Abstract:
An improved tunable diode laser is capable of fast digital line selection over a broad wavelength spectrum, and uses no moving parts. A focusing element, such as a mirror or a lens, used in combination with a micromirror array serves as the retroreflector in a typical Littman-Metcalf laser cavity. This configuration provides arbitrary, simultaneous, and/or sequential line selection capability over a very broad wavelength range. The use of an individually-controllable micromirror array eliminates the high precision mechanical motion of a grating element and improves the overall durability and ruggedness of the device. The present invention can be integrated into any diode laser and has significant application in spectroscopy.

Description:
This invention was made with U.S. Government support under contract #F29601-98-C-0147 awarded by the Department of the United States Air Force. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of tunable diode lasers, and particularly to tunable diode laser configured as a Littman-Metcalf laser cavity with high speed line selection capability. 
     BACKGROUND OF THE INVENTION 
     Tuning for a monochromatic light, such as emission from a laser diode, is most commonly realized by using the Littman-Metcalf external cavity configuration with a diffraction grating and a rotating mirror used to select the specific wavelength. Using this approach, a high degree of precision in the rotation mechanism is required for wavelength selection, and the tuning process is very slow. This original approach as applied to dye laser technology is described in detail in the following non-patent prior art: M. G. Littman and H. J. Metcalf, Applied Optics, vol. 17, no. 14, 2224-2227, Jul. 15, 1978, and P. McNicholl and H. J. Metcalf, Applied Optics, vol. 24, no. 17, 2757-2761, Sep. 1, 1985. 
     Recently, a variety of techniques have been applied to tuning diode lasers. Versatility and low cost are desired specifically in spectroscopic applications. For example, a variety of U.S. Patents exist for laser tuning with alternative configurations of the mirror at the cavity end. U.S. Pat. No. 4,896,325 discloses an alternative cavity configuration in which a pair of mirrors with narrow discontinuities to provide reflective maxima bound the active cavity. These narrow bands of reflective maxima provides means for wavelength tuning which is actively controlled by a vernier circuit. U.S. Pat. No. 4,920,541 discloses an external laser cavity configuration of multiple resonator mirrors used to produce multiple wavelength emission from a single laser cavity simultaneously or with a very fast switching time. U.S. Pat. No. 5,319,668 discloses a tunable diode laser with a diffraction grating for wavelength separation and a moveable mirror at the cavity end for wavelength selection. The pivot points are designed to provide an internal cavity length specific for the production of several wavelengths. Alternative tuning arrangements are possible. U.S. Pat. No. 5,771,252 discloses an external cavity continuously tunable wavelength source utilizing a cavity end reflector moveable about a pivot point for simultaneous rotation and translation for wavelength selection. 
     In addition, several U.S. Patents disclose the use of alternative components in the laser cavity configuration in order to achieve wavelength tuning. U.S. Pat. No. 4,216,439 discloses a spectral line selection technique that utilizes a spectral line selection medium in the gain region of an unstable laser resonator cavity. U.S. Pat. No. 4,897,843 discloses a microprocessor-controlled laser system capable of broadband tuning capabilities by using multiple tuning elements each with progressively finer linewidth control. U.S. Pat. No. 5,276,695 discloses a tunable laser capable of multiple wavelength emission simultaneously or with a very fast switching time between lines by using a laser crystal in the cavity and fine rotation of the cavity end reflective element. U.S. Pat. No. 5,734,666 discloses a wavelength selection apparatus for a laser diode eliminating mechanical motion of a grating by utilizing a laser resonator for wavelength range selection and a piezoelectric-controlled crystal for specific wavelength selection. 
     Recent non-patent prior art also discloses relevant technology. In SPIE vol. 2482, pp. 269-274 by Zhongqi, Zhang, et al., a microprocessor-controlled tunable diode laser that utilizes a stepper motor to rotate the grating for wavelength tuning is described. In addition, in SPIE vol. 3098, pp. 374-381 by Uenishi, Akimoto, and Nagoka, a tunable laser diode with an external silicon mirror has been fabricated with MEMS technology and has wavelength tunability. 
