Passively mode-locked broad-area semiconductor laser generating ultrashort pulses of high energy and high brightness

Abstract

A mode-locked semiconductor laser oscillator configured to emit laser pulses. The oscillator comprises a large area diode supporting multiple spatial modes. The diode is configured to emit a multi-mode light beam. The oscillator also comprises a multi-mode saturable absorber configured to absorb a portion of the multi-mode light beam and to convert the multi-mode light beam into a train of ultrashort optical pulses. The oscillator also comprises an external laser cavity surrounding the diode and the saturable absorber, the external laser cavity is configured to filter the multi- mode light beam such that only a single-mode light beam is reflected back towards the multi-mode diode and allowed to oscillate in the external laser cavity. The laser pulses are emitted via the external laser cavity.

Description

PASSIVELY MODE-LOCKED BROAD-AREA SEMICONDUCTOR LASERGENERATING ULTRASHORT PULSES OF HIGH ENERGY AND HIGH BRIGHTNESSFIELD OF THE INVENTION

[0001] The subject matter described herein generally relates to laser diodes configured to output pulses.BACKGROUND

[0002] Semiconductor lasers represent the industry standard for sources of coherent light due to their high electrical-to-optical efficiency, simple and robust construction, high optical power, tunability, wide range of available wavelengths (covering the entire VIS-NIR range), and low cost. Thus, the generation of ultrashort optical pulses directly from semiconductor lasers with high pulse energy, high peak power and high spatial beam quality (brightness) is highly desired for many applications, such as: 3D micromachining and processing of materials (polymer / dielectric / metal), advanced methods of nonlinear or fluorescence microscopy, precision measurement and frequency metrology, frequency conversion and medical applications. However, the peak power of diode laser oscillators so far is not up to par with standard solid-state or fiber lasers, such as mode-locked Ti: Sapphire lasers or mode-locked Er / Yb fiber lasers. Thus, fiber / solid-state lasers are currently the prevalent solution in the market, despite their high complexity, limited wavelength availability, and higher cost.

[0003] Since the optical power of a laser diode is directly dictated by its physical area, broad area laser diodes (BAL) are an attractive route for high-power lasers competitive with solid-state lasers, while maintaining high customizability and simplicity - requiring only electrical pumping. Unfortunately, the high power from BAL diodes normally comes at the expense of degraded spatial beam quality and coherence, since the wide cross-section of the diode wave-guide is inherently multi-mode spatially along the slow axis of the BAL.SUMMARY

[0004] The subject matter discloses a mode-locked semiconductor laser oscillator that emits a few picosecond pulses (and possibly shorter) with record peak power (450W so far)and pulse energy (0.5nJ so far) directly out of the oscillator (with no amplifier). To achieve this high-power performance the subject matter discloses a high-current broad-area, spatially multimode diode amplifier, placed in an external cavity that enforces oscillation in a single spatial mode. Consequently, the brightness of the beam is near-ideal (M2 = 1.3). Mode locking is achieved by dividing the large diode chip (edge emitter) into two sections with independent electrical control - a gain section and a saturable absorber section. The gain section may be larger than the saturable absorber section. Both the gain section and the saturable absorber section are wide, and therefore spatially multi-mode, where the single-mode beam quality is achieved through the design of the external cavity and its interaction with both sections.

[0005] The gain of the chip is controlled by the forward current through the gain section, and mode locking is invoked and controlled by tuning the reverse voltage on the saturable absorber section. Precise tuning of the reverse voltage on the absorber section tunes the saturation level and recovery time of the absorber, providing a convenient knob to optimize the mode-locking performance for various cavity conditions.

[0006] The semiconductor laser comprises a broad diode defined as a multi-mode light emitting diode, and a saturable absorber located between the broad diode and a lens, such that the saturable absorber receives a multi-mode signal. The saturable absorber may be an integral part of the diode, or a separate part assembled at another location of the laser cavity.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Some embodiments are herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of embodiments. In this regard, the description taken with the drawings makes apparent to those skilled in the art how embodiments may be practiced.

[0008] In the drawings:

[0009] Figure 1 shows a laser semiconductor a linear cavity configured to output laser pulses, according to exemplary embodiments.

[0010] Figure 2 shows a laser semiconductor having a folded cavity configured to output laser pulses, according to exemplary embodiments.

[0011] Figure 3 shows a device for generating entangled photon pairs in a high-flux regime based on a monolithic crystal with dispersion compensation, either by mirror coatingsof inverse dispersion (a) or by a bonded slab of transparent optical medium of inverse dispersion (b), according to exemplary embodiments.

[0012] Figure 4 shows a device for the generation and measurement of quantum entanglement in a wide-band monolithic source, according to exemplary embodiments.

