Lasers have become commonplace in the modem world. Compact disks (CDs) are widely used for entertainment and business, and digital versatile disks (DVDs) are widely used for entertainment. Industrial and scientific uses for lasers include high-resolution spectroscopic analysis, and are commonly implemented in a vast variety of optical sensor systems. Optical communication systems routinely utilize lasers to generate light beams for communicating optical signals.
The process of light amplification by stimulated emission of radiation (LASER) occurs in an optical resonator. The components of a typical optical resonator include: a partially reflective mirror from which the useful portion of the laser beam is emitted; an optical amplifier such as a glass tube filled with an appropriate gas, or a semiconductor device; and a reflective mirror. The laser beam is formed by light resonating between the mirrors located at the two ends of the optical resonator.
Some applications require a laser to emit a beam that can be tuned to different wavelengths. Such applications include, among others: devices that read and write CDs or DVDs by altering the laser beam between a write wavelength and a read wavelength; spectroscopic analysis of a substance using various wavelengths of light; and optical communication systems that utilize wavelength division multiplexing (WDM).
Each of the optical components used in a tunable laser must be precisely manufactured, placed into exact alignment relative to one another, and this alignment must be maintained in order for the laser to properly function. In some situations it is desirable to minimize the number of optical components in an effort to achieve a desired cost target while maintaining an acceptable reliability standard.
In accordance with some embodiments of the invention, a tunable laser includes an optical gain medium, a diffraction grating, and a tuning mechanism. The optical gain medium is configured to emit a light beam from a front facet and a laser beam from a rear facet. The diffraction grating includes a concave reflective diffractive surface positioned to receive the light beam, and is configured to reflect light of a selected wavelength to the front facet of the optical gain medium. The tuning mechanism is configured to adjust the relative position of the optical gain medium and the diffraction grating.
The above and other aspects, features, and advantages of the present invention will become more apparent upon consideration of the following description of preferred embodiments taken in conjunction with the accompanying drawing figures, wherein:
In the following detailed description, reference is made to the accompanying drawing figures which form a part hereof, and which show by way of illustration specific embodiments of the invention. Other embodiments may be utilized, and structural, electrical, as well as procedural changes may be made without departing from the scope of the present invention.
Gain medium 105 may be implemented using known devices for amplifying light including, for example, a semiconductor diode, and plasma-based or liquid-based optical media. The gain medium is shown having an anti-reflection (AR) layer 120 on front facet 125, and a reflective or partially reflective layer 130 on rear facet 135.
Gain medium 105 emits light beam 140 which propagates along optical axis 150. Light beam 140 is a diverging light beam which is incident upon diffraction surface 145 of diffraction grating 110. Resonant cavity 155 is defined by rear facet 135 and the diffraction surface of grating 110.
Reflective diffraction surface 145 is concave with respect to incident light beam 140. Diffraction grating 110 may be implemented using a conventional reflective diffraction grating. Diffraction grating 110 diffracts and thereby focuses a diverging light beam that is incident upon the grating. The concave diffraction grating may have either a spherical diffraction surface or an aspherical diffraction surface. Particular examples of aspherical diffraction surfaces include parabolic and elliptical surfaces. In addition, a suitable diffraction grating has a diffraction efficiency anywhere from about 30 percent to about 93 percent. Examples of various types of conventional diffraction grating configurations that may be used in laser 100 are depicted in
Referring again to
In operation, gain medium 105 emits diverging light beam 140 which propagates along optical axis 150 toward diffraction grating 110. Diffraction surface 145 diffracts light of a selected wavelength, shown in
The concave shape of diffraction surface 145 enables the diffraction grating to reflect the diverging light beam emitted by front facet 125 of the optical gain medium back to the front facet of the optical gain medium as a converging light beam that enters the optical gain medium without the need for any additional imaging components such as a lens or lens and mirror combination. This is not to say that a lens would never be employed in this structure. There may be special circumstances where a lens could provide an enhanced result.
The wavelength selected by the diffraction grating, and thus the wavelength of laser light beam 170, is tuned by adjusting the relative positions of optical medium 105 and diffraction grating 110. Such adjustment of the relative positions changes the incidence angle of light beam 140 with respect to the reflective diffraction grating 110, which changes the selected wavelength. Such adjustment of the relative positions typically additionally changes the length of resonant cavity 155 to ensure that the cavity remains resonant at the changed selected wavelength. Possible adjustments of the relative positions of optical gain medium 105 and diffraction grating 110 include: rotating the diffraction grating about pivot pin 160; translating the diffraction grating along the X axis; translating the diffraction grating along the Y axis; or some combination thereof. Alternatively or additionally, the relative position of the gain medium and the diffraction grating may be adjusted by translating the gain medium along the X axis, the Y axis, or both axes. One specific example is to rotate the diffraction grating about the pivot pin, and to translate the diffraction grating along the X axis relative to the optical gain medium.
The optical resonance process implemented by laser 100 may be summarized as: amplifying light in gain medium 105, emitting light beam 140 from the gain medium, reciprocally reflecting the light beam at a selected wavelength at the diffraction surface of grating 110, and emitting a portion of the amplified light as laser light beam 170. This resonance process is advantageously simple compared to conventional designs for tunable lasers that often require the cavity laser beam to be imaged by a separate lens or some sort of lens, grating, and mirror combination.