     All of the prior art described above is limited in its performance by one or more of the following: requiring mechanical motion, small wavelength range tunability, or specified or limited line selection order. Especially for applications in spectroscopy, broadband wavelength tuning, arbitrary or simultaneous line selection, and limited or no mechanical motion are desired characteristics. 
     OBJECTS OF THE INVENTION 
     Therefore, it is the object of the invention disclosed herein to provide a tunable diode laser with broadband digital line selection capability. 
     It is also an object of the invention disclosed herein to provide a tunable diode laser with fast, broadband, digital line selection capability in arbitrary order. 
     It is also an object of the invention disclosed herein to provide a tunable diode laser with fast, broadband, digital line selection capability in arbitrary order that allows discrete switching between a predetermined series of wavelengths. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved tunable diode laser configuration over the prior art. A focusing element, such as a concave mirror or a lens, used in combination with a micromirror array serves as the retroreflector in a typical Littman-Metcalf laser cavity. This configuration provides both arbitrary and simultaneous line selection capability over a very broad wavelength range. The use of individually controllable micromirrors within the micromirror array eliminates the high precision mechanical motion of the grating element and improves the overall durability and ruggedness of the device. The present invention can be universally adapted to any laser diode device. The advantages that the present invention provides over the prior art are particularly significant in various spectroscopic applications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the invention believed to be novel are set forth in the associated claims. The invention, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is an isometric cutaway view of a digital micromirror linear array showing its individual micromirror elements&#39; interaction with incident light in both resting and +/−θ positions. 
     FIG. 2 is a schematic line drawing detailing the propagation of incident light on an individual micromirror array element in its resting (Off) position. 
     FIG. 3 is a schematic line drawing detailing the propagation of incident light on an individual micromirror array element in its first +θ position. 
     FIG. 4 is a schematic line drawing detailing the propagation of incident light on an individual micromirror array element in its second −θ position. 
     FIGS. 5 a  and  5   b  are a schematic view of the entire tunable diode laser system showing both incident and reflective light paths. 
    
    
     DETAILED DESCRIPTION 
     A primary component of the invention presently described is a digitally controllable micromirror array (MMA). Referring to FIG. 1, the MMA is, for this specific application, a linear array  100  of individually controllable micromirrors (micromirror elements). Alternative micromirror array configurations such as a two dimensional array or other geometric configuration could also be used, however the best mode includes a linear array. A micromirror element in its resting (off) position  104  will provide a normally reflected light wave  106  in response to a first incident light wave  108 . Similarly, a deflected micromirror element  110  in either of its +/−θ positions (see discussion of FIGS. 3 and 4 below) will provide a deflected light wave,  112  in response to a second incident light wave,  114 . 
     The control and reflection properties of the MMA linear array  100  are further described with reference to FIGS. 2,  3 , and  4 . FIG. 2 shows a representative micromirror array element  216  in its resting (off) position. A perpendicularly-incident light wave  218  reflects back along the same axis as a perpendicularly reflected light wave  220 . This resting (off) position is not utilized once the MMA  100  is powered on to either of its +/−θ positions, and so is shown here solely for reference. 
     FIG. 3 shows representative micromirror array element  216  deflected to its first position with a tilt of +θ from its resting position. Perpendicularly incident light wave  218  will reflect in a different direction as a first deflected light wave  322 . 
     FIG. 4 shows representative micromirror array element  216  deflected to its second position with a tilt of −θ from its resting position. Perpendicularly incident light wave  218  will reflect in a different direction as a second deflected light wave  424 . 
     Typically, the normal resting state as illustrated by FIG. 2 is not a position used in an optical system design. Most often, the MMA  100  is used in its first position as shown in FIG. 3 or in its second position as shown in FIG.  4 . FIG. 2 is shown here for the purpose of comparison and to establish the resting position from which the deflection is measured. In the system configuration presented here, the entire MMA  100  is placed at an angle to the incident light. When the individual micromirrors are in the first position, their tilted plane is normal to the incident light. 