[0013] Figures 5A-5C show optional designs of external cavities to output laser pulses, according to exemplary embodiments.

[0014] Figure 6 is a flow chart describing one embodiment of a method for outputting laser pulses.

[0015] Figure 7 is a flow chart describing one embodiment of a method for generating high-energy pulses in the UV by seeding an Excimer amplifier with pulses from the mode- locked diode-laser system.DETAILED DESCRIPTION

[0016] In one embodiment, the technical challenge is to output laser pulses of relatively high pulse energy (>0.1 nano-Joul), having a relatively short pulse duration (<10 picoseconds or shorter) and relatively high average power (>100mW), while the light beam is of sufficient spatial quality, keeping a significant part of the beam’s energy and power.

[0017] In some cases, the high-energy pulses output from the device of the subject matter are achieved with high spatial coherence. Specifically, M2= 1.2 — 1.3 is achieved for all the observed repetition rates (harmonics of the fundamental cavity repetition), indicating a near-ideal spatial mode with high brightness and focusability.

[0018] In general, high-output power and energy require high electric current through the diode. Since the electrical current-density through the device is limited by the damage threshold of the diode, larger current requires diodes with larger physical area, i.e. longer or wider (or both) than standard single-mode diodes. The length of the diode however is limited to a few millimeters by the self-lasing threshold - since the gain of a diode amplifier is directly proportional to its length, too-long devices will have too-high gain in a single-pass, to the point that the emitted power saturates the gain already in a single-pass (a condition termed “self lasing”). A self-lasing amplifier does not require a cavity to lase and is therefore totally uncontrollable within a cavity, as it is guided by its internal lasing and does not respond to external feedback. Thus, to generate an amplifier that will emit high-power without self-lasing, one must have the amplifier transversely wide. Inevitably, wide devices cannot be spatiallysingle-mode (by definition, a wide device supports more than one transverse mode, typically a few tens of micrometers wide, or wider), which dramatically reduces the beam quality of the laser. As a result, standard broad-area laser diodes (BAL) of high power have poor beam quality and low brightness along the slow axis.

[0019] The device and methods disclosed herein overcome this power-brightness compromise and output laser beams that are simultaneously high power / energy and high brightness. The subject matter discloses placing a broad-area, multimode diode amplifier in an external cavity that enforces a single spatial mode, while keeping most of the emitted power. Broad-area diodes can be categorized roughly into two classes based on their physical dimensions: long and narrow vs. short and wide. Long diodes offer significant gain in a single pass (10-30dB or more), which is very convenient to operate, forming the standard industry choice due to their high gain and power. However, long diodes are sensitive to self and parasitic lasing, especially at high currents, reducing their effectiveness in an external cavity, especially with high current and power (e.g. when the gain is >20dB, any feedback, even lower than 1%, from a parasitic reflection may cause involuntary lasing). The subject matter therefore may utilize also shorter yet wider devices of equal area (less than 2mm length), that can carry the same current, but have lower single-pass gain (less than lOdB). Such short and wide devices are widely ignored by the industry today since they appear to have inferior gain and beam quality compared to long and narrow. In an external cavity, however, lower gain devices (of equal current and power) are much more controllable and effective, being insensitive to parasitic lasing from weak internal reflections. Indeed, short-wide diodes are inevitably more spatially multi-mode, which indicates that achieving good beam-quality (single-mode) requires effective and efficient spatial filtering within the external cavity, as explained hereon. Thus, the selection of the length-width dimensions of the diode is guided by the considerations of gain and beam quality. Both “long / narrow” and “short / wide” diodes may be useful for this matter.

[0020] The technical solution of the subj ect matter is a device configured to output laser pulses. The device comprises a multi-mode gain diode and a multi-mode saturable absorber diode (either configured together as a single device with two sections or as two devices located next to each other), such that the absorber absorbs some of the emitted light. The light then passes through a set of lenses and mirrors (and possibly additional elements) that form the external cavity of the laser, whose functionality is to ensure that the fundamental spatial mode of the external laser cavity has good spatial overlap with the gain medium and to act as a spatial filter to resonate only the fundamental mode. In some cases, the device of the subject matterutilizes soft filtering techniques, in which the gain diode acts as its own spatial filter, automatically eliminating any possible mismatch. The soft filtering techniques have been demonstrated to achieve both high-brightness (M2 = 1.3 @ 90% power) and high-power (1.5W) in a single mode CW beam.

