W, resulting in a 2W supercontinuum, with 1mW/nm power spectral density in the visible spectral region, corresponding to approximately 1nJ/nm energy spectral density.

0068A fifth embodiment of the invention provides a method of scaling the energy of optical supercontinuum pulses generated using the optical pulse sources 30, 40 of the previous embodiments, The method comprises reducing the pump pulse repetition rate of the optical pump laser 31, 41 and increasing the pump pulse duration.

0069In general, the magnitude of spectral broadening of an optical source propagating through a given nonlinear optical element is directly proportional to the intensity of that source. For a pulsed laser source, the intensity is given by the pulse peak power: <maths id="math0001" num=""><math display="block"><msub><mi mathvariant="normal">P</mi><mi>peak</mi></msub><mo>=</mo><msub><mi mathvariant="normal">P</mi><mi>avg</mi></msub><mo>/</mo><mfenced><mrow><mi mathvariant="normal">R</mi><mo>*</mo><mi>dT</mi></mrow></mfenced></math><img file="EP2492736B2_D0001.tif" /></maths> where Ppeak is the peak power of the pulse, Pavg is the average power of the laser source, R is the pulse repetition frequency of the source and dT is the temporal duration of the pulse.

0070The energy of the pulse is given by: <maths id="math0002" num=""><math display="block"><mi mathvariant="normal">E</mi><mo>=</mo><msub><mi mathvariant="normal">P</mi><mi>avg</mi></msub><mo>/</mo><mi mathvariant="normal">R</mi></math><img file="EP2492736B2_D0002.tif" /></maths>

0071For a laser based on a MOPA (Master Oscillator, Power Amplifier) design, while the average power of the laser can be scaled the peak power is limited by nonlinear effects (mainly stimulated Raman scattering) within the fibre amplifier system, which has the effect of clamping the peak power, resulting in: <maths id="math0003" num=""><math display="block"><msub><mi mathvariant="normal">P</mi><mi>peak</mi></msub><mo>=</mo><mi>Constant</mi><mo>=</mo><msub><mi mathvariant="normal">P</mi><mi>avg</mi></msub><mo>/</mo><mfenced><mrow><mi mathvariant="normal">R</mi><mo>*</mo><mi>dT</mi></mrow></mfenced></math><img file="EP2492736B2_D0003.tif" /></maths>

0072The method realises that, to a first approximation, the extent of spectral broadening within a pulsed supercontinuum fibre laser can be considered to be limited by the peak power of the pulse, but that the energy of the pulse can be changed by adjusting the pulse repetition rate and the temporal duration of the pulse.

0073The maximum pulse energy of the optical pulse source 30, 40, and hence the maximum pulse energy (or energy spectral density) of resulting supercontinuum pulses, can therefore be increased by reducing the pulse repetition rate of the pump laser 31, 46 and, in the case of the MOPA 46 by either reducing the pulse repetition rate or scaling the average power from the amplifier 45. For a given amplifier design and average power output, an increase in pulse energy can be achieved with a reduction of pump pulse repetition rate and increase of pump pulse duration.

0074The limit of peak power extractable from a given fibre amplifier is determined by the length of the amplifier fibre and the cross sectional area of the mode propagating within this fibre. For standard single mode optical fibres operating at 1µm, the peak power limit can be as low as a few kW, but this can be scaled to tens ofkWatts for large mode area (LMA) fibres where the effective mode field diameter within the fibre mode can be greater than 12µm.

0075The described embodiments make use of the reduction in pulse repetition rate to enable scaling of the energy spectral density of the supercontinuum without the requirement for scaling the power spectral density and therefore the average power from the pulsed optical pump source.

0076From the foregoing therefore, it is evident that the present invention enables the delivery of filtered visible laser sources at a range of pulse repetition rates from 100MHz down to a few Hz or less. This invention has clear benefits over conventional supercontinuum sources for applications where the pulse-to-pulse separation of the source is an important parameter.

0077Various modifications may be made without departing from the scope of the present invention. For example, where the DPSS pump laser may be replaced by a different type of mode-locked laser, and may comprise a MOPA. The PCF may be replaced by a different micro-structured optical fibre, such as a side-hole fibre, hollow-core photonic bandgap fibre or solid-core photonic bandgap fibre, or may be replaced by a different nonlinear optical element, such as a nonlinear crystal like such as LBO, BBO, CLBO, PPLN, PPKTP or any other nonlinear optical crystal phase matched to produce frequency conversion of the pump laser output.

0078In the embodiments comprising a MOPA, the gating means may alternatively be located after the power amplifier.

