Patent Application
20080310465
The present invention relates generally to semiconductor laser systems and methods, and more particularly to a method and a laser system for stabilized frequency doubling.
A laser is an optical source that emits photons in a coherent beam. Laser light is typically a single wavelength or color, and emitted in a narrow beam. Laser action is explained by the theories of quantum mechanics and thermodynamics. Many materials have been found to have the required characteristics to form the laser gain medium needed to power a laser, and these have led to the invention of many types of lasers with different characteristics suitable for different applications.
A semiconductor laser is a laser in which the active medium is a semiconductor. A common type of semiconductor laser is formed from a p-n junction, a region where p-type and n-type semiconductors meet, and powered by injected electrical current. As in other lasers, the gain region of the semiconductor laser is surrounded by an optical cavity. An optical cavity is an arrangement of mirrors, or reflectors that form a standing wave resonator for light waves.
Optical resonators are often called cavities, and the terms are often used interchangeably in optics. Use of the term cavity does not imply a vacuum or air space. A cavity as used in optics may be within a solid crystal. An optical cavity (or optical resonator) is an arrangement of optical components, which allows a beam of light to circulate. In a simple form of semiconductor laser; for example a laser diode, an optical cavity may be formed in epitaxial layers, such that the light is confined to a relatively narrow area perpendicular (and parallel) to the direction of light propagation. There are two basic types of cavities: standing-wave or linear cavities, where the light bounces back and forth between two end reflectors; and ring cavities, where the light may make round trips in two different directions.
The color or frequency of the emitted light may depend on the gain medium. Another method is called frequency doubling. In this method, a fundamental laser frequency is introduced into a nonlinear medium, and a portion of the fundamental frequency is doubled. Frequency doubling in nonlinear material, also called second harmonic generation (SHG), is a nonlinear optical process, in which photons interacting with a nonlinear material are effectively “combined” to form new photons with twice the energy and, therefore, twice the frequency and half the wavelength of the initial photons.
Typically, lasers are developed and tuned to emit a narrow frequency of light with a portion of the laser light fed back into the gain region. Many observations and calculations of the effects that can occur in semiconductor lasers subjected to reflections external to the gain region have been made. Principally, five regimes of feedback effects in lasers have been defined.
The regimes are defined by the behavior of the frequency spectra of the laser subjected to different feedback power level ratios. Generally, these five regimes of operation are experimentally well defined, and the transitions between them may be easily identified. For example, refer to R. W. Tkach et al., “Regimes of Feedback Effects in 1.5-μm Distributed Feedback Lasers,” Journal of Lightwave Technology, vol. LT-4 (11), pp. 1655-1661, November, 1986.
Regime I, the lowest level of feedback, shows a narrowing or broadening of the frequency emission line, depending on the phase of the feedback. The phase of the feedback is critical in Regime I. Any slight change in phase causes emission linewidth instability. Regime II shows instabilities in emission linewidth, depending on the distance to the external reflector. The broadening, which is observed at the lowest levels for out of phase feedback, changes to an apparent splitting of the emission line, arising from rapid mode hopping. The magnitude of the splitting depends on the strength of the feedback and on the distance to the reflector.
Regime III is entered as the feedback is increased further. The emission linewidth in Regime III does not depend on the distance to the reflection; the mode hopping is suppressed, and the laser is observed to operate on a single narrow line. This regime may occupy only a small range of feedback power ratio; for example, from −45 dB to −39 dB, and, consequently, the laser remains sensitive to other reflections of comparable or greater magnitude.
Regime IV is at a feedback level that does not depend on the distance to the reflection and may occur for a distributed feedback laser; for example from −38 dB to −8 dB. The transition from Regime III to Regime IV may occur at higher feedback power ratios for higher laser powers. Referring to
Regime V is defined at the highest levels of feedback (typically greater than −10 dB) with a narrow linewidth emission observed. Typically, it is necessary to use an antireflection coat on the laser facet to reach this regime. In this regime, the laser operates as a long cavity laser with a short active region. If there is sufficient frequency selectivity in the cavity, the laser operates on a single longitudinal mode with narrow linewidth emission for all phases of the feedback. In regime V, the laser is relatively insensitive to additional external optical perturbations.