[0021] For mode locking, this cavity design is very instrumental as well, as it allows to achieve record results for a single diode oscillator, delivering 0.55nJ pulse-energy at a repetition rate of 380MHz and 450W peak-power together with a 1 ,4ps pulse duration, all in a highly pure spatial mode (M2 = 1.3). This was achieved directly out of the oscillator (no amplification) in a design that uses no special fabrication with only off-the-shelf components.

[0022] The saturable absorber (SA) is configured to convert the continuous wave output into a train of ultrashort optical pulses. The saturable absorber is a passive optical component that exhibits variable absorption properties depending on the intensity of incident light. At low light intensities, the saturable absorber absorbs a fraction of the light intensity, which introduces losses into the laser cavity and hampers the laser’s operation, but as the intensity increases, the saturable absorber’s absorption decreases due to saturation, which is a nonlinear optical effect that occurs when the intensity of the beam is sufficient to excite a considerable fraction of absorbers in the medium, thereby reducing the number of available absorbers (and absorption). The reduction of loss at high intensities drives the laser to concentrate the emitted energy in short intense pulses, where the duration of the pulse is dictated by the absorption lifetime of the excited state and by the gain dynamics. The saturable absorber may be defined as a medium with a limited and controlled density of absorbers and can be embodied in a wide variety of options, where most options include some host matrix medium with a dopant absorber of controlled density. However, in the case of diode lasers, some exemplary embodiments use an unpumped section of diode itself (with no forward current). When a diode section is not pumped, it induces loss on the passing light (instead of gain). Furthermore, the saturation level of the absorbing diode can be controlled in real time by applying a reverse voltage, which effectively increases the saturation level, thereby driving the laser towards short pulses of high-intensity.

[0023] In some embodiments of the saturable absorber, the absorber is multimode in space (as part of the wide, multimode gain chip) and experiences the wide optical mode in the gain medium. This configuration counters prior art approaches, in which saturable absorbers are preferably placed at a tight focus of the external laser cavity (which normally coincides with the Fourier plane of the gain medium) to minimize the saturation power. Also, when positioned at a Fourier plane, the saturable absorber itself can act as a spatial filter that selectsa tightly focused mode. In contrast to prior art approaches, placing the saturable absorber at the plane of the wide gain medium resulted in record results that combine beam quality and power of the laser pulses. This apparent discrepancy has two reasons: First, the wide cross-section of the saturable absorber indeed pushes the saturation power up, which in turn pushes the pulse energy and peak power up. Second, and most important, the saturable absorber in this wide configuration affects not only the temporal profile of the pulse, but also the spatial profile like an effective Kerr-lens. Apparently, the refractive index of the saturable absorber medium varies transiently with the light pulse due to the transient variation in carriers density within the semiconductor that occurs upon the absorption of the leading edge of the pulse. This effective Kerr-lens, which is unique to a multimode saturable absorber, can be powerful in an external cavity to control and guide the mode-locking process and outcome.

[0024] In general, each arm of the external cavity contains a minimum of two elements: a fast lens and an end-mirror. The fast lens is placed near the diode with a sufficient numerical aperture (usually short focus smaller than 10mm) to collect the wide-angle emission of the fast axis, collimating or focusing the beam along the fast axis onto the end mirror. An additional slow lens of longer focus can be placed between the fast lens and the end-mirror to operate primarily on the slow axis (its operation varies between the hard and soft filtering configurations). At least one of the lenses is cylindrical along one axis, while the other can be either cylindrical along the other axis or spherical. The lenses can be replaced by other focusing optical elements, such as spherical / parabolic mirrors, diffractive lenses, aspheric lenses, etc. The external laser cavity may contain additional optical elements, such as folding mirrors, imaging arrangements, polarization control elements, output coupling, etc.

[0025] Figure 1 shows a diode laser in a linear cavity configured to output laser pulses, according to exemplary embodiments of the technology described herein. The laser cavity comprises a laser diode having a gain component 140 and a saturable absorber 150. The gain component 140 is configured to amplify the light passing through it by stimulated emission in phase with the incident light.

[0026] The external cavity comprises two collimating lenses 130 and 132 configured to convert the diverging beam emitted by the laser diode into a collimated (parallel) beam with minimal divergence. The laser cavity also comprises two focusing lenses 120 and 122 configured to focus the laser beam to a small spot size at a specific distance from the lens. None of the collimating lenses 130 and 132 and the focusing lenses 120, and 122 are located between the gain component 140 and the saturable absorber 150. The collimating lenses 130 and 132 are located closer to the gain component 140 and the saturable absorber 150 than the focusinglenses 120 and 122. The laser also comprises an output coupler 160 which is configured to allow a portion of the laser light to exit the external laser cavity and form the output beam.