     The MMA employed for this purpose is a well established in the prior art as a packaged array of individually-controlled micromirrors. The array of micromirrors is formed over an array of individually-addressable electrodes. There are various designs for the micromirror support mechanisms that provide various tilt or deflection directions, any of which is suitable for use in accordance with the invention. Only one will be described here, as a non-limiting illustration of the desired micromirror array functionality. 
     Each micromirror is supported by at least one hinge device and is controlled by at least one address electrode. There is a definable gap between the address electrode and the micromirror to allow for tilting or deformation of the micromirror. A common configuration is a squared micromirror supported by two hinges at two corners of the micromirror diagonally opposite each other, and controlled by address electrodes at the two remaining corners. Such a dual control system enables the bi-directional tilt or deformation of the micromirror element  216  as shown in FIGS. 3 and 4. The MMA  100  described in detail above is an integral component of the present invention. 
     FIG. 5 a  details the incident path of MMA  100  while FIG. 5 b  details the reflective path of MMA  100 . Referring now to FIG. 5 a,  a diode laser source  526  emits a set of output light waves  528 , over a broad spectrum of wavelengths that is collimated by collimating means  527  such as the illustrated lens set. A diffraction grating  530  is positioned in front of the light wave output  528  of laser diode source  526  and collimating means  527  so as to diffract output light waves  528 . The angle of diffraction is dependent on wavelength of light. The majority of the light is diffracted into the first order and diffracted light waves  532  are passed through or reflected from a focusing element  534 , such as a lens or a mirror, for example, in order to transform the diffraction angle of diffracted light waves  532  to linear spatial dispersion according to wavelength. 
     As such, a set of focused light waves of a first wavelength  536  and a set of focused light waves of a second wavelength  538  and so on, are individually brought to points of focus in the same plane but linearly displaced from one another. All wavelengths present in the output light waves  528  of the diode laser source  526  are separated in this manner. Although the total number of wavelengths present can be numerous, only two are shown in the figure for ease of discussion. 
     The focal plane of focusing element  534 , which is also the plane in which the light waves of first and second wavelengths,  536  and  538  respectively, are focused, is where the micromirror array  100  is located. As shown in FIG. 5 a,  the MMA  100  is slightly tilted in this plane in order to achieve a normal reflection back into the system when the micromirrors are placed in their first position. Each of the separated wavelengths, such as focused light waves of first wavelength and second wavelength,  536  and  538  respectively, is focused on and thus controlled by a single micromirror array element such as  104  in FIG.  1 . 
     As stated, each wavelength is associated with an individual element of the MMA  100 . Control module  540  sends a control signal  542  to the MMA  100  setting the configuration of its individual elements to control the selection of wavelengths of the diode laser source  526 . 
     Any representative micromirror array element  216  upon which a wavelength not selected for lasing is focused, is moved into the second (−θ) position, as shown in FIG.  4 . Now referring to FIG. 5 b,  the light  544  reflected from such a −θ micromirror element is deflected away from focusing element  534  and collected by a light trap  546 . (Only one such deflecting −θ micromirror element is illustrated here. It is understood that in practice a plurality of such deflections can and do occur.) 
     In order to select a wavelength for lasing, the representative micromirror array element  216  upon which the desired wavelength is focused, is switched to the first (+θ) position, as represented in FIG.  3 . Again referring to FIG. 5 b,  such a +θ micromirror array element  216  will reflect the selected wavelength light  548  back through focusing element  534  to the diffraction grating  530  and provide selected wavelength input  550  into the laser diode source  526  to promote amplification and thus lasing at that particular wavelength. Once amplified, the laser output  552  is produced along the zeroth order of the diffraction grating  530 . 