An anti-reflection coating may be needed in order to achieve a higher power ratio feedback. An anti-reflection coating (AR coating) is a dielectric coating applied to an optical surface in order to reduce the optical reflectivity of that surface in a certain wavelength range. In most cases, the basic operation principle is to introduce one or more additional optical interfaces so that the reflected waves from all the different interfaces largely cancel each other by destructive interference.
One disadvantage of the prior art is that stability of the laser may be dependent upon the cavity configuration. Any slight variation in distance or temperature may cause an erratic laser output. Another disadvantage of the prior art is in frequency doubling, also termed second harmonic generation (SHG), a single frequency emission system may be an advantage for communication signals but not an advantage for illumination uses or display systems.
These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by preferred embodiments of the present invention, which may emit a plurality of stable second harmonic light in closely related frequencies, which are generally unaffected by small variations in external cavity length and temperature.
Illustrative embodiments provide a semiconductor laser system that comprises a semiconductor gain region operating within the coherence collapse regime, an intra-cavity nonlinear optical medium, and a feedback reflector. The intra-cavity laser system comprises a semiconductor gain region operating in the coherence collapse regime and producing broad frequency fundamental light, a nonlinear resonator optically coupled to the semiconductor gain region, and a coherence collapse reflector optically coupled to the nonlinear resonator and to the semiconductor gain region. The nonlinear resonator doubles a first portion of the broad frequency fundamental light and emits a plurality of second harmonic light frequencies external to the laser system. A second portion of the broad frequency fundamental light is reflected into the semiconductor gain region with a feedback power ratio sufficient to cause the semiconductor gain region to operate in the coherence collapse regime.
Illustrative embodiments further include a method of generating a plurality of second harmonic light frequencies in an intra-cavity laser. The method comprises operating a laser gain region in coherence collapse and producing broad frequency fundamental light in the laser gain region. A first portion of the broad frequency fundamental light is fed back into the laser gain region; this forces the laser gain region into coherence collapse. Further, a second portion of the broad frequency fundamental light is doubled to form a plurality of second harmonic light frequencies. The plurality of second harmonic light frequencies is emitted from the intra-cavity laser.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.
The present invention will be described with respect to preferred embodiments in a specific context, namely an edge emitting distributed Bragg reflector as the laser gain region with a total internal reflection crystal resonator, and a coherence collapse feedback reflector. Other laser types and configurations may also be applied to the illustrative embodiments.
Optical communication systems, as well as other laser applications, may require a narrow linewidth emission; therefore, lasers have been typically operated in the feedback power ratio of Regime V or Regime III. This enables operation with a narrow linewidth emission. Illustrative embodiments provide a system and a method of operating an extended cavity laser in the feedback power ratio of Regime IV. The broadened frequency emission of the gain region operating in the coherence collapse regime beneficially increases the power and stability of the doubled frequency emission. Operating in the coherence collapse regime, the gain region produces an infrared light across broad frequency emission linewidth (in the range of 50 GHz). Within this broad emission, there may then be a plurality of frequencies that are mode matched to a nonlinear resonator. The nonlinear resonator may act as a frequency doubling resonator. The nonlinear resonator then doubles a portion of each of the accepted modes of the broad frequency fundamental light and emits a plurality of second harmonic frequencies of each of the accepted modes of the fundamental light. In this example, frequencies are blue or green light. A portion of the miss-mode matched infrared light is reflected back into the gain region. An antireflective coating facet on the laser gain region may aid in increasing the amount of feedback power into the laser gain region, thereby allowing a sufficient feedback power ratio to force the gain region into the coherence collapse regime. To those of ordinary skill in the art it is obvious that there are some fractional light losses in these operations and that while not addressed directly, these fractional light losses are within the scope of this invention. The plurality of second harmonic frequency light is emitted from the laser cavity.
With reference now to
a shows a laser spectrum with a feedback power ratio of essentially −40 dB. Relaxation oscillation side-bands 100 first appear in this spectrum.