[0027] In some exemplary embodiments of the subject matter, the gain diode is designed and / or manufactured to be “angle-cut”, where the entrance and exit facets of the diode form an angle relative to the beam. An angle cut suppresses self-lasing by deflecting the parasitic reflections of the diode-facets and preventing them from returning into the diode, which can cause self-lasing. Specifically, the angle 0 between the facet and the beam is selected such that the residual reflection from the exit facet will be deflected to miss the entrance facet, such that tan 0 > where w, I are the width and the length of the diode waveguide, respectively. In a specific embodiment this “angle-cut” is achieved using a 5mm long diode that is cut in the shape of a parallelogram, whose long axis is placed at an angle of 3. 6° relative to the beam line and whose entrance and exit facets form an angle of 13. 7° to the laser beam in the external laser cavity (to ensure refraction at 3. 6° internally in the diode).

[0028] The diode chip is divided to two sections with independent electrical control: A longer section of the diode is operated with forward current and acts as a gain medium, while a shorter section is driven in reverse voltage and acts as a variable saturable absorber. The diode facets may be AR-coated (to have a reflection lower than 0.2%). The diode facet may be anglecut to effectively mitigate self lasing. The spherical lens and the cylindrical lens are configured to stabilize the fast and the slow axes of the beam, while one arm also contains an output coupler (OC) that couples some of the external laser cavity light out, forming the laser beam. The output coupler may be variable (V.O.C), composed of a polarizing beam-splitter and a wave plate (either quarter-wave plate if the cavity configuration is linear or half-wave plate in a ring cavity configuration), allowing to tune the amount out out-coupled power in order to optimize the emitted energy and mode-locking performance.

[0029] The gain component 140 is driven in forward current to provide amplification, whereas the absorber section 150 is driven in reverse voltage to tune the absorber’s saturation level to induce passive mode-locking and optimize the absorber’s performance. This diode chip may be placed in the external cavity configuration that comprises two identical cavity arms around the diode chip. The length of the arms (« 400mm in this realization) acts as a soft spatial filter that enforces oscillation in a single spatial mode. One arm also includes an output coupler (OC) that couples some of the external laser cavity light out, forming the laser beam. The output coupler may be a variable output coupler 160, implemented by a polarizing beamsplitter and a rotating quarter-wave plate for optimizing the output power and mode-lockingperformance. To stabilize the external laser cavity for both the fast and slow axes, the diode facet is located slightly before the focal plane of a spherical "fast" lens (fs= 9mm), thereby imaging the diode facet in the fast axis onto the end mirror of the external laser cavity (at 400mm distance). An additional cylindrical "slow" lens of a longer focus (fc= 75mm) is placed to form a telescope for the slow axis (together with the spherical lens). This arrangement stabilizes the slow-axis mode of the external laser cavity and enforces single mode operation.

[0030] The pulses output from the device of the subject matter are characterized with energies that surpass previously published work by 1-2 orders of magnitude, while the peak power of these laser pulses is 1-2 orders of magnitude higher than all ps-range sources, comparable only to fs-range oscillators that have much lower pulse energy.

[0031] Figure 2 shows a laser semiconductor having a folded cavity, where the external cavity has only one arm, configured to output laser pulses, according to exemplary embodiments. In the folded cavity design, the laser diode comprises a high reflective (HR) component 210 physically touching or relatively close to the gain component 220. In some exemplary embodiments of the subject matter, there are no lenses or other components configured to deflect light between the gain component 220 and the HR component 210.

[0032] In the folded cavity design, the laser semiconductor comprises a saturable absorber 230 located near the gain component 220. The laser cavity comprises a single set of lenses, collimating lens 240 and focusing lens 250 located between the saturable absorber 230 and the output coupler 260.

[0033] The device of the subject matter implements both the standard slit-based hard filter and modified cavity design-based soft filter, as schematically shown in Figure 5 A. In both approaches, the fast axis of the diode is inherently single-mode due to the diode structure. The slow axis, however, is generally multi-mode. The multi-mode operation can be described as an incoherent sum of plane waves that propagate in different directions out of the diode and are focused onto different points on the end mirror.

[0034] In the hard filtering approach, the slow lens forms a Fourier transform of the diode aperture on the end mirror, where a slit is located for spatial filtering. The slit allows only a narrow range of plane waves to be reflected back into the external laser cavity, thereby enforcing a single mode with a wide profile at the diode facet. The magnification ratio of the fast and slow lenses can be chosen to compensate for spatial astigmatism. While figures 1-2 outline cavities in a linear configuration, it should be clear that embodiments include ring cavity configurations.

