Patent Description:
This invention relates generally to amplifier system for multi-photon microscopy, and more particularly to a microjoule amplifier system for three-photon microscopy utilizing existing femtosecond lasers used for two-photon microscopy as a seed source.

In the lifesciences, fluorescence microscopy is used to study biological samples like individual cells or complex structures like the brain. Scattering in these samples limits the imaging depth. It is generally true that light with longer wavelength is scattered less in tissue hence allowing it to penetrate deeper into e.g., the brain. Two-photon (2p) microscopy makes use of this and allows imaging about <NUM> deep into e.g., the brain. In two-photon microscopy, the wavelength needed to excite a fluorophore or fluorescent protein is doubled compared to single photon excitation as e.g., used in confocal microscopy. Three-photon (3p) microcopy takes this approach one step further by using even longer wavelength light. The downside of going from single to two- to three-photon excitation is that the probability of such an event happening becomes less and less likely and hence requires higher laser intensity. For 3p microcopy, µJ laser pulses producing Megawatt of peak power are required. Systems capable of producing these peak power levels are expensive and operate with low repetition rates.

Most scientists, however, need, besides the 3p imaging capability, the ability to do <NUM>-photon imaging in order to acquire images at a high frame rate to capture dynamic processes. 3p is only needed when they need to go deep into the sample. A 2p laser is a workhorse in most neuroscience lab. An inventive idea behind this disclosure is to produce a device which can be used in conjunction with a ubiquitous 2p light source to produce light pulses in the µJ regime.

<CIT> discloses a system for producing ultrashort tunable pulses based on ultra broadband OPA or OPG in nonlinear materials. The system parameters such as the nonlinear material, pump wavelengths, quasi-phase matching periods, and temperatures can be selected to utilize the intrinsic dispersion relations for such material to produce bandwidth limited or nearly bandwidth limited pulse compression. Compact high average power sources of short optical pulses tunable in the wavelength range of <NUM> - <NUM> and after frequency doubling in the wavelength range of <NUM> - <NUM> can be used as a pump for the ultra broadband OPA or OPG. In certain embodiments, these short pump pulses are obtained from an Er fiber oscillator at about <NUM>, amplified in Er fiber, Raman-shifted to <NUM> - <NUM>, stretched in a fiber stretcher, and amplified in Tm-doped fiber. To produce short pulses in the <NUM> - <NUM> wavelength range, the pulses are frequency-doubled with a chirped frequency doubler for nearly bandwidth-limited output.

<CIT> discloses a laser assembly for generating laser output light at an output wavelength of approximately <NUM> includes a fundamental laser, an optical parametric system (OPS), a fifth harmonic generator, and a frequency mixing module. The fundamental laser generates fundamental light at a fundamental frequency. The OPS generates a down-converted signal at a down-converted frequency. The fifth harmonic generator generates a fifth harmonic of the fundamental light. The frequency mixing module mixes the down-converted signal and the fifth harmonic to produce the laser output light at a frequency equal to a sum of the fifth harmonic frequency and the down-converted frequency. The OPS generates the down-converted signal by generating a down-converted seed signal at the down-converted frequency, and then mixing the down-converted seed signal with a portion of the fundamental light. At least one of the frequency mixing, frequency conversion or harmonic generation utilizes an annealed, deuterium-treated or hydrogen-treated CLBO crystal.

<CIT> discloses a fiber Chirped Pulse Amplification (CPA) laser system includes a fiber mode-locking oscillator for generating a laser and stretching into a few hundreds ps-<NUM> ns pulse by a fiber stretcher for projecting into an acoustic-optic (AO) functioning as a pulse picker for generated a modulated laser with a reduced repetition rate for projecting to a multiple stage amplifier. The multiple stage amplifier further includes a first high-energy amplifier implemented with a large mode area (LMA) fiber for amplifying the modulated laser up to a uJ level laser and a second amplifier implemented with a PCF based Yb doped fiber to further amplify the uJ level laser to a mJ level laser.

<CIT> discloses a fiber laser system includes a fiber mode-locking oscillator, a fiber stretcher, a multistage amplifier chain, a pulse picker, and a compressor wherein at least a device for performing a pulse shaping, a spectral shaping and a polarization shaping and a combination thereof is implemented in the fiber mode-locking oscillator, the fiber stretcher, the multistage amplifier chain, the pulse picker, and the compressor for managing and reducing nonlinear effects in the fiber laser system. The combinations of pulse shaping, spectral shaping and polarization shaping in different stages of the fiber laser system enables the fiber laser system to generate a short pulse of <<NUM> fs and a high energy laser in a range between <NUM> uJ to over mJ and an average power from <NUM> W to <NUM> W.

Disclosed are ideas to produce an add-on device which turns widely used high repetition rate lasers used for 2p microscopy into a light source which can be used for 3p microscopy. The add-on encompasses a device to reduce the pulse repetition rate of the high repetition rate (><NUM>) laser source (laser or OPO) to less than <NUM> which allows for higher pulse energies while maintaining reasonable average powers. If the high repetition sources operate below <NUM> the add-on shifts or broadens the seed light to cover <NUM> to <NUM> before amplification. If the high repetition rate source operates at or around <NUM> the add-on might only need to amplify the pulse after downshifting the repetition rate. In another implementation the add-on shifts or broadens the <NUM> light to cover the spectral range out to <NUM> before amplification.

