Narrowing a spectral linewidth of a high-power semiconductor laser diode

Abstract

Methodology for reducing the linewidth of a multimode diode laser across a wide range of operating wavelengths that maintains high output power is based on retroreflective feedback introduced by a combination of a dispersive prism and a thin optical etalon and that provides high degree of operational flexibility over the use of a volume Bragg grating. An external cavity created by the etalon introduces spectral selectivity with a period equal to the etalon's free spectral range and covers a spectral range including both visible and near-infrared regions. The spectral selectivity caused by the prism reduces the linewidth more than 10-fold to 100 pm and below for different etalon thicknesses and operating currents in the presence of temperature fluctuations.

Description

NARROWING A SPECTRAL LINEWIDTH OF A HIGH-POWER SEMICONDUCTOR LASER DIODECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of and priority from the U.S. Provisional Patent Application No. 63 / 622,316 filed on lanuary 18, 2024, the entire contents of which are incorporated by reference herein, for all purposes.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with government support under Grant Number 2116275 awarded by the National Science Foundation. The U.S. government has certain rights in the invention.TECHNICAL FIELD

[0003] The present invention relates to external cavity semiconductor laser diode systems and, more particularly, to such system configured to generate high-power light output possessing spectral linewidth below 100 pm and narrower for direct use in Raman gas spectroscopy.RELATED ART

[0004] Tailoring of spectral output characteristics of light produced by laser diode gain chips with the use of optical feedback has been utilized for quite some time. Typically, a frequency- selective element is positioned at the output of the laser diode (usually, separated from the laser diode by a collimation lens) to force the laser diode to operate in a narrowed range of frequencies. Optionally, the tuning of a center frequency can be implemented via mechanical and / or electrical adjustments. Depending on the particular implementation of the so-called external cavity, the reduction in laser linewidth can be substantial, sometimes down to a linewidth on the of order of MHz (-106nm) or below. Over the years, this capability has enabled substantial advances in various applications including tunable laser spectroscopy, holography, Raman spectroscopy, and metrology.

[0005] The operational limitations of the so-structured external cavity (EC) laser diode apparatus - and, in particular, the insufficient power of the produced optical output - are well recognized and arise in part due to the single-mode nature of the underlying laser diodes. Structures based on larger, multimode, wave-guided laser diodes can also be spectrally stabilized, but - when operated at high power (> 1 W) levels - require very specialized (uniquely designed to be used with a particular laser diode) components such as a volume Bragg gratings or tapered amplifiers, for example, to achieve any substantial reduction of the spectral linewidth. Yet, numerous applications such as Raman gas spectroscopy could benefit from high (multi-watt) power, low cost, and spectrally narrowed (~cnT1wide) output light from the laser diode (particularly in the blue spectral region because the Raman scattering cross section grows with the fourth power of frequency).SUMMARY OF THE INVENTION

[0006] An embodiment of the present invention provides a laser apparatus that includes a semiconductor laser diode configured to produce a spatially astigmatic light output, an optical system positioned to receive light generated by such laser diode and to transform this light output into a substantially collimated beam of light, and a spectral frequency-selective system. The spectra frequency-selective system is positioned to intersect light produced by the laser diode and to produce a light output of the apparatus, in transmission through the spectral frequency-selective system. The spectral frequency-selective system includes first and second substantially parallel to one another external dielectric interfaces defining an external cavity for the laser diode, and two intermediate dielectric interfaces (each of which is positioned between the first and second external dielectric interfaces and the optical system and inclined with respect to an axis of the substantially collimated beam of light). Optionally, in more than one implementation of the invention, the first and second external dielectric interfaces are uncoated surfaces of an optical plane-parallel plate that defined a Fabry -Perot etalon. The laser apparatus in configured to generate the light output having a linewidth narrower than 100 pm. In at least one specific case, each of the first and second external dielectric interfaces and each of the two intermediate dielectric interfaces separates an optically transparent material from air. Optionally, each of the first and second external dielectric interfacesmay be substantially perpendicular to the axis of the substantially collimated beam of light. Alternatively or in addition, first and second external dielectric interfaces may include only two external dielectric interfaces. Substantially in every implementation of the apparatus, the semiconductor laser diode may be structured as an in-plane multimode semiconductor laser diode. Optionally, a spectral dispersion plane of the spectral frequency-selective system is oriented with a fast axis of the laser diode. Alternatively or in addition, embodiments of the invention provide for use of any embodiment of the laser apparatus alluded to above for reducing a linewidth of a semiconductor laser diode by at least 10-fold (preferably, 15-fold, more preferably, 20-fold, and most preferably, 50-fold or more), and / or for use of such embodiment that includes tuning a central wavelength of operation of the laser apparatus with a spectral step substantially equal to a free spectral range defined by the first and second external dielectric interfaces.