[0035] Figure 3 shows a device for generating entangled photon pairs in a high-flux regime based on a monolithic crystal with integral dispersion compensation to ensure a broad optical bandwidth of phase matching. Dispersion compensation can be achieved either by optical coatings of inverse dispersion (negative Group delay dispersion (GDD) ) on the endfacets of the nonlinear crystal (a), or by bonding a slab of transparent optical medium of inverse GDD to the nonlinear crystal (b), according to exemplary embodiments. The compensation slab 115 may be formed of any optical medium, whose dispersion sign is opposite to that of the non-linear crystal, and its length is chosen such that the two dispersions will substantially or exactly cancel (the crystal and the slab). The compensation slab 115 may be made of BK7 glass, fused silica, or other glass types having a dispersion of the opposite sign to that of the non-linear crystal.

[0036] The compensation slab 115 enables utilization of the full spectrum of phase matching in the crystal 130 and ensures the widest optical spectral bandwidth, allowed by the compensation of the dispersion within the monolithic source (>5THz optical bandwidth). Another possibility to obtain dispersion compensation in a monolithic source is by using a dielectric mirror coating with inverse dispersion on the end mirrors. In this case, the coating type may be a GTI - Gires-Tournois Interferometer that can be implemented at any wavelength and shortens the cavity length, simplifying mechanical implementation and ensuring minimal internal losses.

[0037] Dispersion compensation in a monolithic source can be achieved by bonding the compensation slab 115 with inverse material dispersion and suitable thickness. This option reduces the optical coating requirements.

[0038] Characterizing the actual performance is essential for any light source. Specifically, for a wide-band source of entangled photons, a wide-band measurement of the entanglement / squeezing level and the purity of the entangled quantum state is required. Such measurement is important not only during development stages but also for the end-user to monitor the source and detect faults if deviations occur. The optimal measurement for this purpose is parametric homodyne detection, which measures the entanglement and the induced squeezing simultaneously across the entire optical bandwidth (conceived and demonstrated by an article at Nature Communications, 609 (2018)). In particular, parametric homodyne sends the entangled pairs generated in the source under test into an identical source, and the intensity of the pairs (or spectrum) is measured at the output of the second source. Here, photon pairs from the first source can be either amplified or annihilated in the second source, depending on the relative phase between the pump and the photon pairs, thereby forming a quantum nonlinearinterferometer of photon pairs, known as SU1,1. The contrast of the observed interference is the "quantum witness" of entanglement or squeezing (as explained in the above-identified article). Specifically, when the sources are identical and the entanglement is ideal, the contrast of interference is 100%.

[0039] The challenge in measuring the photons is how to generate an identical source. The main difficulty arises from the fact that the proposed source is a monolithic cavity that generates a comb structure of frequency modes, where for a good measurement it is necessary that both the measured and the measuring sources will have perfectly matching comb structures; i.e. precise interferometric alignment of the cavity lengths and phases is required. The solution of the subject matter is to use the tested source as the measuring device, thus ensuring automatic alignment between the measured and the measuring sources (as they are the same source). Fortunately, in the proposed monolithic crystal, this is possible if the cavity is arranged to be a ring cavity. Specifically, entangled pairs are generated in a ring cavity only in the direction of the pump (say clockwise), indicating that the opposite direction (counterclockwise) can be used for measurement. In fact, when the emitted beams (pairs and pump) are reflected into a ring cavity, the pairs and pump automatically couple in the opposite direction, thereby forming the optimal measurement process.

[0040] In some embodiments, the pump laser beam enters the crystal 130, whose facets 131, 132 are polished and coated with low-loss reflective coatings for the PDC light to form the cavity (a high reflector on side 131 and a partial reflector on side 132). To maximize the generated bandwidth, the cavity mirrors may also be designed to compensate for the crystal dispersion. The resulting output is a high-flux beam of entangled photon pairs, with a comb- spectral structure of discrete resonant frequencies. Dispersion compensation enables high conversion efficiency across the entire optical spectrum while maintaining phase matching. For example, the two photons have frequencies that sum to the pump-frequency. The returning pump beam serves as feedback for locking the laser frequency to the monolith, ensuring low noise.

[0041] In some embodiments, the system comprises an isolator 135 and locking electronics 145 configured to actively adjust the pump laser frequency to match the resonance conditions in the monolithic crystal 130. The need for locking depends on the passive stability of the laser frequency (the monolithic cavity is inherently stable due to its construction with only slow drifts that can be corrected by temperature tuning).

[0042] Figure 4 shows a device in which the pump laser 140 is tilted relative to the main axis of the crystal 130. Measuring is achieved by tilting the pump beam at a predefinedangle and utilizing total internal reflection from the crystal facets 131 and 132. Since the crystal 130 is relatively narrow and long, the required tilt angle is small (in the range of 6-8 degrees in our realizations), so the phase matching conditions will remain almost identical and will only require minor adjustments in the crystal's operating temperature. Additionally, the confocal geometry of the cavity, which is central to the spatial stability, will also ensure that the slight astigmatism resulting from the tilt on the curved end mirror will hardly affect the spatial mode of the cavity.

