Patent Description:
The application space for ultrashort pulse lasers has become extremely broad, including but not limited to machining and processing of materials, high-field laser-matter interactions, ultrafast time-resolved spectroscopy, nonlinear microscopy, and etc. Each of the listed applications benefits from rugged, compact, and robust pulsed sources capable of outputting reproducible single mode (SM) pulses in a femtosecond (fs) - picosecond (ps) time range.

Lasers that emit pulses of tens of femtoseconds in duration (<NUM> fs = <NUM>-<NUM> s) are used mainly for research and development, where requirements for environmental stability are not strict. Output pulse energy for such lasers may vary in a very broad range, from several nanojoules and higher. Their repetition rate also varies broadly, from <NUM> to <NUM>. There are many industrial applications that do not require high pulse energy, given such short pulse duration, but are in need of reliable lasers, simple in handling to allow operation by users not skilled in laser technology, like biologists, chemists, or physicians, and tolerant to industrial environment. Laser-assisted processing of optically transparent solid materials, like glasses or sapphire, which includes bulk modification for <NUM>-D structuring, surface etching, and direct bonding may require pulse energies as low as <NUM>-<NUM> nanojoules (nJ), combined with a few hundred or sub-hundred femtosecond pulse duration. Analytical applications require even lower pulse energy in combination with shorter pulse duration to ensure nondestructive interaction. These applications include multiphoton microscopy and spectroscopy that benefit not only from short pulse duration and high beam quality, but also from controllable spectral phase and temporal pulse profile for selective chromophore excitation when an ensemble of different chromophores is investigated. Laser-assisted crystallization or nucleation finds application in pharmaceutical industry and. Two-photon polymerization for lithography and <NUM>-D printing or micro-structuring uses pulse energies from a single nJ to tens of nJs, given the pulse duration is less than <NUM> fs. Similar pulse parameters are in use for photoporation and transfection, which became widely used processes in today's gene manipulation procedures performed on living cells. The number of applications where low-energy femtosecond pulses can be successfully used is quickly growing.

The common feature of all the aforementioned applications is the requirement for a "cold" process where heat dissipation is suppressed or strongly mitigated and can be effectively controlled in both time and volume. This is achieved by using shorter pulses < <NUM> fs and preferably <<NUM> fs, which ensures minimal or no thermal effect caused by radiation on the region of interest. The realization of "cold" interaction regimes allows maximizing purely nonlinear processes without destruction associated with accumulation of energy excess, inherent to longer pulses.

The development of fs pulse lasers cannot be separated with the development of the titanium-doped sapphire {Ti:sapphire) laser gain material. Ultrafast laser pulses require a broad spectral bandwidth. The Ti:sapphire, with its gain bandwidth spanning the near-infrared (near-IR) from <<NUM> to ><NUM>, is the champion in this regard.

Prior art Ti:sapphire based pulse laser systems are disclosed inter alia in <NPL>.

Nevertheless, in recent years other technologies have made inroads. For example, fiber lasers, disclosed in this application, are now generating pulses <<NUM> fs in duration, as are other ytterbium (Yb)-crystal based lasers. There are a few reasons for using fiber lasers instead of Ti:sapphire gain media. One of the reasons includes low cost, compactness, simplicity with tunability and pulse duration and reliability.

Another reason is that the broad bandwidth of Ti:sapphire requires a large wavelength interval between the pump light and the laser light-that is, a large quantum defect. This also means significant thermal dissipation. In Yb-doped lasers (including Yb-fiber), the quantum defect is several times smaller, as is made possible by the far narrower gain bandwidth.

Yet the narrow gain spectrum of the Yb fiber gain media can also be very problematic. Having the broad gain spectrum, Ti:sapphire gain media are capable of maintaining the evolution of fs pulses without the necessity of spectrally broadening these pulses beyond a Ti:sapphire oscillator before the pulses are amplified in a Ti:sapphire amplifier which has the same broad gain spectrum as the oscillator.

In contrast, Yb fiber oscillators and amplifiers have a substantially narrower gain spectrum. The Yb oscillator can be configured to output coherent broadband pulses. But further pulse amplification is not possible because Yb-doped fibers, as its nature dictates, can amplify only a small spectral region. The rest of the broad spectral line remains unaffected. But most of the ultrafast laser applications require the amplified broadband fs pulse.

Several methods of generating ultrafast pulses in the fiber gain media are well known. One of these methods - passive mode locking - is part of the disclosed subject matter. The key to the passive modelocking is the presence in a ring cavity of at least one component that has a nonlinear response to increasing peak intensity.