[0007] Embodiments of the invention additionally provide a method, which includes the step of reducing a spectral linewidth of a semiconductor laser diode at least 10-fold performed with the use of substantially every embodiment of the laser apparatus alluded to above. Such reducing includes: operating the semiconductor laser diode to produce light having the spectral linewidth that exceeds at least one nanometer at an output thereof and substantially collimating the light with an optical system to form a substantially collimated beam of light; placing an optical prism defining two intermediate dielectric interfaces to transmit such substantially collimated beam of light; and positioning a Fabry-Perot etalon formed by the first and second external dielectric interfaces to provide optical feedback to the semiconductor laser diode and to form a laser apparatus output in transmission therethrough. In at last one embodiment of the method, the process of placing may include placing the optical prism with a spectral dispersion plane thereof to be substantially parallel to a fast axis of the laser diode and / or the process of positioning may include positioning the Fabry- Perot etalon substantially perpendicularly to an axis of the substantially collimated beam of light incident thereon. Optionally, each of the placing and positioning may include using uncoated dielectric interfaces. Alternatively or in addition, and substantially every implementation of the method may include varying a drive current and / or a temperature of the semiconductor laser diode to tune a central wavelength of a spectral linewidth of the optical output produced by the laser apparatus output with a spectral step defined by a free-spectral range of the Fabry -Perot etalon whilesubstantially maintaining a spectral linewidth of the laser apparatus output below 100 pm and a power of the laser apparatus output above at least 1 W. At least one of the embodiments of the method may additionally include performing Raman gas spectroscopy with the use of an embodiment of the laser apparatus. Optionally, implementation of the method is carried out under the circumstances that ensure a central wavelength of the spectral linewidth to be between about 439 nm and about 443 nm.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The invention will be more fully understood by referring to the following Detailed Description of Specific Embodiments in conjunction with the Drawings, of which:

[0009] FIG. 1 A is a schematic of an external cavity (EC) laser diode apparatus in which optical feedback is provided by a Fabry-Perot etalon (FPE) that forms the EC while additional intracavity spectral selection is made with the use of an optical prism. FIG. IB illustrates the same with addition of some flexure adjusters.

[0010] FIG. 2 illustrates effect of the optical etalon feedback on the laser diode spectrum in absence of the optical prism in an embodiment of the apparatus of the invention. Several plots corresponding to several values of the thickness of the etalon are presented, with the linewidth (measured at the full-width-half-maximum, FWHM, level) marked in each case. The diode was operated at a current of 2.5 A. With feedback, the output power was greater than 4 W. Etalons of different thickness, as indicated in a legend, were made of borosilicate glass (BK7, D263), and fused silica (FS).

[0011] FIG. 3 illustrates normalized spectra of the output produced by the apparatus of FIG. 1A containing both the optical prism (equilateral, SF11 glass) and FPE (0.3 mm-thick fused silica plane-parallel optical plate). The numbers above the peaks indicate the FWHM linewidth values, in pm, obtained from Gaussian fits (dashed lines). The legend indicates the associated drive currents at which the laser diode was operated, in Amperes, and optical power, in Watts.DETAILED DESCRIPTION