[0043] The Optical parametric oscillator (OPO) beam of broadband squeezed light may be generated in a collinear configuration from the tilted pump laser 140, which enters the crystal 130 at an angle relative to the crystal’s elongated axis. Both the OPO and the pump beams is reflected in the bottom or top wall of the crystal. The pump beam exits through mirror 131 due to anti -refl ection for the pump wavelength and the OPO output beam exits at the facet 132 via partial reflection mirror. The OPO output beam (along with a residue of the pump) is reflected back from an external mirror 175 tilted at an angle that is perpendicular to the direction in which the laser beam exits the facet 132. The external mirror 175 results in the beam entering the facet 132 into the ring cavity at the same angle it exited the facet 132, thereby rotating in the ring-cavity in the opposite direction, back towards the facet 131, then to the upper wall of the crystal 130 and out of the facet 132 towards spectrometer 105. This backwards rotating OPO beam interacts with a backwards rotating pump that enters through mirror 131 (either the reflection of the original pump or a from a fresh source). A filter 160 is located between the facet 132 and the spectrometer 105, separating the pump from the pairs of photons. The pairs of photons continue to the spectrometer 105 while the pump moves towards phase control device 170.

[0044] In some exemplary cases, the preferred configuration for the crystal is monolithic due to the crystal’s geometry, which ensures stability and high resistance to manufacturing or alignment errors and allows efficient alignment of the pump laser without the need for full spatial mode overlap. This necessitates polishing the back facets 131, 132 of the crystal 130 to a spherical mirror finish with a focal point equal to the crystal’s length. Conversely, the crystal length determines the spectral spacing of the monolithic cavity's frequency-comb and the efficiency of pair generation. Considerations of manufacturing simplicity also have an impact.

[0045] In some exemplary cases, a monolithic crystal for frequency conversion requires coatings on the crystal (and other elements in the monolith, if applicable) that separate the response to photon pairs from the response to the pump laser. Specifically, the back mirrorneeds to be highly reflective (HR) for photon pairs but anti-reflective (AR) for the pump laser. The front mirror requires partial reflection (PR) for pairs and full reflection for the pump laser, and inner surfaces (if present) require AR coating for both waves.

[0046] Figures 5 A-5C show optional designs of external cavities to output laser pulses, according to exemplary embodiments. Figure 5A shows a single-mode fast axis, having a spherical lens placed near the diode that images the fast axis onto the end-mirror. Figure 5B shows a hard slit-filtering external cavity having Fourier transform formed by a spherical fast lens, imaged by slow lens onto the end mirror with slit. The slit is configured to filter only the desired single spatial mode. Figure 5C shows a soft-filtering, having diode’s aperture acting as spatial filter, enforced single-mode operation.

[0047] The soft filtering approach operates such that the slow axis is not focused on the end-mirror, but rather collimated. The slow lens is moved away from the focusing condition, either towards the fast lens (if the fast lens is spherical) or away from it (for a cylindrical fast lens), which gradually reduces the allowed beam divergence at the diode plane, until finally only the lowest divergence mode can survive (the fundamental “plane-wave” mode). The device operates in a plane configuration, employing the diode’s aperture as its own spatial filter; i.e. only one spatial mode can be transmitted through the diode with low loss, while other modes miss the diode and suffer loss (overlap at an insufficient level with the gain medium upon return). Specifically, the position of the slow lens in the external laser cavity may be shifted, such that the diode’s image acts as an entrance-pupil for the back-propagating beam from the high-reflecting mirror at the end of the cavity arm. Single mode operation is enforced when the two-way distance from the effective pupil to itself exceeds the Rayleigh range of a single mode beam, the single-mode beam’s waist matches the pupil. This condition is reached when the slow-lens and the fast-lens form an anamorphic telescope for the slow-axis. The filtering mechanisms operate therefore between two spatial stability limits - the hard slitfiltering in Figure 5B is equivalent to a concentric cavity and the soft filtering configuration of Figure 5C is equivalent to a planar cavity.

[0048] Figure 6 shows a method for outputting laser pulses, according to exemplary embodiments.

[0049] In operation 610, a cavity light beam may be amplified in a large area diode capable of supporting multiple spatial modes. The multi-mode operation can be described as an incoherent sum of plane waves that propagate in different directions out of the diode and are focused onto different points on the end mirror.