The laser gain region 202 may be an edge emitting laser, a grating outcoupled surface emitting (GSE) laser, or a vertical cavity surface emitting (VCSEL) laser. Laser gain region 202 may be configured with, for example, a distributed Bragg reflector; a lateral distributed Bragg reflector; or a Fabry-Perot resonator, among other configurations. Laser gain region 202 may be a distributed feedback laser.
Nonlinear resonator 204 may include a nonlinear crystal, such as KNbO3 or Cr3+:LiSrAlF6 (Cr:liSAF). Those of ordinary skill in the art are aware of many nonlinear materials for use in a nonlinear resonator. All of these materials are within the scope of this invention. Nonlinear crystal resonator 204 may comprise mirror components as well as a nonlinear crystal. The mirrors may be a part of nonlinear crystal resonator 204 in which successive passes of the fundamental frequency beam, in this example an infrared beam, reinforce each other so that the infrared intensity circulating inside the resonator becomes many times higher than the intensity incident upon the resonator. Nonlinear crystal resonator 204 may be, for example, a standing wave resonator or a bow-tie type ring resonator, as well as other types of nonlinear resonators known to those of ordinary skill in the art. Nonlinear crystal resonator 204 may alternately be of a total internal reflection configuration, without reflectors or mirrors external to the nonlinear crystal.
A coherence collapse feedback reflector 206 may be a single reflector or a system of reflectors that may reflect sufficient power into the laser gain region to force the laser into coherence collapse. Feedback reflector 206 reflects light with a feedback power ratio in the coherence collapse regime, back into the laser gain region 202 of laser cavity 100. As discussed in the background section above, the coherence collapse regime feedback range is defined by the distinctive appearance of the laser spectra in the regime. At the lower feedback power ratio, transition to the coherence collapse regime satellite modes, separated from the main mode by the relaxation oscillation frequency, occurs. As the feedback power ratio is increased within the coherence collapse regime, the laser spectrum is broadened. As the feedback power ratio is increased further than the coherence collapse regime, into Regime V, the spectrum narrows. For a distributed feedback laser, the coherence collapse regime is from about −38 dB to −8 dB. Different laser configurations may perform in the coherence collapse regime with a feedback power ratio in a different range.
The spectrum of light path 210 and 212 (not shown) may be the frequencies not in resonance with the nonlinear resonator and other reflected light.
While the gain region of the IR laser is shown in this embodiment as edge emitting laser 402 with a lateral distributed Bragg reflector 404, many other embodiments are possible. For example, the gain region of the IR laser may be a surface emitting laser VSCEL or a grating outcoupled surface emitting laser (GSE), or a distributed feedback laser, among other types of semiconductor lasers. Laser cavity 400 may also have phase tuning and frequency tuning capabilities.
Nonlinear crystal 412 is a frequency doubling crystal, such as frequency doubling crystal resonator 204 in
Infrared reflector 416 reflects infrared frequencies back into laser gain region 402. The reflected IR light follows the same light path back into the IR gain region, providing a sufficient feedback power ratio to cause the IR gain region to function in the coherence collapse regime.
Further, an anti-reflective coating 418 may coat the emitting edge of gain region 402. The coating permits the returning reflected IR beam to enter the IR gain region with negligible reflection.
While the gain region of the IR laser is shown in this embodiment as edge emitting laser 502 with a distributed Bragg reflector 504, again many other embodiments are possible. For example, the gain region of the IR laser may be a surface emitting laser VSCEL or a grating outcoupled surface emitting laser (GSE), or a distributed feedback laser, among other types of semiconductor lasers. Laser cavity 500 may also have phase tuning and frequency tuning capabilities.
Nonlinear crystal 518 is a frequency doubling crystal, such as frequency doubling crystal resonator 204 in
Infrared reflector 516 reflects infrared frequencies back into laser gain region 502. The reflected IR light follows the same light path back into the infrared gain region, providing a sufficient feedback power ratio to cause the infrared gain region to function in the coherence collapse regime.
Further, an anti-reflective coating 522 may coat the emitting edge of gain region 502. The coating permits the returning reflected IR beam to enter the IR gain region with negligible reflection.
Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. For example, many of the features and functions discussed above can be implemented in alternative components. As another example, it will be readily understood by those skilled in the art that the nonlinear crystal resonator may be varied by type and configuration while remaining within the scope of the present invention.
Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.
