The present invention relates generally to microlasers and associated fabrication methods and, more particularly, to Q-switched microlasers and associated fabrication methods.
Modern electro-optical applications are demanding relatively inexpensive, miniaturized lasers capable of producing a series of well-defined output pulses. As such, a variety of microlasers have been developed which include a microresonator and a pair of at least partially reflective mirrors disposed at opposite ends of the microresonator to define a resonant cavity therebetween. The microresonator of one advantageous microlaser includes an active gain medium and a saturable absorber that serves as a Q-switch. See, for example, U.S. Pat. No. 5,394,413 to John J. Zayhowski, which issued on Feb. 28, 1995, the contents of which are incorporated in their entirety herein. By appropriately pumping the active gain medium, such as with a laser diode, the microresonator will emit a series of pulses having a predetermined wavelength, pulse width and pulse energy.
As known to those skilled in the art, the wavelength of the signals emitted by a microlaser is dependent upon the materials from which the active gain medium and the saturable absorber are formed. In contrast, the pulse width of the laser pulses emitted by a conventional microlaser is proportional to the length of the resonator cavity. As such, longer resonator cavities will generally emit output pulses having greater pulse widths. Further, both the pulse energy and average power provided by a microlaser are proportional to the pulse width of the pulses output by the microlaser. All other factors being equal, the longer the microresonator cavity, the longer the pulse width and the greater the pulse energy and average power of the resulting laser pulses.
Conventional microlasers, such as those described by U.S. Pat. No. 5,394,413, are end pumped in a direction parallel to the longitudinal axis defined by the resonator cavity. In this regard, the longitudinal axis of the microresonator cavity extends lengthwise through the resonator cavity. Since the resonation cavity is generally a rectangular solid, the longitudinal axis is oriented so as to be orthogonal to the pair of at least partially reflective mirrors that define the opposed ends of the resonant cavity. As such, conventional microlasers are configured such that the pump source provides pump signals in a direction perpendicular to the at least partially reflective mirrors that define the opposed ends of the resonant cavity. The effective length of the resonator cavity is therefore equal to the physical length of the resonator cavity.
While the microlaser can be fabricated such that the resonator cavity has different lengths, a number of factors contribute to generally limit the permissible length of the resonator cavity. See, for example, U.S. Pat. No. 5,394,413 that states that the resonator cavity, including both the saturable absorber and the gain medium, is preferably less than two millimeters in length. In particular, a number of electro-optical applications require microlasers that are extremely small. As such, increases in the length of the resonator cavity are strongly discouraged in these applications since any such increases in the length of the microresonator cavity would correspondingly increase the overall size of the microlaser.
In addition, the length of passively Q-switched microlasers is effectively limited by the requirement that the inversion density must exceed a predetermined threshold before lasing commences. As the physical length of the resonator cavity increases, greater amounts of pump energy are required in order to create the necessary inversion density for lasing. In addition to disadvantageously consuming more power to pump the microlaser, the increased pumping requirements create a number of other problems, such as the creation of substantially more heat within the microlaser which must be properly disposed of in order to permit continued operation of the microlaser. In certain instances, the heat generated within the microlaser may even exceed the thermal capacity of the heat sink or other heat removal device, thereby potentially causing a catastrophic failure of the microlaser.
Since the pulse width and correspondingly the pulse energy and average power of the pulses output by a microlaser cavity are proportional to the length of the resonator cavity, the foregoing examples of practical limitations on the length of the resonator cavity also disadvantageously limit the pulse width and the corresponding pulse energy and average power of the pulses output by conventional microlasers. However, some modem electro-optical applications are beginning to require microlasers that emit pulses having greater pulse widths, such as pulse widths of greater than 1 nanosecond and, in some instances, up to 10 nanoseconds, as well as pulses that have greater pulse energy, such as between about 10 xcexcJ and about 100 xcexcJ, and greater average power, such as between 0.1 watts and 1 watt. As a result of the foregoing limitations on the length of the resonator cavity and the corresponding limitations on the pulse widths, pulse energy and average power of the pulses output by the conventional microlasers, conventional microlasers do not appear capable of meeting these increased demands.
A microlaser is therefore provided according to one embodiment of the present invention that is capable of supporting a zig-zag resonation pattern in response to pumping of the active gain medium so as to effectively lengthen the microresonator cavity without having to physically lengthen the microresonator cavity. As such, the microlaser of these embodiments can generate pulses having greater pulse widths and correspondingly greater pulse energies and average power levels than the pulses provided by conventional microlasers of a similar size.
According to the present invention, the microlaser includes a microresonator having an active gain medium and a Q-switch, such as a passive Q-switch proximate to and, in one embodiment, immediately adjacent to the active gain medium. In advantageous embodiments, the active gain medium and the Q-switch are integral such that the microresonator may be a monolithic structure. The microresonator extends lengthwise between opposed end faces. The microlaser also includes first and second reflective surfaces disposed proximate respective ones of the opposed end faces to define a microresonator cavity therebetween. While the first and second reflective surfaces can be coated upon respective ones of the opposed end faces of the microresonators, the first and second reflective surfaces can also be formed by mirrors that are spaced from respective ones of the opposed end faces. The microlaser can also include a pump source for introducing pump signals into the active gain medium via at least one of the end surfaces of the microresonator such that the zig-zag resonation pattern is established within the microresonator cavity.
In one advantageous embodiment, the opposed end faces are each disposed at a nonorthogonal angle xcex1, such as between about 30xc2x0 and about 45xc2x0, relative to a line perpendicular to a longitudinal axis defined by the microresonator cavity and extending between the opposed end faces. In one embodiment, the opposed end faces are each disposed at the same nonorthogonal angle xcex1 relative to the longitudinal axis such that the opposed end faces are parallel. In another embodiment, the opposed end faces are oriented in opposite directions by the same nonorthogonal angle xcex1. As a result of the nonorthogonal relationship of the opposed end faces, the microlaser of either embodiment is capable of supporting the zig-zag resonation pattern in response to pumping of the active gain medium via at least one of the end surfaces of the microresonator.
By supporting the zig-zag resonation pattern, the effective length of the microresonator cavity is increased relative to conventional microlasers having substantially the same physical size that do not support a zig-zag resonation path. In this regard, the effective length of the microresonator cavity of the present invention is the length of the zig-zag resonation path established by the microlaser which is significantly longer than the linear resonation paths established by conventional microlasers that extend parallel to the longitudinal axis of the resonator cavity. As such, the microlaser of the present invention can emit pulses having a longer pulse width and correspondingly greater pulse energies and average power levels than the pulses emitted by conventional microlasers of the same physical size.
In order to permit the pump signals to be received by the active gain medium without being reflected from the end face, the microlaser can include an antireflection coating on the end face through which the pump signals are delivered for permitting pump signals having a predetermined range of wavelengths to be received by the active gain medium. The microresonator also generally includes a plurality of side surfaces extending between the opposed end faces. In order to further facilitate resonation within the microresonator cavity, the plurality of side surfaces can be roughened, such as by grinding, to thereby diffuse light.
In order to permit the microlaser to emit signals of a predetermined lasing wavelength via one of the opposed end faces, the first reflective surface is preferably highly reflective for laser signals having the predetermined lasing wavelength. In contrast, the second reflective surface is preferably only partially reflective for laser signals having the predetermined lasing wavelength. As such, the microlaser can emit laser pulses having the predetermined lasing wavelength via the second reflective surface.
In one embodiment, the microlaser also includes a heat sink upon which at least the microresonator is mounted and a housing in which at least the microresonator is disposed. In this embodiment, the housing includes a window through which laser signals generated by the microresonator are emitted.