[0050] In operation 620, a portion of the multi-mode light beam may be absorbed at a multi-mode saturable absorber section to generate pulses. Embodiments disclose tuning the reverse voltage on the absorber to tune the saturation level and recovery time of the absorber. In some embodiments, the saturable absorber receives a multi-mode signal. In some embodiments, the saturable absorber is placed at the plane of the wide gain medium resulted in record results that combine beam quality and power of the laser pulses.

[0051] In operation 630, the multi-mode light beam may be filtered in external cavity surrounding the diode and the saturable absorber to favor single spatial mode. The filtering may be done by passing the light beam through a set of lenses and mirrors (and possibly additional elements) that form the external cavity of the laser. The filtering may be done using a hard slit-filtering external cavity having Fourier transform formed by a spherical fast lens, imaged by slow lens onto the end mirror with slit. The slit is configured to filter only the desired single spatial mode. The filtering may be done using a soft-filtering technique, having diode’s aperture acting as spatial filter, enforced single-mode operation.

[0052] In operation 640, the laser pulses may be emitted via the external laser cavity. In some embodiments, the laser pulses output with high spatial coherence. Specifically, the laser pulses may have spatial coherence in the range of M2= 1.2 — 1.3, indicating a near-ideal spatial mode with high brightness and focusability. In some embodiments, the laser pulses may be output via an output coupler 160 configured to allow a portion of the laser light to exit the external laser cavity and form the laser pulses. In some embodiments, the laser pulses have relatively high pulse energy (>0.1 nano-Joul), have a relatively short pulse duration (<10 picoseconds or shorter) and relatively high average power (>100mW).

[0053] In operation 650, a single-mode light beam may be reflected back towards the multi-mode diode to oscillate in the external laser cavity. The light beam may be reflected through a set of lenses and mirrors (and possibly additional elements) that form the external cavity of the laser. In some embodiments, a narrow range of plane waves is reflected back into the external laser cavity using a slit in the cavity, thereby enforcing a single mode with a wide profile at the diode facet. The diode is the gain medium that amplifies the light which passes through the diode both backward and forward. The term oscillation is very common in lasers to denote the operation of the laser above the threshold, where the laser generates coherent radiation through lasing. The Saturable absorber is within the cavity (just like the gain), so they both operate together when the pulse evolves in the cavity where in every round trip the light is shaped in time by the saturable absorber and in space by the spatial filtering of the external cavity.

[0054] Figure 7 is a flow chart describing one embodiment of shows a method for generating high-energy pulses at UV wavelengths by amplifying the output of the mode-locked diode laser (with appropriate frequency conversion) in an Excimer UV amplifier, according to exemplary embodiments. UV laser signals are used in ophthalmology surgeries and metallurgy processing, as well as in other technologies. Currently, there is a technical challenge to obtain short pulses of high temporal and spatial coherence at the UV range, as the existing sources of UV pulse signals have low beam quality and low temporal coherence / quality (Excimer lasers). However, Excimer amplifiers can easily amplify low-energy seed pulses to very high energy, while maintaining the coherence properties of the seed. Since the mode-locked diode-laser described above generates high-coherence pulses, it can be utilized to generate the needed seedpulses in UV, as outlined in figure 7.

[0055] In operation 710, a mode-locked diode-laser is arranged to emit light at a subharmonic of the desired UV wavelength for amplification in the excimer amplifier. For that purpose, the gain medium may be selected and the external cavity parameters tuned to emit the desired wavelength. The laser signal may be output in a wavelength according to properties of the diode. For example, a mode-locked diode laser will be arranged to emit at 702nm wavelength, whose second harmonic at 351nm can be amplified in a XeF excimer amplifier.

[0056] In operation 720, the output of the laser diode may be frequency harmonic generated to the excimer UV wavelength. The frequency harmonic generation may be a multiplication may be by a whole number, for example doubling or tripling the laser diode's output signal. The multiplication may be done to match the diode's signal properties to the properties of the desired signal. For example, the diode outputs signals of 702 nanometers and the output of the laser diode is doubled to 351 nanometers to fit the wavelength of the excimer.

[0057] In operation 730, the multiplied output of the laser diode may be inputted into the excimer amplifier. This way, the laser signal inherits the good quality of the laser diode and enjoys the amplified power of the excimer device.

[0058] In operation 740, the amplified UV signal is output from the excimer device. The excimer device may output the UV signal having energy in the range of milli-joule or more with high quality beam - spatial and temporal. The output may be transferred via a transparent wall in the body of the excimer device.

[0059] All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of anyreference in this application shall not be construed as an admission that such reference is available as prior art. To the extent that section headings are used, they should not be construed as necessarily limiting.