     As a reference for the description of the deflection of representative micromirror array element  216  and for a complete understanding of its incorporation into the present invention, the following convention is used. Since the tilting direction of the micromirror array elements  216  is somewhat arbitrary and dependent on the actual laser system layout, the first position will always be defined to represent deflection into the laser cavity. Similarly, the second position will always be defined to represent deflection away from the laser cavity. 
     Specific wavelengths within the spectral range of the laser diode source  526  can be selected for amplification and laser output by deflecting their associated micromirror array element  216  into the first position. Similarly, specific wavelengths within the spectral range of the laser diode source  526  can be deselected for amplification by deflecting their associated micromirror array element  216  to their second position. This is the most basic system configuration in which the present invention is implemented. The control module  540  would serve as the user interface. Wavelength selection by the user, would be converted by the control module  540  to a specific MMA  100  configuration of deflection into the first and second positions, and a control signal  542  sent to the MMA  100  for implementation. 
     This basic system implementation can be naturally expanded to allow for amplification of multiple wavelengths at the same time by simultaneously deflecting all micromirror array elements  216  desired for amplification to the first position, and all others to the second position. 
     Moreover, the present invention can also be configured for fast digital wavelength selection. This system configuration can be implemented through the control module  540  by programming a series of control signals  542  to be sent to the MMA  100  to control and activate selected micromirrors and thus their corresponding wavelengths in a specific sequence. This allows for switching between a selected wavelength or set of wavelengths and the next within the switching time of the micromirror array elements, typically on the order of 20 μsec. 
     An alternative embodiment of the present invention utilizes alternative technology to provide the function of the MMA  100  in the present invention. The MMA  100  can be replaced by a combination of a mirror and a mask array with all other system elements remaining the same. The mask array comprises individual mirror mask elements that can each turn on or off selected wavelengths focused on the mirror region behind them by allowing or preventing light from passing through them. Due to the mechanical nature typical of such mask arrays and inherent size limitations, this alternative embodiment is not best suited for laser diode and/or spectroscopic applications, but may become more practical for such use as the technology for mask array mirrors is improved, and so is considered to be within the scope of this invention and its associated claims. Electronic mask arrays, which become transparent under one electronic condition (digital “0” current) and opaque under a second electronic condition (digital “1” current) are also suited to this purpose when coupled with a suitable mirror. Also suitable to replace MMA  100  would be an array of “semi-mirrors” which are reflective under one electronic condition and transparent or opaque (non-reflective) under a second electronic condition. A classic example of this technology is liquid crystals which are used in a wide variety of display applications from computer monitors to digital wristwatches. Liquid crystals in various configurations can be transparent, opaque or reflective as described above. 
     Similarly, another alternative embodiment of the present invention replaces MMA  100  with a combination of a mirror and a spatial light modulator with all other system elements remaining the same. This configuration would also provide individual mirror elements whose light path could be controlled. As in the alternative embodiment above, this configuration is not best suited for laser diode and spectroscopic applications primarily due to signal loss, but may also become more practical with advances in this technology and so is considered to be within the scope of this invention and its associated claims. 
     The key point, is that MMA  100 , or whatever other device is used as a substitute (equivalent) for MMA  100 , must provide the functional ability to selectively reflect back in a given direction, or not reflect back in that given direction, light which impinges upon each localized region of that device. Generically, the MMA  100 , and any other suitable device which provides this fundamental functionality, shall be referred to as a “locally-controllable reflectivity array means.” The local, selectable reflective elements of such an array (such as the individual micromirrors of MMA  100 ) will be generically referred to as “localized reflective elements” of that array. 
     The advantages of the present invention as compared to prior art listed above include the applicability to any wavelength laser diode, no mechanical movement in the preferred embodiment, fast digital laser line selection, arbitrary laser line selection, and multiple laser line selection at once all in one tunable diode laser system. 
     While only certain preferred features of the invention have been illustrated and described, many modifications, changes and substitutions will occur to those skilled in the art. It is, therefore, to be understood that this disclosure and its associated claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.