Unlike other laser systems, mode-locked fs fiber lasers are sensitive to the group velocity dispersion (GVD) when propagating through any medium. The GVD is a characteristic of a dispersive medium used to determine how the medium will affect the duration of an optical pulse traveling through it. More specifically, the GVD is the frequency dependence of the group velocity of light in a transparent medium. In the context of ultrashort pulses, the GVD is manifested by a giant chirp due to the effects of nonlinearity, e.g. self-phase modulation.

The term "giant chirp" as used here refers to the key characteristic of the resulting oscillator output pulses. Typically, the high energy giant chirped output pulse will have a duration at least tens of picoseconds and more, which eliminates the need for an external pulse stretcher and thus allows the use of increased power levels in the mode-locked oscillator. The giant chirped pulses can be compressed with a compressor to femtoseconds in order to extract higher pulse energies, but such a compression is not simple as explained immediately below.

Assume that Yb fiber laser generated pulses each acquire a giant chirp having a full width (FW) of <NUM>. Using a pair of gratings, it is possible to compress each pulse to, for example, a τFWHM <NUM> fs pulse. But is the <NUM> fs duration the lowest limit for the given band? No. Theoretically, of course, any fully coherent pulse may be compressed to the lowest possible duration for a given time-bandwidth-product of this pulse which is referred to as either transform limited (TL) or Fourier transform (FT) pulse. For example, the same <NUM> chirped pulse has the transform limited τFWHM of about <NUM> fs. The question is where those parasitic <NUM> fs are acquired.

The duration of an ultrashort laser pulse is limited by its bandwidth, spectral shape and the degree to which all its frequencies are in phase. The generation of transform-limited pulses depends on how accurately phase distortions inside and outside the laser can be measured and corrected. The task for producing TL pulses is rather simple: compensate the giant chirp.

The chirp, propagating through optical fibers and fiber components, acquires linear and nonlinear components of the GVD, wherein the nonlinear component corresponds to a third- or higher order dispersion (TOD) resulting from the frequency dependence of the GVD. The linear dispersion component compensation of the chirp with static compressors, such as volume and fiber Bragg gratings (VBG, FBG respectively) or surface gratings, is well known. However static compressors may not only be ineffective when dealing with the nonlinear component, but they are also known to introduce the additional and significant TOD. The mechanism correcting the nonlinear component is referred to as a pulse shaper.

The techniques realizing the compression are also well known to one of ordinary skill in the pulsed laser arts and include, among others, the used in the situations similar to the following.

There are a few known programmable pulse shapers provided with algorithms, such as the multiphoton intrapulse interference phase scan (MIIPS), that are used to assist in compensating nonlinear (and sometime linear) components in the following manner. The programmable pulse shaper incorporates a liquid crystal spatial light modulator (SLM) capable of introducing a well-calibrated phase function across the laser spectrum. The reference phase function is one that causes a local correction to the second derivative - group dispersion (GD) measured in fs<NUM> or ps<NUM> - of the spectral phase and higher order derivatives. As it scans across the spectrum of the laser, the maximum intensity in the second-harmonic signal scans as well such that the phase shift introduced by the modulator has the same value but opposite sign to the original pulse maximum of the second harmonic. In the absence of phase distortions, there is a linear relation between the two; phase distortions change this relation. The results obtained through the tracking phase deviation form a correction function. Upon double integration of the correction function, the deviations yield the phase distortions. Once phase distortions are measured, the pulse shaper corrects a pulse shape.

The difficulty of operating the MIIPS-controlled pulse shaper is well known to one of ordinary skill in the ultra-short pulse laser arts. Typically, the tuning and operation of the pulse shaper requires a team of physicists all with the PhD degree. Different operational regimes of a pulse generator, i.e., different pulse durations, output pulse energies and pulse shapes both in time and frequency domains, require different phase masks or phase and amplitude masks. Also, a single phase mask working well for one of the group of identically configured pulse laser generators unlikely properly operates in combination with another pulse generator of the same group. It all leads to an unacceptable cost and labor inefficiency.

One of the known known programmable pulse shapers is an acousto-optical modulators (AOM) in which a radio-frequency electrical signal drives a piezoelectric transducer launching a traveling acoustic wave. Modulator action is based on diffraction of the light beam from refractive index changes induced by the traveling acoustic wave. The diffracted beam is shifted in frequency by an amount equal to the electrical drive frequency, ideally with an amplitude and phase that directly reflect the amplitude and phase of the RF drive. A liquid crystal phase modulator (LCoS) can also be utilized. Alternatively, an acousto-optical programmable dispersive filter (AOPDF) can be used. In AIPDF the acoustic wave co-propagates with the optical field to rotate the field polarization and thereby, change the optical delay. By using an electronic arbitrary waveform generator to drive the AOM, the acoustic profile can be controlled, and programmable pulse shaping achieved. However, both AOMs and AIPDFs require synchronization with the laser source and typically may not run at the desired high pulse repetition rate (PRR), which somewhat limits their application.