[0060] The foregoing detailed description has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit to the precise form disclosed. Many modifications and variations are possible in light of the above teaching (for example, all the embodiments above were considered in linear cavity configurations, but can be easily modified to ring-cavity, as standard in laser design). The described embodiments were chosen in order to best explain the principles of the proposed technology and its practical application, to thereby enable others skilled in the art to best utilize it in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope be defined by the claims appended hereto.

Claims

CLAIMS1. A mode-locked semiconductor laser oscillator configured to emit laser pulses, comprising: a large area diode configured to operate in multiple spatial modes, the diode is configured to emit a multi-mode light beam; a multi-mode saturable absorber configured to absorb a portion of the multi-mode light beam and to convert the multi-mode light beam into a train of ultrashort optical pulses; at least one mirror or lens that define an external laser cavity surrounding the diode and the saturable absorber, the external laser cavity is configured to filter the multi-mode light beam such that only a single-mode light beam is reflected back towards the multi-mode diode and allowed to oscillate in the external laser cavity; wherein the laser pulses are emitted via the external laser cavity.

2. The device of claim 1, wherein: the saturable absorber is an integral part of the multi-mode diode.

3. The device of claim 1, wherein: the external laser cavity comprises a slit located near a Fourier Plane of a gain medium, such that the slit filters the Fourier transform, allowing only a single spatial mode to oscillate in the external laser cavity.

4. The device of claim 1, wherein: the diode comprises a gain section driven in forward current, in order to provide amplification, whereas the absorber section is driven in reverse voltage to control the absorption lifetime and saturation level.

5. The device of claim 4, wherein: the gain section is larger than the saturable absorber section.

6. The device of claim 4, wherein: a gain of the oscillator is controlled by forward current through the gain section, and mode locking is invoked and controlled by tuning reverse voltage on the saturable absorber.

7. The device of claim 4, wherein: the external laser cavity comprises a plurality of collimating lenses and focusing lenses; wherein the collimating lenses are located closer to the gain section and the saturable absorber than the focusing lenses.

8. The device of claim 1, wherein: the gain diode is angle-cut, wherein the diode comprises an entrance facet and an exit facet; wherein the entrance facet and an exit facet form an angle other than 90 degrees relative to the single-mode light beam to prevent parasitic lensing from residual reflections.

9. The device of claim 8, wherein: at least one of the diode's facets include anti -reflective coating.

10. The device of claim 1, wherein: the external laser cavity comprises two identical cavity arms around the diode.

11. The device of claim 10, wherein: each arm of the external laser cavity comprises a fast lens and an end-mirror.

12. A method for emitting laser pulses, comprising: emitting a multi-mode light beam from a large area diode configured to operate in multiple spatial modes; absorbing a portion of the multi-mode light beam at a multi-mode saturable absorber; converting the multi-mode light beam into a train of ultrashort optical pulses at the multi-mode saturable absorber defined by a at least one mirror or lens; filtering the multi-mode light beam by an external laser cavity surrounding the diode and the saturable absorber; reflecting a single-mode light beam back towards the multi-mode diode to oscillate in the external laser cavity; and emitting the laser pulses via the external laser cavity.

13. The method of claim 12, further comprising:controlling a gain of the oscillator is controlled by forward current through a gain section of the large area diode, controlling mode locking by tuning reverse voltage on the saturable absorber.

14. A device configured to emit laser pulses, comprising: a large area diode configured to operate in multiple spatial modes, the diode is configured to emit a multi-mode light beam; a multi-mode saturable absorber configured to absorb a portion of the multi-mode light beam; at least one mirror or lens that define an external laser cavity surrounding the diode and the saturable absorber, the external laser cavity is configured to filter the multi-mode light beam such that only a single-mode light beam is reflected back towards the multi-mode diode and allowed to oscillate in the external laser cavity; wherein the laser pulses are emitted via the external laser cavity; wherein the large size of the diode and the high pumping current provide a sufficient level of saturation.

15. The device of claim 14, wherein: the saturable absorber is an integral part of the multi-mode diode.

16. The device of claim 14, wherein: the external cavity comprises a slit located near a Fourier Plane of a gain medium, such that the slit filters the Fourier transform, allowing only a single spatial mode to oscillate in the external laser cavity.

17. The device of claim 14, wherein: the diode comprises a gain section driven in forward current, in order to provide amplification, whereas the absorber section is driven in reverse voltage.

18. The device of claim 14, wherein: the gain diode is designed and / or manufactured to be “angle-cut”, where the entrance and exit facets of the diode form an angle relative to the beam.

19. The device of claim 14, wherein:at least one of the diode's facets include anti -reflective coating.

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