[0012] The related art recognizes a diffraction grating to be the most commonly used feedback optic in external cavity diode lasers: diffraction gratings are among the most spectrally- dispersive elements and can also possess high diffraction efficiency. At the same time, it is also well known that commercial off-the-shelf gratings are not designed for high-intensity operation and easily suffer optical damage easily. (Although it is possible to partially circumvent this limitation by beam expansion, the power loss incurred due to grating feedback, even in a Littrow configuration, can easily be of order 30%-50%.) Volume Bragg gratings (VBG), while having been demonstrated to provide the desired optical feedback in a multimode laser diode based external cavity apparatus, are very unique in that a specific VBG has to be designed and implemented for each particular laser diode source and, as such, a given VBG cannot be simply re-used to construct different EC laser diode-based apparatus that utilize high-power laser diode emitters differing in central wavelengths of light output that such emitters produce. Optical prisms have also been used in external cavity laser diodes and are known to tolerate high optical power because their dispersion relies on bulk material properties while their angular dispersion characteristics are recognized to be significantly poorer than the dispersion provided by the gratings. (Prisms are also generally much less lossy than diffraction gratings and for this reason have been employed as intracavity spectral selection elements in gas or dye lasers, for example. At longer wavelengths, for example, prisms find use as intracavity dispersion compensating elements in ultra-fast lasers, with total losses below 1%.)

[0013] Implementations of the present invention address the deficiencies of related art. In particular, the inability of the external -cavity laser diode systems of related art to ensure generation of a Watt-scale light-output that has linewidths sufficiently narrow to satisfy the needs of Raman gas spectroscopy is solved by devising a laser diode-containing apparatus the optical feedback for which is provided by an external combination of a Fabry-Perot etalon (FPE) and an optical prism. The optical surfaces of such combination of the optical elements - positioned externally to the high- power (Watt-level, and typically producing an astigmatic output beam, as known in the art) laser diode - are preferably uncoated to ensure the reliable operation of such optical feedback system across the wide spectral range covering both visible and near infrared spectral windows. To be particularly useful in some applications of Raman gas spectroscopy, such external cavity laserapparatus may be configured to generate light outputs in the blue portion of the optical spectrum with linewidths of less than 100 pm and down to just a few tens of pm, depending on the specifics of operational parameters. (It is appreciated that - considering the spectrally multimode nature of the laser diode emitters used in embodiments of the invention - the term “linewidth” as used herein refers to a width of an overall spectral band of the output from the embodiment of the external cavity laser diode apparatus configured according to the idea of the invention, and the actual values of the full-width-half-maximum, FWHM, measure of such spectral width are often beyond the spectral resolution of the utilized for such measurement grating spectrometer. The overall spectral band of the output generally includes a comb of modes defined by the external optical resonator, while the increase of magnitude of the optical feedback provided to the multimode emitter may reduce the number of such modes in the comb of modes.)

[0014] To this end, FIG. 1 schematically illustrates an embodiment of a laser diode apparatus 100 the external cavity of which is limited by the FPE 110 and contains an optical prism 120. The light output of a highlight astigmatic multimode laser diode 124 (Nichia NUBM44) is collected by the collimating lens 128, with the fast axis of the light output oriented in the plane of the drawing as indicated (as shown by the arrow 132). As known in the art, the fast axis of the output of the in-plane (or, as known in the art, edge emitting) laser diode 124 is the axis substantially transverse to the active layer of the emitter of the laser diode 124, along which the divergence of the light output, propagating outside of the laser diode 124 is higher that along an axis parallel to the active layer (~ a slow axis). The laser source 124 generated blue light with a bandwidth (measured as the FWHM) level of several nanometers when operated at a diode current of about 1 A to about 3 A. After collimation of the light output by the aspheric lens 128, a halfwave plate 136 was employed to establish a p-polarization for light incident on the prism 120 at near the Brewster’s angle. In at least one specific implementation, a high-density glass such as Schott SF10 or SF11 was used to the prims 120 that feature dispersion with Abbe numbers as low as 28 and 26, respectively. (Losses of optical power on propagation inside the prism are material-dependent and in the blue spectral range SF11 is among the most transmissive high-dispersion glasses available off-the-shelf.) Reflections from the FPE 110 created frequency-dependent feedback, thereby spectrally narrowing the light throughput 140 (collected in transmission through the FPE 110, withthe optional cylindrical expander devised with the use of two cylindrical lenses, as will be understood by a skilled person). Inset schematically illustrates the indices of optical materials of the FPE 110 and the surrounding medium. FIG. IB is a photograph of the prism-etalon external cavity laser diode 100 implemented with flexure adjusters (here, I1E is about 1.5).