Regardless of the configuration, a programmable pulse shaper is a complex tool used in the laboratory. Typically, a fiber laser ultrashort pulse system is assembled in situ from different components: a mode-locked oscillator, which outputs short pulses, stretcher, and a programmable pulse shaper facilitating pulse compression. The assembled system is bulky and requires complicated tuning which is typically done by a team of highly educated personnel.

A need exists for a fs fiber laser system suitable to satisfy requirements of "cold start" and other applications, and enhanced by the capability of programming the spectral phase and thereby, the temporal pulse shape. Other prior art ultrashort pulse laser systems are disclosed inter alia in <NPL>, <CIT> , and <NPL>),.

Another need therefore exists for a reliable fs fiber laser system capable of maintaining the evolution of amplified broadband pulses.

A further need exists for an industrial ultrashort laser unit which integrates a mode-locked fs fiber laser, programmable pulse shaper and a computer, operative to control and coordinate the operation of the fiber laser and pulse shaper, into a single unit.

The invention is defined in the appended independent claims.

The invention as disclosed in independent claim <NUM> concerns a pulse configurable laser unit, PCLU, outputting uniform ultrashort pulses, comprising:.

The invention as disclosed in independent claim <NUM> discloses a method of generating ultrafast pulses by the PCLU of claims <NUM>-<NUM>, comprising:.

The above and other features of the disclosed laser unit are further discussed in detail to become more readily apparent in conjunction with the following drawings:.

<FIG> illustrates a basic configuration of commercial coherent laser unit <NUM>. The latter includes a laser source or fiber pulse generator <NUM> which is configured with a passively mode-locked master fiber oscillator <NUM> and, optionally, power fiber amplifier <NUM>. The oscillator <NUM> and amplifier <NUM> have thus a MOPFA architecture with the oscillator <NUM> being a seed. The laser unit <NUM> outputs a train of single mode (SM) broadband coherent pulses. For example, each pulse may have a FWHM of up to <NUM>, pulse duration τ of <NUM> - <NUM> ps, average power Pav from <NUM> to <NUM> mW and pulse energy Ep of at least <NUM> MW. The output ps pulses each are generated with a giant chirp that only increases when the ps pulses interact with a fiber medium. The chirp development involves a gradual formation of linear and nonlinear components. The compensation of both chirp components leading to the compression of chirped pulses to respective TL pulses, can be performed by a combination of static compressor <NUM> operative to compensate a major part of linear chirp and programmable pulse shaper <NUM> or sometime only by pulse shaper <NUM> dealing with the remaining part of the linear chirp and high order dispersion. Both pulse generator <NUM> and pulse shaper <NUM> are controllable by a common or respective individual computers <NUM>. The static compressor <NUM> and pulse shaper <NUM> are preferably mounted in a laser head <NUM> which can be coupled to pulse generator module <NUM> by a flexible cable. Alternatively, all three main components may be enclosed in the same housing.

Turning to <FIG>, static compressor <NUM> is positioned downstream from shaper <NUM>. It is important to remember that while the static compressor compensates a linear chirp component, it also introduces additional nonlinearities that have to be taken into account. <FIG> shows an alternative configuration of laser unit <NUM> in which compressor <NUM> is located between pulse generator <NUM> and shaper <NUM>. Similar to <FIG>, shaper <NUM> of <FIG> compensates the nonlinear chirp and the remaining portion of the linear chirp.

The selection of any of the above configurations of unit <NUM> shown in <FIG> depends on the parameters of pulses output by pulse generator <NUM> and the modulation depth of pulse shaper <NUM>. It is perfectly reasonable to expect unit <NUM> to operate in a range of pulse durations varying between about <NUM> fs to about <NUM> ps. The laser unit <NUM> of any of <FIG> is operative to compress broadband coherent ps pulses from pulse generator <NUM> to respective nearly TL pulses. For example, if generator <NUM> outputs a Gaussian FWHM <NUM> broadband <NUM> ps pulse centered at <NUM> wavelength, it can be effectively compressed at the output of unit <NUM> to about <NUM> - <NUM> fs. For all practical purposes, the latter can be referred to as a TL pulse since theoretically the shortest pulse duration for the <NUM> pulse is <NUM> fs and even less.