0079This specification also includes the subject matter of the following clauses which are not intended to define the scope of protection:

Clauses

0080<ol id="ol0001" ol-style=""><li>1. An optical pulse source comprising: <ul id="ul0003" list-style="none" compact="compact"><li>an optical pump laser operable to generate optical pump pulses at a pump pulse repetition rate;</li><li>a nonlinear optical element;</li><li>gating means provided between the pump laser output and the a nonlinear optical element, and operable to selectively control the launch of pump pulses into the nonlinear optical element at a variable, lower repetition rate, to thereby selectively control the generation of optical supercontinuum pulses within the nonlinear optical element and their repetition rate.</li></ul></li><li>2. An optical pulse source according to clause 1, wherein the nonlinear optical element comprises one or more nonlinear optical crystals or a microstructured optical fibre.</li><li>3. An optical pulse source according to clause 2, wherein the microstructured optical fibre is one of highly non-linear photonic crystal fibre, side-hole fibre, hollow-core photonic bandgap fibre and solid-core photonic bandgap fibre.</li><li>4. An optical pulse source according to any of clauses 1 to 3, wherein the optical pump laser comprises a mode-locked laser or a mode-locked fibre oscillator.</li><li>5. An optical pulse source according to clause 4, wherein the optical pulse source further comprises an optical fibre amplifier configured to receive pump pulses from the modelocked fibre oscillator and operable to amplify the pump pulses, the modelocked fibre oscillator and the amplifier together forming a master oscillator power amplifier (MOPA), and the gating means is provided between the modelocked fibre oscillator and the optical fibre amplifier.</li><li>6. An optical pulse source according to any of clauses 1 to 5, wherein the gating means comprises an optical modulator element such as an acousto-optic modulator, an electro-optic modulator or a semiconductor saturable absorber mirror (SESAM) modulator.</li><li>7. An optical pulse source according to clause 6, wherein the optical modulator element is synchronised with the output of the optical pump laser, to selectively control the repetition rate of pulses launched into the nonlinear optical element, to thereby selectively control the output pulse repetition rate.</li><li>8. An optical pulse source according to any of clauses 1 to 7, wherein the source further comprises a wavelength tunable optical bandpass filter configured to receive an optical supercontinuum pulse and to wavelength filter the pulse at a predetermined wavelength.</li><li>9. An optical pulse source according to clause 8, wherein the wavelength tunable optical bandpass filter is configured to receive a plurality of optical supercontinuum pulses in a pulse set and to wavelength filter each pulse at a respective one of a corresponding plurality of predetermined wavelengths, to thereby form a wavelength interleaved optical pulse set.</li><li>10. An optical pulse source according to clause 9, wherein the wavelength tunable optical bandpass filter is configured to filter a plurality of such pulse sets in series to thereby form a repeatingly wavelength interleaved optical pulse train.</li><li>11. An optical pulse source according to any of clauses 1 to 10, wherein the source further comprises second gating means provided after the nonlinear optical element configured to pulse-pick received optical pulses to a lower pulse repetition rate.</li><li>12. An optical pulse source according to clause 11, wherein the second gating means comprises an acousto-optical modulator or a wavelength tunable optical bandpass filter.</li><li>13. An optical pulse source according to clause 12, wherein the wavelength tunable optical bandpass filter comprises an acousto-optic tunable filter.</li><li>14. An optical pulse source according to clause 12 or clause 13, wherein a single wavelength tunable optical bandpass filter is configured to perform both wavelength filtering and pulse-picking.</li><li>15. An optical pulse source according to any of clauses 11 to 14, wherein the second gating means is further operable to reduce the optical intensity of one or more optical pulses in the pulse set or train.</li><li>16. A method of scaling the energy of optical supercontinuum pulses generated by an optical pulse source according to any of clauses 1 to 15, the method comprising the steps of: <ul id="ul0004" list-style="none" compact="compact"><li>reducing the pump pulse repetition rate of the optical pump laser; and</li><li>increasing the pump pulse duration.</li></ul></li><li>17. A method of scaling the energy of optical supercontinuum pulses according to clause 16, wherein the method comprises reducing the pump pulse repetition rate to less than 20MHz and increasing the pump pulse duration to greater than 10 picoseconds.</li><li>18. A method of scaling the energy of optical supercontinuum pulses according to clause 17, wherein the pump pulse duration is at least 200 picoseconds.</li><li>19. An illumination source for stimulated emission depletion microscopy, the illumination source comprising an optical pulse source according to any of clauses 1 to 15 configured to generate output optical pulses having a repetition rate in the range 1MHz to 10MHz and an energy spectral density of at least 100 picojoules per nanometer, the optical pulse source being operated to scale the energy of the optical supercontinuum pulses by reducing the pump pulse repetition rate of the optical pump laser and increasing the pump pulse duration.</li><li>20. An illumination source for stimulated emission depletion microscopy according to clause 19, wherein the optical pump laser is operable to generate optical pump pulses having a pulse repetition rate of less then 20MHz, and a pump pulse duration of at least 200 picoseconds.</li><li>21. An optical fluorescence microscope based on Stimulated emission and depletion, the microscope comprising an illumination source according to clause 19 or clause 20, the illumination source being configured to provide both excitation illumination and stimulated emission illumination.</li></ol>