1. Field
This disclosure is generally concerned with optical sciences and specifically concerned with mid-IR lenses having high numerical apertures and simultaneously short focal lengths, and further with coupling of such lenses to specialized optical sources.
2. Related Technology
Lasers are optical sources which provide light of remarkable characteristics. Coherence, high intensity, narrow bandwidth, among others are characteristics common to some lasers which permit their use in conjunction with complex systems to achieve remarkable results. Of particular interest is the highly collimated narrow beam produced by some lasers. Gas lasers have optical resonators defined by two mirrors arranged to produce very narrow beams of planar wavefronts (high spatial coherence). The optical output from such arrangements is highly useful in a great many systems partly because of the nature of the output beam.
In contrast to gas lasers, laser diodes are optical sources which convert electrical current to photonic output at a semiconductor PN junction which operates as a laser gain medium. As the geometry associated with such PN junction is generally planar, diodes arranged as lasers sometimes have an asymmetric stripe cross section resulting in output beams having a different degree of divergence in two orthogonal directions. Laser diodes have been used in conjunction with complex optics to condition an output beam to improve its symmetry.
In many optical systems, it is desirable to have an axially symmetric collimated beam of planar wavefronts. Yet it can be difficult to arrive at such output from some lasers; especially where a gain medium is constrained by certain geometric features. In those cases, a laser output beam may have less than ideal symmetry and beam shape. Whereas some gas laser systems have an output beam shape which depends on the resonator configuration, diode laser systems sometimes have an output beam conditioned by optics external to the resonator cavity. In some cases, special cooperation between a laser and its output optics can improve system performance.
Laser diode output beams have been coupled to external systems via micro optical elements. This is certainly true in the case where diode lasers are coupled to fiber optic systems. In such systems the micro optics used may be made from rather conventional materials since laser diode spectra, with very few exceptions, include only those wavelengths suitable for transmission by conventional optics. The art is crowded with semiconductor lasers coupled to external systems via micro optics elements.
Highly specialized structures recently being used as laser gain media include those known as quantum well stack or sometimes as “quantum cascade”, QC or QCL, laser systems. These structures are semiconductor devices but are strictly not diodes. Rather, a unipolar structure of many layers provide for selective transitions between allowed energy states. When formed in accordance with prescribed designs and coupled with an appropriate optical resonator, a quantum well stack makes an excellent gain medium having good laser performance. In particular, quantum well gain media based laser devices may be arranged with exceptionally wideband gain profiles in the highly useful mid-IR spectrum. In some versions, these lasers are highly tunable and have exceptionally high power output.
However, as in the case with a diode laser beam produced in a quantum well gain medium device may have less than perfectly ideal shape and beam characteristics. The geometry of the gain medium sometimes imparts unwanted influence on the output beam shape. A quantum well stack gain medium is generally formed as a thin stripe semiconductor at the core of a waveguide. The waveguide supports resonant modes which help shape the overall output beam. In particular, a quantum well stack may include an emission facet from which an optical beam leaves the gain medium in a direction substantially normal to the facet surface. The long wavelength or “mid-IR” output beam from a quantum well based laser is typically highly divergent and can be somewhat asymmetric about orthogonal directions. Further, these devices have been constructed by experts in a fashion whereby a sufficiently wide waveguide will support a plurality of transverse oscillation modes. Transverse modes have a greater portion of optical energy ‘off-axis’. This off-axis energy can be lost when a laser is coupled to inefficient systems which suffer aperture clipping effects.
Experts have now made many systems, instruments, experiments, based upon a quantum cascade lasers and in each and every case without exception, the output of the laser is handled in a fashion whereby the beam is conditioned for use and coupling with other subsystems and experiment or test components. Most generally, the output of a quantum cascade laser is coupled to an experiment by a collimation lens or a parabolic mirror. A quite common way to couple QCLs to external systems is via a mirror having a 50 millimeter focal length and a 50 millimeter clear aperture. While operable, this arrangement suffers from significant edge clipping and the loss is merely tolerated. Since these systems emit mid-IR wavelengths, special lenses which transmit these wavelengths are necessary. Materials such as zinc-selenide, germanium, chalcogonide, or other mid-IR transparent material are used to form these lenses.
As the infrared related optical sciences are well developed and quite mature, a great body of lens technologies exists in this area. Particularly, IR lenses configured for imaging tasks are plentiful. With only a cursory effort, an Internet search will generate hundreds of manufactures and designers for lenses useful in IR imaging systems. Imaging lenses are generally compound lenses made up of several individual pieces which function together to perform a high performance imaging relationship with an image plane. While an imaging lens can be used for laser collimation, this arrangement is certainly less than ideal.