<FIG> illustrates the schematic of pulse generator <NUM> configured as a self-starting passively mode-locked laser which is disclosed in greater detail in <CIT>. The pulse generator <NUM> is based master oscillator <NUM>, further referred to ring resonator <NUM> in which outputs of respective fiber amplifiers <NUM> and <NUM> seed one another. Between the first and second amplifiers <NUM> and <NUM>, two or more identical groups or chains of fiber elements are coupled together to a define ring cavity <NUM>. Besides the fiber amplifier, each chain includes a fiber coil <NUM>, <NUM> which provides respective periodic spectral and temporal broadening of signal and narrow line filters <NUM>, <NUM> operative to spectrally filter the broadened signal. The filters may have substantially the same bandwidth but confiugred so that respective spectra overlap one another. Alternatively, one of the filters may be confiugred with a bandpass which is at most five (<NUM>) times broader than the bandpass of the other filter with the obligatory overlap of their spectrums which ensures the laser's self start. The illustrated configuratoin of self starting pulse generator <NUM> providing sequential spectral broadening and filtering leads to generation of giant chirped pulses with a substantially uniform duration, spectrum and amplitude.

The ring resonator <NUM> further includes one or more isolators <NUM>, providing the unidirectional guidance of light around the waveguide, and one or more output couplers <NUM> positioned immediately downstream from respective fiber coils <NUM>, <NUM>. The output couplers each guide the chirped pulse outside ring resonator <NUM>. To intensify the desired population inversion in a gain medium of the amplifiers, i.e., to start the operation of the inventive pulse generator, one or two CW pumps <NUM> are coupled to respective amplifiers. All of the above-disclosed components are interconected by single transverse mode (SM) passive fibers. The amplifiers <NUM> and <NUM> each may be based on a SM or MM fiber doped with an appropriate rare earth ions, such as ytterbium (Yb). All fibers have respective substantially uniformly dimensioned cores with mode field diameters (MFD) which substantially match one another. In use, the illustrated scheme is caharacterized by nonsaturated start-up and saturated steady-state pulse generation (modelocked) phases.

<FIG> illustrates an optical schematic of pulse generator <NUM> configured with additional optical fiber elements which desirable to provide higher pulse energies. As mentioned above, all fibers of ring resonator <NUM> have a uniform small core diameter typically not exceeding <NUM>. While the cores of respective fibers can be increased to support propagation of SM light with greater MFD, the pulse energy as a function of MFD may become prohibitively high at the start of generator <NUM> since pulses guided through the schematic of <FIG> in the beginning are overly long. The longer pulses provide higher energy that can jeopardize the integrity of the cavity components.

To avoid prohibitively excessively high pulse energies and yet to obtain maximum possible pulse energies at the output of ring resonator <NUM>, the disclosed pulse generator <NUM> further includes an additional amplifying cascade <NUM> which is a linear analog of ring cavity <NUM> of <FIG>. The amplifying cascade <NUM> may be provided as a separate module or mounted together with the ring resonator and includes the same very components as pulse generator <NUM>. Thus, like ring resonator <NUM>, additional amplifying cascade <NUM> is configured with filters <NUM>, <NUM> with overlapping spectra, SM passive fibers interconnecting the components, isolators <NUM> and amplifiers <NUM> and <NUM>. The opposite ends of the active fiber of amplifying cascade <NUM> are coupled to respective input and output SM fibers <NUM> with all three fibers having respective cores with matching core diameters. The latter are greater than the diameter of all other fibers located upstream from amplifying cascade <NUM>. The doped core fiber of amplifier <NUM> is typically, but not necessarily tapered at the downstream end thereof to provide higher thresholds for nonlinear effects.

Regardless of the configuration of pulse generator <NUM>, disclosed unit <NUM> may be configured with a pulse picker <NUM> operative to control the desired number of pulses and optionally frequency at which unit <NUM> outputs a train of ultrafast pulses. The pulse picker <NUM> is located either upstream from additional amplifying cascade <NUM> or downstream therefrom.

<FIG> illustrates the optical schematic of the chirp-compensating mechanism including two stages - a static pulse compressor <NUM> and pulse shaper <NUM> of <FIG>. In the shown schematic, the static pulse compressor <NUM> receives broadband coherent chirped pulses from pulse generator <NUM> which initially propagate through guiding mirrors M2 and M1 before being compressed by a static compressor, which includes two transmission gratings G1 and G2 and a roof mirror RM. The operation of the static compressor provides for correcting a linear chirp component in broadband coherent chirped pulses. The mechanism of the static compressor is well known to one of ordinary in the laser arts and operates to at least minimize the second order dispersion (SOD) referred hereto as the linear chirp component of the chirped pulse. The partly compressed pulses further propagate in a direction opposite to the initial one from mirror M2 to M1. However, the static compressor does not address the nonlinear component of the chirped pulse. Moreover, it adds about <NUM>. 73x10<NUM> fs<NUM> of the third-order dispersion (TOD).