[0015] According to the idea of the invention, since angular dispersion of the prism 124 substantially low, a secondary spectral selection is needed to reduce the multimode laser linewidth below 1 nm if the prism is employed. Such secondary spectral selectivity is provided by the FPE 110 of thickness E that simultaneously serves as a feedback element if positioned after the prism, as shown in FIG. 1 A. The effect of the FPE 110 is to return light to the laser diode 124 from its two parallel dielectric interfaces (surfaces) 110A, 110B separated by a distance LE, thereby creating an external cavity with the facet of the gain chip of the laser diode source 124.

[0016] Spectral selectivity without the optical prism. For a low-power single-spatial-mode laser diode without the prism, such a feedback etalon is known to result in tunable singlelongitudinal mode operation. With the multimode laser diode source 124 of FIG. 1 A without the prims 124, however, such etalon feedback creates spectrally-periodic transmission through the external cavity and thus enables laser operation at spectrally-equidistant bands (FIG. 2). The spectral separation of such bands is equal to the etalon’s free spectral range (FSR), in Hz,

[0017] FSR = c / (2nELE),

[0018] where c conventionally denotes the speed of light in vacuum, and nE= 1.5 is the index of refraction of the material of the etalon 110 (glass). The diode was operated at a current of 2.5 A. With feedback, the output power was greater than 4 W. Etalons of different thickness, as indicated in a legend, were made of borosilicate glass (BK7, D263), and fused silica (FS). Accordingly, as seen from different plots of FIG. 2. different etalon thicknesses of 0.14, 0.4, 0.5, and 2 mm yielded a laser output spectrum with spectral peaks separated by about 480, 170, 125, and 45 pm, respectively (in substantial agreement with the expected theoretical separations of 470, 160, 130, 33 pm, respectively). As the skilled artisan will readily appreciate from FIG. 2, the use of etalons of different thicknesses indeed facilitates lasing at spectral regions that are following the external cavity transmission modulation (although such modulation is not strong due to the approximately 4% reflectance of a single uncoated surface of the etalon 110). In the spectrum oflight emerging from the laser diode without any feedback whatsoever (represented by the upper trace of FIG. 2), a periodic modulation can also be observed. Such modulation is due to the cavity formed between the front and back facets of the laser diode gain chip itself. The periodic spectral features (the longitudinal cavity mode peaks) associated with the cavity formed by the gain chip back facet and each individua / etalon surface is not visible on the spectral scale of FIG. 2.

[0019] Spectral selectivity with the use of the optical prism. When the optical prism 120) in one specific case - the equilateral prism) is introduced into the EC the FPE 110 and the laser diode 124, as in the configuration shown in FIG. 1 A, the spectral transmission of the FPE 110 and that of the prism 120 combine. Here, each of the two dielectric interfaces that define lateral surfaces of the prism 120 is placed at an angle to the local axis of the light beam propagating from the laser diode 120, while the two dielectric interfaces defining the etalon 110 are oriented substantially normally to the local axis of the light beam incident onto the etalon. For a given prism 120, the spectral selectivity occurs in association with its angular dispersion, i.e., the prism’s transmission bandwidth is defined by the degree to which light is returned into the waveguide of the gain chip of the laser diode 124 after reflection from the etalon 110 and a second pass through the prism 120. To minimize the overall lasing bandwidth, the thickness of the etalon 110 must be appropriately chosen. If the etalon 110 is too thick, then the prism’s transmission bandwidth will encompass more than one etalon transmission peak. If the etalon 110 is too thin, a single spectral peak will result, but its bandwidth will be sizeable.