In addition to IR imaging lens systems, lenses may be arranged for non-imaging or general purpose optical beam handling. Sometimes these lenses are available as “singlets” or single element pieces. In one example of interest, laser collimation applications might call for a plano-convex lens operable at mid-IR wavelengths. Indeed, these lenses are widely available and may be purchased from suppliers of IR lenses. However, laser collimation singlets are designed for and are most suitable for laboratory use with optical beams having large cross-section; that is, laser collimation singlets are most generally large diameter devices of approximately 1 inch. These lenses designed for use in conjunction with special precision optical fixtures designed specifically to couple with low vibration optical benches and permit adjustable alignment and advanced stability features. These lenses typically have a clear aperture of at least 20 millimeters that may be designed with various focal lengths, but typically not with a focal length less than about 20 millimeters. The ratio of focal length to aperture is sometimes called “f#” or ‘f number’. In rare cases, IR singlets having f-number as low as 1 can be found but it has been exceedingly difficult and complex to achieve f-numbers less than 1. These are certainly not available on commercial markets. This is particularly true in the case of longer wavelengths of IR. While near IR systems enjoy the possibility of use of many lens materials, mid-IR systems are highly restrictive in this regard. Materials which transmit mid-IR wavelengths further complicate the possibility of manufacture of micro-optics having low f-number.
As many existing quantum cascade laser systems demand a collimated beam, a mid-IR singlet lens is placed in relation to the quantum cascade lasers such that the emission surface is at or near the focal point of the lens. Lenses which transmit mid-IR light are readily available having a focal length approximately 1 inch and an aperture or diameter of about 1 inch. This is the most commonly used optic for coupling a quantum cascade laser to related subsystems.
When a zinc selenide lens of diameter=1″, and focal length=1″ is used to collimate the output of a quantum cascade laser, the lens is placed with its focal plane at the emission aperture of the laser. Because the divergence of the laser output is greater than the numerical aperture of the lens, some light is lost at the lens extremities due to aperture clipping. While the amount of light lost may be as high as 30% of the laser output, this has been tolerated in published systems. This can be even more problematic for lasers which operate with transverse modes. In systems running with excited transverse modes, a greater percentage of optical energy is off-axis and lost at the lens aperture.
Accordingly, it is very desirable to provide for improved coupling between a mid-IR collimation lens and quantum well gain medium based lasers. First, it is desirable to improve matching of the numerical aperture such that the lens can receive the entire output of the laser. Second, it is highly desirable to reduce the laser energy which is off-axis and susceptible to losses at system apertures (high spatial frequency filtering/clipping). Lasers having a greater portion of energy on-axis can be more effectively coupled to lenses having a numerical aperture less than 1.
Due partly to materials limits, manufacturing processes available, and industrial experience in IR arts, it has been heretofore impossible to realize mid-IR singlets with numerical aperture greater than about 0.6. For most optical IR systems, this is not a problem as the physical nature of common IR systems does not demand lenses with such performance characteristics. Until the arrival of unipolar quantum well optical sources having characteristic highly divergent mid-IR output beams, and further the arrival of new applications in view of this particular source, there has been little or no effort to discover new devices characterized as high numerical aperture, short focal length, mid-IR lenses, and any such efforts have failed; and indeed left a great and unfulfilled need. This is clearly evidenced as skilled practitioners regularly choose inferior output couplings—i.e. those available.
Examples of infrared lenses available commercially include those described in the literature as follows:
Newport Corporation catalog pages including IR lenses describes AMTIR-1 low cost NIR to Mid-IR lenses arranged as singlets in 1″ configurations. These lenses have a low NA or F/# no better than 1.
Computar Varifocal lenses have short focal lengths but these lenses are arranged as multi-element compound lens systems for IR imaging.
ISP optics catalog pages include ZnSe IR singlets characteristic of the state of the art for IR optics useful in the mid-IR spectrum. These optical elements are as small as 0.5 inch, however, in all cases their f/# is quite large and in no case are these lenses provided for short focal length applications. The lenses are all configured as ‘thin’ lenses having large working distances (and large focal lengths) which contribute to complex optical set-ups suitable for laboratory use only.
Wysokski et al present one of a great many systems representative in the art of a laboratory set-up on optical benches having large optical elements held in specialized optical mounting apparatus. A collimating lens is CL a 1″ diameter f/0.6 lens. This is the typical way that a quantum cascade laser is coupled to an external optical experiment.
Another representative illustration of a quantum cascade laser coupled to an experimental set-up in the commonly used configuration is nicely described in the drawing of Hensley's paper: “Recent Updates in QCL-based Sensing Applications” from Physical Sciences, Inc. An optical bench is presented with stabile optical element holders and mounting systems which accommodate 1″ optics common in the industry. The set-up requires a large optical table base for coupling all elements of the optical train which are separated by great distances exposing them to misalignment and vibration problems thereby requiring careful regulation of the apparatus in a guarded environment.