The compression-adaptive stage - folded-4f pulse shaper <NUM> - is commonly used for fs pulse shaping and enables further refinement of the laser pulse profile by dispersing the laser spectrum across the linear phase modulator so as to directly manipulate the spectral phase regardless of the laser repetition rate. In other words, pulse shaper <NUM> is operative to deal with the nonlinear chirp.

The pulse shaper <NUM> is configured with a dispersive transmission grating G3, collimating lens or double lens L that make an incident beam including several color beamlets which propagate parallel to one another. The parallel beamlets of the shown red, blue and green colors are incident on a pixelated linear 1D or 2D SLM. Finally, the dispersed beam is reflected back parallel to the incident beam, but the reflected beam is vertically spaced from the incident beamlets while being guided through grating G3, mirrors M3 and M4 towards the output of the compressor mechanism, gaining nearly a TL pulse or with modulated phase shift. Since all beam shaping optical components are positioned in a vertical plane, the overall footprint of the compressor mechanism is small. As shown, the compressor mechanism is dimensioned to be about <NUM> x <NUM> which is very compact. The pulse shaper <NUM> is controlled by computer <NUM> executing a software based on MIIPS algorithm, which measures the residual spectral phase and corrects for TOD and higher-order dispersion, and is executed by computer <NUM>.

When a 2D LCOS SLM is used, one can shape both spectral phase and amplitude by utilizing "diffractive shaping" mode. In this implementation, the broadband light is spectrally dispersed and focused along one axis of the LCOS SLM (e.g., horizontal) but it is not focused along the other, orthogonal axis (vertical). A periodic, sawtooth phase pattern, similar to profiles on plane-ruled surface gratings, is encoded along every column of SLM pixels. The laser light is diffracted by this encoded pattern. The spectral phase, added to the diffracted light, can be altered within the full <NUM>-2π range by shifting the periodic pattern along the vertical axis, independently for every SLM pixel column. The depth of the periodic phase modulation for any given SLM pixel column determines the light diffraction efficiency for the corresponding spectral component, and thereby, the laser spectrum at the shaper output.

Depending on the number of pixels of the SLM, the use of the static compressor can be unnecessary since the SLM with a large number of pixels, for example <NUM>-<NUM>, is operative to deal with both linear and nonlinear chirp components. Instead of the shown folded-4f pulse shaper based on the LCOS SLM, the pulse shaper may be based on an AOM or AOPDF. The operation of the disclosed schematic is the same as when the nonlinearly chirped component is compensated by pulse shaper <NUM> before compressor <NUM> deals with the linear chirp component.

Referring to <FIG>, the pulse shaper <NUM> also offers the capability of reshaping TL pulses by modifying the phase mask or profile applied to the SLM, as disclosed immediately below. Thus pulse shaper <NUM> may operate in many industrial applications requiring pulses with arbitrary shapes different from the nearly Gaussian shape of the TL pulse. For example, as can be seen in <FIG>, the phase mask is modulated to form flat top pulses. Practically any pulse shape may be formed utilizing pulse shaper <NUM> and its software readable media.

The pulse shaping mechanism may be realized by two different architectures: open loop control and adaptive control. In the open loop configuration, the desired pulse shape at the output of the unit is introduced by the end user through the user interface. Considering that the input pulse has known characteristics, the desired transfer function is known. It would not be difficult for one of ordinary skill to program the SLM using computer <NUM> (<FIG>) to provide the desired transfer function if the pulse shaper is precisely calibrated, i.e. the wavelength/pixel and voltage/phase relationship for each pixel are determined and stored in the memory of computer <NUM>. With this approach, in addition to TL pulses, other different pulse shapes including triangular, rectangular, parabolic or any other pulse shape can be formed and then stored in the library of computer <NUM>.

<FIG> illustrate an exemplary method of tuning the disclosed laser unit so that it could meet the customer's requirements to the duration, shape and/or amplitude of the compressed ultrashort pulse. Specifically, the ultimate goal of this method is to provide a customer with freedom to obtain ultrashort pulses of the desired duration, shape and/or amplitude by controllably manipulating a phase and/or amplitude in the disclosed pulse configurable laser unit. For example, in a TL pulse, all the frequencies are locked, and have net zero impedance. In this case, the spectral phase <NUM> of <FIG> is practically horizontal or flat. Generally, a pulse shaper can be defined by a number of parameters, such as the input bandwidth, frequency resolution, maximum delay and others all stored in the memory of computer <NUM>. Additionally, the wavelength to pixel and voltage to phase correspondence are also stored in the memory of computer <NUM> of <FIG>.

The tuning of the disclosed laser unit is based on the fact that a pulse with a close to Gaussian shape, as shown in <FIG>, is output by ring resonator <NUM> and further guided through the schematic of <FIG> which has the desired length of the dispersive media. The latter includes all of the components of pulse generator <NUM> and delivery fibers guiding the broadband coherent chirped pulses to the two-stage compressor. If the schematic of <FIG> is used, then the dispersive media also includes all fibers and components of the linearly configured amplifying cascade <NUM>. All of the fibers - passive and active - are standard fibers used by the manufacturer of the disclosed unit.