[0020] In practice, the best results for the chosen materials of the prism and the etalon were obtained with an etalon thickness between about 0.15 mm and about 0.3 mm. FIG. 3 shows the spectrum of the output 140 of light from the apparatus 100 that was equipped with the fused silica etalon of 0.3 mm thickness (produced by LightMachinery Inc.), for five different laser diode currents. The corresponding output powers are indicated in the legend of FIG. 3. Generally, the linewidths (measured as FWHM) were narrower at lower currents, and in each case below 100 pm. (Notably, as the skilled person will appreciate, while only several spectral linewidths achieved with the use of specific feedback are illustrated in FIG. 3, the general ability to reduce an initial linewidth of the multimode laser diode to a specific FWHM value as a result of establishing the optical feedback as discussed here substantially is not limited.) The threshold current was 0.3 A and theoutput power at threshold was 0.1 W. Thus, the operation reported above is in a regime well above threshold. A linewidth of about 70 pm (about 3.6 cm'1) was registered at a current of about 2.15 A, with the output power of 3 W carried by the beam 140. Such twenty -fold reduction in linewidth as compared with the linewidth of the original, not modified laser diode 124 (-the bare laser diode linewidth) can only be realized when the spatial orientation of the laser diode 124 is that illustrated in FIG. 1, i.e. when the spectral dispersion plane of the prism 120 (and, therefore, of the whole spectral frequency-selective system combining the prims and the FPE) is substantially aligned with the fast axis of the laser diode 124 - that is, when the fast axis of the laser diode is substantially in the spectral dispersion plane of the prism 120. (The spectral dispersion plane, in this case, is defined as a plane in which a dihedral angle is formed by the two lateral surfaces of the prism through which the light propagates.) When the laser diode 124 is rotated by 90 degrees, the so- substantial narrowing could not be demonstrated because of the multimodal nature of the waveguide of the laser diode along the slow axis.

[0021] Overall, as the skilled artisan will appreciate, the present work describes a methodology to achieving spectrally narrowed (down to a few cm1in width) laser diode light output with output power of over 3 W. Its working principle is based on the combination of a dispersive prism and a thin feedback etalon to provide optical feedback to the high-power multimode off-the-shelf consumer laser diode (chosen, in this case, to operate in a blue spectral region). In accordance with an example of the embodiment described with reference to FIG 1 A, IB, an external cavity laser diode apparatus is provided configured to generate light operationally suitable for use in Raman gas spectroscopy. While specific values chosen for this embodiment are recited, it is to be understood that, within the scope of the invention, the values of all of parameters may vary over wide ranges to suit different applications.

[0022] Generally, the choice of operational parameters of an embodiment of the apparatus suited for a specified application may be informed by the following non-exhaustive list of considerations:

[0023] - Cavity length. A shorter overall length of the EC arrangement shown in FIG. 1 is generally preferred, since the light beam output from the laser diode 124 diverges along the direction defined by the diode’s slow axis due to the multimodal nature of the laser operation alongthe slow axis. The EC length realized in the setup of FIGs. 1 A, IB was approximately 40 mm. The astigmatic distribution of the light output 140 could be additionally corrected using a cylindrical beam expander (placed after or before the feedback etalon 110) as shown in FIG. 1 A.

[0024] - Light Collimation. The aspheric high numerical aperture (NA ~ 0.5 ... 0.6 or so) lens 128 was mounted on a fine-adjustable flexure arm and positioned in front of the laser diode 124 to collimate the beam along the fast axis of the laser diode 124. Being single-mode along this direction, the laser diode beam can be collimated substantially completely along this axis. As the skilled artisan will appreciate, depending on the value of operating current optimal position of the lens along the axis of the beam may differs somewhat, which is likely associated with a change in the lens’ index of refraction, and which can be dynamically corrected if so desired.

[0025] - Diode Current. For the recording of spectra of the light output from the overall EC apparatus (such as those shown in FIG. 3) at different driving currents, the spatial orientation of the etalon 110 may require some readjustment for each operating current to ensure that no more than one spectral peak is observed. Because the diode gain redshifts with increase of the drive current, the optimal center frequency of operation shifts accordingly to a longer (redder) wavelength as seen in FIG. 3. Notably, such feature emphasizes a major operational benefit of the proposed methodology of narrowing the spectral linewidth of a high-power multiple laser diode over that achieved with the use of a volume Bragg grating: in advantageous contradistinction with the VBG, which is fabricated for a fixed wavelength of operation, the combination of the very same spectral frequency-selective system defined by the combination of the optical prism and the FPE can be used for with the laser diode operated under different conditions or with different laser diodes. For example, employing the fact that the spectral selectivity of the FPE used in the spectral frequency- selective system of the apparatus is defined by a multiplicity of spectral bands separated from one another by the FSR of the FPE, the central wavelength of operation of the laser apparatus can be adjusted or tuned with a spectral step substantially equal to a free spectral range defined by the two external dielectric interfaces forming such FPE. (In addition, unlike the VBG that employs multiple dielectric boundaries or interfaces to provide the optical feedback into the laser diode, the chosen FPE utilizes only two dielectric interfaces.) The threshold current of the laser diode 124 was about0.3 A and the output power at threshold was about 0.1 W - therefore, the operation of the overall system 100 discussed above was observed in a regime well above threshold.