Each optical component within a laser unavoidably disperses, absorbs or otherwise dissipates some of the light. However, since laser 100 has fewer components than traditional lasers, the amount of light that is lost is reduced. A reduction in the amount of dissipated light provides one or more of the following benefits: it increases the light output of the laser, permits the laser to be operated with less power, and increases the range of wavelengths in which the laser can effectively operate.
In one embodiment of the invention, adjustment of the relative position of gain medium 105 and diffraction grating 110 is performed infrequently. For example, adjustment may occur during an initial calibration process after the laser is manufactured and then re-occur only infrequently, if at all, during maintenance or repair operations in the field. Such an embodiment can use a positioning mechanism having a threaded rod that engages the diffraction grating. In this embodiment, the diffraction grating is adjusted (rotated, translated relative to the optical gain medium, or both) by rotating the rod, possibly manually, or by means of a stepper motor or the like.
In an alternative embodiment, the adjustment of the relative position of the gain medium and diffraction grating occurs dynamically during the operation of the laser. This may be accomplished using, for example, the just-described threaded rod adjustment mechanism under the control of an electronic position-control circuit that receives feedback as to the wavelength currently being emitted by the laser.
As shown in these figures, the basic shape of reflective diffraction surface 145 is concave and is defined by a series of diffraction grooves 200.
Astigmatism defines the condition in which the tangential and sagittal foci of a diffraction grating are not coincident, which causes a line image at the tangential focus. A diffraction grating with diffraction grooves that are uniform and parallel typically causes a diffracted light beam to have a large amount of astigmatism. As a result, light beams diffracted by such gratings result in undesirable line images. As a line image propagates, it becomes increasingly more difficult to focus the image on a desired optical element, such as front facet 125 of gain medium 105. Accordingly, in accordance with embodiments of the invention, diffraction surface 145 includes diffraction grooves 200 that have increasing pitch and curvature radius which enables the grating to correct astigmatism in incident light beam 140 and diffracted light beam 165.
In general, diffraction grooves have particular characteristics such as pitch, curvature, and profile. Groove pitch 210 (
Used in a tunable laser, an astigmatism-correcting diffraction grating advantageously increases the tuning range of the wavelengths of the beam that the laser can emit, or increases the efficiency with which the laser operates, or both. The specifics regarding the particular pitch and curvature radii of the various diffraction grooves that will correct astigmatism is known. See, for example, “Diffraction Gratings,” pages 222-227, by M. C. Huntly, 1982, Academic Press, and “Diffraction Gratings and Applications,” pages 255-275, by Evgeny Popov and Erwing G. Loewen, 1997, Marcel Dekker.
Diffraction grating 110 is shown as a circular diffraction grating, but other geometries (for example, elliptical, rectangular, and the like) may also be used. In addition, a diffraction grating having diffraction grooves with a sinusoidal groove profile is depicted in
Each diffraction groove profile will exhibit a particular diffraction efficiency. Accordingly, the selection of a particular groove profile for the diffraction grating is typically determined by the diffraction efficiency requirements of the laser application being implemented.
The grooves of the diffraction surface may be formed using any suitable technique. For example, angular grooves can be formed by passing a diamond-tipped scribe over the surface of a diffraction grating, or by using conventional ion-beam milling technology. Photolithography is another well-known technique that may be used to form the grooves of the diffraction grating.
Using a diffraction grating that has diffraction grooves with increasing pitch and radius curvature is helpful in many applications, as noted above, but such a diffraction grating is not an essential feature of the present invention. Diffraction gratings having diffraction grooves that are uniform and parallel may also be used.
In operation, laser 410 generates an exposure beam that is expanded by beam expander 420. The exposure beam enters beam splitter 440 where the beam is split into two exposure beams, 470 and 472. Exposure beams 470 and 472 are reflected by mirrors 450 and 452, respectively. Imaging components 460 and 462 respectively direct exposure beams 470 and 472 through spatial filters 430 and 432 onto surface 145 of diffraction grating 110. This optical configuration causes exposure beams 470 and 472 to overlap and interfere with each other, according to the well-known principles of light wave interference and holography.
The interference pattern formed by exposure beams 470 and 472 can be modified by changing the position of spatial filters 430 and 432 relative to diffraction grating 110, or by changing the incidence angle of exposure beams 470 and 472. A change in the interference pattern results in a corresponding change in the groove pitch, or the radius of curvature of the diffraction grooves, or both. As previously described, the amount of astigmatism and other aberration correction provided by the diffraction grating depends upon the pitch and curvature radii of the various diffraction grooves of the grating. Accordingly, a diffraction grating that can correct a particular level or type of aberration may be formed by changing the position of spatial filters 430 and 432, or the incidence angle of exposure beams 470 and 472, or both.
Concave surface 145 is coated with a suitable positive or negative photoresist. When exposed to the interference pattern produced by exposure beams 470 and 472, the photoresist records the interference pattern. A suitable positive photoresist development process, for example, removes portions of the photoresist that were exposed to the interference pattern, leaving portions of the photoresist that were not exposed to the interference pattern. To form a reflective diffraction grating, a reflective metal layer, for example, may be formed over the patterned surface using known deposition techniques.
While the invention has been described in detail with reference to disclosed embodiments, various modifications within the scope of the invention will be apparent. It is to be appreciated that features described with respect to one embodiment typically may be applied to other embodiments. Therefore, the invention properly is to be construed only with reference to the claims.