Determination of a chirp for pulse generator <NUM>, operating in a given regime, i.e., pump power and overlap between filters, is an initial step of tuning the laser unit to be shipped. Once the phase is determined for each pixel, the value with the opposite sign, which nullifies the determined chirp, is stored in the memory of computer <NUM>.

The MIIPS technique uses pulse shaper <NUM> to place a series of reference phase patterns <NUM> on a chirped laser pulse and then monitors the spectrum of the second harmonic generation (SHG) response from these reference phase patterns (chirps). This is done to calculate the phase shape of the pulse as a function of wavelength and apply the necessary phase pattern to cancel the chirp of the input beam so as to output a transform limited pulse.

It is possible thus to form a library stored in the memory of computer <NUM> and including a plurality of different phase and, if necessary, amplitude masks <NUM> which correspond to certain pulse parameters and thus are configured to compensate the dispersion of the pulse. If the user using an interface requires a new pulse generator regime characterized by a new pump power and filter overlap, computer <NUM> generates a controlling signal providing the desired change of the laser parameters. The memory of computer <NUM> also has a software operative to retrieve the desired mask corresponding to a new operating regime and apply it to pulses at the output of the pulse generator so as to receive a TL pulse.

While the above-discussed example relates to TL pulses, pulse shaper <NUM> can provide other pulse shapes including regular and irregular shapes. The regular pulse shape may, for example, be a flat top pulse, elliptical, train of pulses with a predetermined duty cycle and etc. The irregular pulse shape may include, for example, a two peak pulse having peaks with different amplitudes. Such an automatic correspondence between controlled pulse parameters at the output of the pulse generator, which are stored in the memory of the computer, and corresponding masks, which are also stored in the memory, substantially facilitates the operation and eliminates the need for highly specialized operators. The more complicated pulse shapes at the output of the disclosed pulse shaper may require the application of both phase and amplitude masks to pulses at the output of the pulse generator. For example, if instead of a TL pulse, a simple flat top pulse of the desired duration is needed at the output of the inventive unit, it can be obtained by applying a different phase mask to increase only a liner chirp. However, if one of or both leading and trailing edges of top-flat pulse are required to have a certain angle and curvature, the use of both phase and amplitude masks likely will be needed.

The computer <NUM> may also have a software operative to help the user knowing only the shape of the desired pulse, but clueless regarding a concrete mask which would provide the pulse with the desired shape in a frequency domain. Using the iterative method, the desired mask can be determined and stored in the library of computer <NUM>.

Returning to <FIG>, the initial goal is to compress a broadband pulse <NUM> at the output of pulse generator <NUM> of respective <FIG> and <FIG> to its near TL shape. By simply applying different chirps <NUM> via the interface, the operator can determine which of the chirps provides pulse <NUM> at the input of shaper <NUM> with the desired flat spectral line and minimal modulation, as shown in respective <FIG>. Once the mask is determined, pulses <NUM> can be compressed to a nearly TL pulse. In the example, a normal chirp shown in red in <FIG> is optimal for obtaining a near transform-limited pulse. The measurements shown in <FIG> are made at different locations upstream from the input into the compressor of <FIG>.

While the desired characteristics of pulse generator <NUM> of <FIG> advantageously, but not necessarily, remain constant to provide a reliable self-start, the configuration of <FIG> obviously may be easily varied so as to have different operational regimes corresponding to different pump powers and mutual position of filters. Each of these regimes and corresponding phase masks, which provide respective nearly TL pulses, also can be stored in the library. It is not unusual, when a nearly TL pulse (within <NUM>% of the theoretical TL pulse) has parasitic low-intensity peaks that can be minimized or completely removed by applying amplitude masks to the nearly formed TL pulse. These masks can be stored in the library.

Typically, the laser system including programmable pulse shaper <NUM> of <FIG> is configured with two processors or computers controlling respective pulse generator and programmable shaper. Adding a full size computer, such as computer <NUM> in <FIG>, that can simultaneously control pulse generator <NUM> and two-stage pulse compressor <NUM>, <NUM> renders the disclosed unit more flexible and the laser system more efficient.

Typically, if not for a preset including the desired phase applied to the peripheral segments in the above example, a team of specialists including individuals with PhD in physics and computer science in addition to other specialists in the art of the unit's application would be needed to achieve the desired result. Thus, the disclosed unit with a database of stored chirps, which correspond to different phase and amplitude masks, offers a unique opportunity for the customer to efficiently perform a great variety of tasks. Furthermore, the phase mask is device specific which necessitates tuning of each new unit at the manufacturing facility which certainly saves the customer a good fortune. Yet if the customer needs a pulse shape that is not part of the stored information, it is always possible to manipulate the phase and/or amplitude modulating thus a pulse shape, as discussed below.