[0026] - Diode Temperature. For the measurements with results illustrated in FIG. 3, no active temperature control was employed. The laser diode 124 was set up on an aluminum plate that was mounted on an optical table. Thus, heat generated by the diode was dissipated primarily through the metal holders in a slow fashion, and the temperature of the mount was slowly rising. Nevertheless, the spectral output remained stable unless the temperature changed by at least a few degrees °C. Some coarse temperature stabilization may be preferred for stable long-term operation.

[0027] The skilled artisan will readily appreciate that further improvements could be introduced into the apparatus 100 to a narrow the spectral linewidth even further and to increase the power of the output 140. In particular, the uncoated prism 120 as currently utilized still produced optical losses of about 5% that weaken the feedback and could be eliminated with an optimized prism apex angle and / or anti-reflective coatings. Interface Fresnel losses can be (and were, in the discussed embodiments) dramatically suppressed by operating the prism near Brewster’s angle as far as light incident thereon is concerned. Furthermore, a more dispersive prism or a combination of two prisms could be employed to reduce the prism filtering bandwidth further. Finally, coating of the surfaces of the glass-plate etalon 110 to achieve a surface reflectivity greater than 4% could provide strengthened feedback and thus further spectral narrowing.

[0028] Understandably, and regardless of whether so described an embodiment of the apparatus of the invention may include a processor controlled by instructions stored in a memory. The memory may be random access memory (RAM), read-only memory (ROM), flash memory or any other memory, or combination thereof, suitable for storing control software or other instructions and data. Those skilled in the art should also readily appreciate that instructions or programs defining the functions of the present invention may be delivered to a processor in many forms, including, but not limited to, information permanently stored on non-writable storage media (e.g. read-only memory devices within a computer, such as ROM, or devices readable by a computer I / O attachment, such as CD-ROM or DVD disks), information alterably stored on writable storage media (e.g. floppy disks, removable flash memory and hard drives) or information conveyed to a computer through communication media, including wired or wireless computer networks. Inaddition, while the invention may be embodied in software, the functions necessary to implement the invention may optionally or alternatively be embodied in part or in whole using firmware and / or hardware components, such as combinatorial logic, Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs) or other hardware or some combination of hardware, software and / or firmware components.

[0029] For the purposes of this disclosure and the appended claims, the expression of the type “element A and / or element B” is defined to have the meaning that is equivalent to “at least one of element A and element B”.

[0030] The use of the terms "substantially", "approximately", "about" and similar terms in reference to a descriptor of a value, element, property or characteristic at hand is intended to emphasize that the value, element, property, or characteristic referred to, while not necessarily being exactly as stated, would nevertheless be considered, for practical purposes, as stated by a person of skill in the art. These terms, as applied to a specified characteristic or quality descriptor means "mostly", "mainly", "considerably", "by and large", "essentially", "to great or significant extent", "largely but not necessarily wholly the same" such as to reasonably denote language of approximation and describe the specified characteristic or descriptor so that its scope would be understood by a person of ordinary skill in the art. In one specific case, the terms "approximately", "substantially", and "about", when used in reference to a numerical value, represent a range of plus or minus 20% with respect to the specified value, more preferably plus or minus 10%, even more preferably plus or minus 5%, most preferably plus or minus 2% with respect to the specified value. As a non-limiting example, two values being "substantially equal" to one another implies that the difference between the two values may be within the range of + / - 20% of the value itself, preferably within the + / - 10% range of the value itself, more preferably within the range of + / - 5% of the value itself, and even more preferably within the range of + / - 2% or less of the value itself.

[0031] References throughout this specification to "one embodiment," "an embodiment," "a related embodiment," or similar language mean that a particular feature, structure, or characteristic described in connection with the referred to "embodiment" is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment4"in an embodiment, “ and similar language throughout this specification may, but do not necessarily, all refer to the sameembodiment. It is to be understood that no portion of disclosure, taken on its own and in possible connection with a figure, is intended to provide a complete description of all features of the invention.