The novelty here is that this add-on system turns a generic 2p laser system into a 3p laser system hence providing unique value to the customers as they can enjoy the benefit of both techniques.

In one embodiment, the present invention provides an amplifier system, including: an optical parametric oscillator (OPO) producing light pulses with a first repetition rate; a pulse picking device configured to reduce the pulses from the first repetition rate to a second repetition rate; a pulse stretching module configured to increase the pulse duration of the pulses from the pulse picking device; an amplifier configured to provide gains to the longer duration pulses; and a pulse compressing module configured to reduce the pulse duration of the amplified pulses.

In one embodiment, the present invention provides an amplifier system, including: a femtosecond laser outputting light pulses with a first repetition rate; a wavelength shifting module configured to shift or broaden the wavelength of the light pulses; a pulse picking device configured to reduce the pulses from the first repetition rate to a second repetition rate; and a pulse stretching module configured to increase the pulse duration of the pulses from the pulse picking device; an amplifier configured to provide gains to the longer duration pulses; and a pulse compressing module configured to reduce the pulse duration of the amplified pulses.

Relative terms such as "lower," "upper," "horizontal," "vertical," "above," "below," "up," "down," "top" and "bottom" as well as derivative thereof (e.g., "horizontally," "downwardly," "upwardly," etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion.

This disclosure describes the best mode or modes of practicing the invention as presently contemplated. This description is not intended to be understood in a limiting sense, but provides an example of the invention presented solely for illustrative purposes by reference to the accompanying drawings to advise one of ordinary skill in the art of the advantages and construction of the invention. In the various views of the drawings, like reference characters designate like or similar parts.

The first idea presented here is to use existing Optical Parametric Oscillator based, high repetition rate (><NUM>) femtosecond sources as a seed source for a lower repetition rate amplifier producing pulse energies on the order of micro Joule, capable of amplifying several <NUM> of bandwidth and operating between <NUM> and <NUM>. As shown in <FIG>, a device reducing the pulse repetition rate <NUM> is placed in between the OPO <NUM> and the amplifier <NUM>. This device could be a Pockels cell or an acousto-optic modulator (AOM). The pulse picking device <NUM> is followed by a pulse stretching module <NUM> which increases the pulse duration in a reversible fashion from the femtosecond regime to the picosecond (<NUM> to <NUM> ps) time scale in order to reduce the peak power of the laser pulse. The module <NUM> is followed by one or multiple amplifier stages <NUM>. The amplifier stages could be based on fiber, fiber rods, bulk glass or crystal. The gain process could be parametric or non-parametric. The amplifier stage(s) <NUM> is/are followed by a pulse compressing module <NUM> which reduces the pulse duration again down to the femtosecond scale. An optional wavelength shifting module <NUM> may be placed between the OPO <NUM> and the pulse picking module <NUM>.

In another embodiment the compression can have the ability to over compensate so the output pulse is negatively chirped, i.e., higher frequencies are in the leading edge of the pulse. The purpose would be to pre-compensate the dispersion in the 3p microscope.

In another embodiment the original seed light from the OPO is either spectrally broadened in a highly non-linear fiber or frequency shifted to cover the <NUM> to <NUM> spectral region before amplification. The broadening could happen after or before changing the repetition rate.

As shown in <FIG>, the second idea proposed is to make use of a femtosecond laser <NUM> operating below <NUM> as a seed source for the µJ amplifier system. Therefore, the wavelength of the high repetition rate source needs to be shifted or broadened by a wavelength shifting module <NUM> to cover the spectral range of <NUM> to <NUM>. An optional preamplifier module <NUM> may be placed between the femtosecond laser <NUM> and the wavelength shifting module <NUM>. The broadening could or could not require additional preamplification. In one implementation a photonic crystal fiber (PCF) could be used to broaden the spectrum to cover the desired range. The pulse picking device could be place before or after the broadening stage. The output of the PCF could benefit from spectral filtering to optimize the overlap with the gain spectrum. The seed light might or might not go to the pulse stretching module before entering the amplification stages described above.

In another implementation the light of the high repetition rate source is frequency shifted via nonlinear processes like Raman shifting before it is used as seed pulse. After the frequency shifting, the light might or might not go to the pulse stretching module before entering the amplification stages described above.

Claim 1:
An amplifier system, comprising:
an optical parametric oscillator (OPO) (<NUM>) producing femtosecond duration light pulses with a first repetition rate greater than <NUM> and tuning range between <NUM> to <NUM> and an average power of > 1W at the peak of the tuning range;
a pulse picking device (<NUM>) configured to reduce the pulses from the first repetition rate to a second repetition rate less than <NUM>;
a pulse stretching module (<NUM>) configured to increase the pulse duration of the pulses from the pulse picking device to picosecond duration pulses;
an amplifier (<NUM>) configured to provide gains to the longer duration pulses; and
a pulse compressing module (<NUM>) configured to reduce the pulse duration of the amplified pulses to femtosecond duration pulses;
wherein the system generates pulse energies, suitable for <NUM> photon spectroscopy, on the order of micro Joule, amplifies several <NUM> of bandwidth, and shifts or broadens the tuning range between <NUM> and <NUM> to a range between <NUM> and <NUM>.