With the inventive system, it is straightforward to use disclosed shaper to alter the spectral phase to the received chirped pulse and thereby, generate the desired optical waveforms or pulse shapes. The simplest example, as shown in <FIG>, is to add a linear chirp (apply a parabolic spectral phase mask) in order to adjust the pulse duration or even the arrival time for the red- and blue- parts of the spectrum. <FIG> illustrates measured autocorrelation function (ACF) profiles for several applied chirp values, targeting <NUM> fs, <NUM> fs, and <NUM> fs FWHM pulse duration if estimated from ACF width using sech2 fit. The corresponding SOD values, used to generate the phase masks for the shaper, are <NUM> fs<NUM>, <NUM> fs<NUM>, and <NUM> fs<NUM> at <NUM>, respectively. For the latter, <FIG> shows the corresponding parabolic phase profile, added to the dispersion compensation mask. The expected time-domain intensity profile of the pulse is provided in <FIG>. Note that either positive or negative chirp can be applied, and the SOD magnitude can be as high as <NUM> fs<NUM>.

Another example is the use of sinusoidal phase modulation for reshaping of the TL pulse into a burst of sub-pulses. The number of sub-pulses and their relative magnitude strongly depend on the phase modulation amplitude. Their time period (spacing) is equal to the spectral phase modulation "frequency" (measured in ps), which can be continuously tuned. For the pulse burst in <FIG>, the modulation amplitude is set to <NUM>. 45π, which leads to a pulse sequence with three main sub-pulses of similar magnitude. The calculated phase mask in <FIG> and expected time-domain intensity profile in <FIG> are given for <NUM> ps spacing between pulses. The latter can be adjusted continuously, with a high precision, by simply tuning the modulation frequency of the sinusoidal phase mask. <FIG> shows experimental ACF traces for <NUM> ps, <NUM> ps, and <NUM> ps spacing between pulses. They match simulated ACF profiles for the corresponding parameters (not shown).

In other words, <FIG> illustrate an exemplary method of breaking the TL pulse of <FIG> into a burst of sub-pulses of <FIG> by applying a sinusoidal phase mask of <FIG>. Changing the parameters of the sinusoidal phase mask, the number of sub-pulses, each of which is closed to a TL pulse, and time delay between the adjacent sub-pulses can be controllably varied.

Phase shaping is an attractive, lossless way to bring selectivity into nonlinear optical processes. It can be illustrated on the second harmonic generation (SHG) process as shown in <FIG>. When ultrashort laser pulses are compressed down to a TL pulse and focused on a thin nonlinear crystal, the dominant contribution into the bell-shaped SHG spectrum cornes from sum-frequency generation (SPG) across the laser spectral bandwidth. Laser photons at frequencies ω+Δ and ω-Δ, where Δ is an arbitrary detuning, add up to produce SHG photons at 2ω and interfere constructively with other contributions because their nonlinear phase φ(ω+Δ)+φ(ω-Δ) is independent of detuning Δ; here, φ(ω) is the spectral phase of the laser pulse. Then, if a phase mask with a proper symmetry is applied, such as, e.g., φ(ω) ∝ (ω-ω<NUM>)<NUM>, constructive interference is preserved only for ω<NUM>, corresponding to frequency 2ω<NUM> in the SEIG spectrum.

The latter can be understood by recalling that third order dispersion (TOD) corresponds to a parabolic group delay (in the frequency domain). Spectral bands at frequencies symmetric relative to ω<NUM>, i.e., at ω<NUM>+Δ and ω<NUM>-Δ, are equally delayed and therefore, continue contributing to the signal at 2ω<NUM> frequency through the SFG process. For other SHG frequencies the timing between paired spectral bands drifts suppressing their contribution into the nonlinear signal.

Experimentally, one observes a narrowed SHG spike when a cubic phase is applied. If there are no other phase distortions, the SHG peak is closely tracking the point of symmetry for the cubic phase. That is if the phase mask is centered at <NUM>, as in <FIG>, the SHG peak is expected to be at <NUM>. By changing the center frequency/wavelength of the cubic phase mask, one effectively tunes the two-photon excitation band. Note that the strength of the two-photon peak when TOD is applied is similar to what one gets using compressed pulses; i.e., the entire spectral bandwidth of the shaped pulse contributes towards the two-photon transition at 2ω<NUM>.