[0032] While the invention is described through the above-described exemplary embodiments, it will be understood by those of ordinary skill in the art that modifications to, and variations of, the illustrated embodiments may be made without departing from the inventive concepts disclosed herein. Disclosed aspects, or portions of these aspects, may be combined in ways not listed above. Accordingly, the invention should not be viewed as being limited to the disclosed embodiment(s).

Claims

CLAIMSWhat is claimed is:

1. A laser apparatus comprising: a semiconductor laser diode configured to produce a spatially astigmatic light output, an optical system positioned to receive a light output produced by the laser diode and to transform said light output into a substantially collimated beam of light; and a spectral frequency-selective system positioned to intersect light produced by the laser diode and to produce a light output in transmission through the spectral frequency-selective system, wherein said frequency-selective system includes: first and second substantially parallel to one another external dielectric interfaces defining an external cavity for the laser diode, and two intermediate dielectric interfaces, each positioned between the first and second external dielectric interfaces and the optical system and inclined with respect to an axis of the substantially collimated beam of light, wherein the laser apparatus in configured to generate said light output having a linewidth narrower than 100 pm.

2. A laser apparatus according to claim 1, wherein each of the first and second external dielectric interfaces and each of the two intermediate dielectric interfaces separates an optically transparent material from air.

3. A laser apparatus according to one of claims 1 and 2, wherein each of the first and second external dielectric interfaces is substantially perpendicular to the axis of the substantially collimated beam of light.

4. A laser apparatus according to one of claims 1 to 3, wherein the first and second external dielectric interfaces include only two external dielectric interfaces.

5. A laser apparatus according to one of claims 1 to 4, wherein the semiconductor laser diode is an in-plane multimode semiconductor laser diode.

6. A laser apparatus according to one of claims 1 to 5, wherein a spectral dispersion plane of the spectral frequency-selective system is oriented with a fast axis of the laser diode.

7. A laser apparatus according to one of claims 1 to 6, wherein the two external dielectric interfaces are uncoated surfaces of an optical plane-parallel plate that form a Fabry -Perot etalon.

8. Use of the laser apparatus according to one of claims 1 to 7 for reducing a linewidth of a semiconductor laser diode by at least 10-fold.

9. Use according to claim 8, comprising: tuning a central wavelength of operation of the laser apparatus with a spectral step substantially equal to a free spectral range defined by the two external dielectric interfaces.

10. A method comprising: with the use of the laser apparatus according to one of claims 1 to 7: reducing a spectral linewidth of a semiconductor laser diode at least 10-fold by:operating the semiconductor laser diode to produce light having the spectral linewidth that exceeds at least one nanometer at an output thereof and substantially collimating the light with an optical system to form a substantially collimated beam of light; placing an optical prism defining two intermediate dielectric interfaces to transmit said substantially collimated beam of light; positioning a Fabry -Perot etalon formed by two external dielectric interfaces to provide optical feedback to the semiconductor laser diode and to form a laser apparatus output in transmission therethrough.

11. A method according to claim 10, wherein said placing includes placing the optical prism with a spectral dispersion plane thereof to be substantially parallel to a fast axis of the laser diode.

12. A method according to one of claims 10 and 11, wherein said positioning including positioning the Fabry-Perot etalon substantially perpendicularly to an axis of the substantially collimated beam of light incident thereon.

13. A method according to one of claims 10 to 12, wherein each of said placing and said positioning includes using uncoated dielectric interfaces.

14. A method according to one of claims 10 to 13, further comprising varying a drive current and / or a temperature of the semiconductor laser diode to tune a central wavelength of a spectral linewidth of an output produced by the laser apparatus with a spectral step defined by a free- spectral range of the Fabry-Perot etalon while maintaining a spectral linewidth of the laser apparatus output below 100 pm and a power of the laser apparatus output above at least 1 W.

15. A method according to one of claims 10 to 13, further comprising: performing Raman gas spectroscopy with the use of the laser apparatus.

16. A method according to claim 15, wherein a central wavelength of the spectral linewidth is between 439 nm and 443 nm.

Images