Thus, <FIG> illustrate another example of reshaping the TL pulse by applying a nonlinear phase mask having a relatively flat central region and two curved end regions. The resulting pulse is extended in time, as shown in <FIG> and, in addition, the pulse is modulated having a series of peaks with the leading peak having a highest intensity, as seen in <FIG>.

Such a reshaping may be advantageously used in multiple applications in chemistry, biology and medicine when coloring a substance, molecules and etc., by markers or luminophores. Different markers can be excited in response to respective different wavelengths. For example, searching for the pathological formations, it is known that the markers of one color is attached to a healthy tissue, whereas the markers of a different color are in contact with the pathological tissue.

The disclosed unit may operate generating low energy fs pulses to avoid damage to any tissue. The fs pulses are characterized by a broad spectral width with the fs pulse each having about <NUM> width. As known to one of ordinary skill, the entire visible diapason is less than <NUM> with the clearly visible spectral region not exceeding <NUM>. Thus an individual fs pulse can excite the markers of different colors corresponding to respective different tissues which, of course, complicates the discrimination process and affects the diagnosis.

The disclosed laser unit, however, solves this problem by applying such a mask that can selectively excite a very narrow spectral region. If the phase mask applied to the pulse in <FIG> was flat, the SHG would have a single continuous spectral range between <NUM> to about <NUM>, as shown in <FIG> by the dot line. But since the disclosed unit can controllably change the phase mask from, for example, the flat one to the one shown in <FIG>, the same very spectrum would have individual narrow SHG spectral regions illustrated by respective peaks of different colors in <FIG>. Thus, despite the great number of spectrally close markers, it is possible to selectively or consecutively excite one marker after another while analyzing individual response signal intensities until the marker corresponding to the tissue to be searched is identified.

<FIG> illustrates the above disclosed method of tuning and operating of inventive unit <NUM> of <FIG>. The laser source <NUM> operates in different operational regimes depending on the intensity of pump light coupled into a gain medium. The parameters of respective operational regimes are stored in a compartmentalized memory storage of computer <NUM>.

The generated pulses each while propagating through a dispersive medium, i.e., all fibers and fiber elements constituting laser source <NUM>, acquires a large group delay dispersion or giant chirp. The giant chirp is regime specific. The giant chirp includes a linear component corresponding to the SOD and nonlinear component corresponding to TOD or higher orders dispersion. Both chirp components should be corrected in order to form a TL pulse. The active phase control realizes the correction of the components.

The <NUM>-stage pulse compressor realizing the phase control includes static compressor <NUM> and programmable pulse shaper <NUM> which are operative to correct respective linear and nonlinear chirp components. A resulting pulse is nearly a TL. However, its waveform is not ideal exhibiting parasitic intensity peaks which can be suppressed by applying an amplitude mask to the nearly TL pulse which may be sufficient to obtain the desired TL pulse. The phase and amplitude masks corresponding to respective operational regimes of the laser source form the library stored in the compartmentalized memory of the computer.

Upon obtaining the desired TL pulse for respective regimes, their shape can be altered to generate other optical waveforms corresponding to respective operational regimes. This is realized by applying phase or phase/amplitude masks to the TL pulse. Once the desired waveform is formed, corresponding masks are stored in the memory of the computer.

Claim 1:
A pulse configurable laser unit, PCLU, (<NUM>) outputting uniform ultrashort pulses, comprising:
a fiber pulse generator (<NUM>) operating in a plurality of operational regimes to output a train of coherent uniform amplified giant chirped broadband pulses along a light path,
wherein the fiber pulse generator (<NUM>) is configured with a self-starting passively mode-locked fiber seed having two identical groups of optical elements which define a ring resonator,
wherein the optical elements of each group include an Yb doped active fiber, fiber coil (<NUM>, <NUM>) and narrow line filter (<NUM>, <NUM>) the narrow line filters of respective groups being configured to have respective spectra overlapping one another, the elements being optically coupled to one another by a plurality of passive fibers.;
a two-stage compressor including
a static compressor (<NUM>), which is configured to compensate for a linear chirp component of each giant chirped broadband pulse, and
a pulse shaper (<NUM>) which is provided with a programmable spatial light modulator, SLM, to correct for a non-linear chirp component of the giant chirped broadband pulse,
wherein the two-stage compressor outputs a train of transform limited, TL, ultrashort coherent pulses in each of the operational regimes of the fiber pulse laser generator; and
one or more computers (<NUM>) configured for executing a software for selectively retrieving a phase mask corresponding to a given operational regime from a library of phase masks, which is stored in a memory of the CPU, and operating the pulse shaper (<NUM>) to apply the retrieved mask across to the programmable SLM of the pulse shaper (<NUM>) so as to compensate for the nonlinear chirp component of the chirped pulse of the compressed TL ultrashort coherent pulses.