Abstract:
Start-up methods for frequency converted light sources and projector systems comprising frequency converted light sources are described herein. The start-up methods generally comprise modulating the frequency converted light source over three degrees of freedom (two spatial dimensions and one wavelength dimension). Specifically, fast oscillation of an axis of an adjustable optical component is performed simultaneously with fast oscillation of a wavelength of the semiconductor laser while a second axis of the adjustable optical component is incrementally stepped and the output intensity of the frequency converted light source is monitored for each step. This start-up method allows for three linear searches to be used to rapidly locate the appropriate control settings for the frequency converted light source.

Description:
CROSS REFERENCE 
       [0001]    The present disclosure claims priority to U.S. Provisional Application 61/369,346, filed Jul. 30, 2010 and entitled “Start-Up Methods for Frequency Converted Light Sources”. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present disclosure generally relates to semiconductor lasers, laser controllers, frequency converted light sources, and other optical systems incorporating semiconductor lasers. More specifically, the present invention relates to methods for aligning frequency converted light sources that include, inter alia, a semiconductor laser optically coupled to a second harmonic generation (SHG) crystal, or another type of wavelength conversion device, with adaptive optics. 
         [0004]    2. Technical Background 
         [0005]    Short wavelength light sources can be formed by combining a single-wavelength semiconductor laser, such as an infrared or near-infrared distributed feedback (DFB) laser, distributed Bragg reflector (DBR) laser, or Fabry-Perot laser, with a light wavelength conversion device, such as a second harmonic generation (SHG) crystal. Typically, the SHG crystal is used to generate higher harmonic waves of the fundamental laser signal. To do so, the lasing wavelength is preferably tuned to the spectral center of the wavelength converting SHG crystal and the output of the laser is preferably aligned with the waveguide portion at the input facet of the wavelength converting crystal. 
         [0006]    Waveguide optical mode field diameters of typical SHG crystals, such as MgO-doped periodically poled lithium niobate (PPLN) crystals, can be in the range of a few microns while semiconductor lasers used in conjunction with the wavelength conversion device may comprise single-mode waveguides having diameters of approximately the same dimensions. As a result, properly aligning the beam from the semiconductor laser with the waveguide of the SHG crystal such that the output of the SHG crystal is optimized may be a difficult task which requires that the position of the beam of the semiconductor laser be precisely controlled along two axes on the input face of the SHG crystal. 
         [0007]    Similarly, the phase matching bandwidth of SHG crystals are typically narrow, generally less than 1 nm. For example, for a 12 mm long PPLN crystal, the phase matching bandwidth may be about 0.16 nm. As such, the wavelength of the semiconductor laser must be precisely controlled to properly align the semiconductor laser with the SHG crystal and obtain the desired output intensity and wavelength from the SHG crystal. 
       SUMMARY 
       [0008]    In one embodiment shown and described herein, a start-up method for a frequency converted light source comprising a semiconductor laser, a wavelength conversion device, and an adjustable optical component arranged to optically couple an output beam of the semiconductor laser with an input facet of the wavelength conversion device is described. The start-up method includes directing the output beam of the semiconductor laser onto an input facet of the wavelength conversion device with the adjustable optical component. The wavelength of the output beam of the semiconductor laser is modulated over a wavelength alignment range corresponding to at least a portion of the phase-matching bandwidth of a waveguide portion of the wavelength conversion device as the output intensity of a wavelength converted output beam emitted from the wavelength conversion device is monitored. The adjustable optical component is oscillated over a first angular range about a first scanning axis such that the output beam of the semiconductor laser is dithered over the input facet of the wavelength conversion device on a first dithering trace. The angular orientation of the adjustable optical component is adjusted about a second scanning axis to vary a position of the first dithering trace over the input facet of the wavelength conversion device on a first scan line as the wavelength of the output beam is modulated and the adjustable optical component is oscillated about the first scanning axis. A first alignment set point on the first scan line is determined based on the output intensity of the wavelength converted output beam as the position of the first dithering trace is varied over the first scan line. Thereafter, the adjustable optical component is oscillated over a second angular range about a second scanning axis such that the output beam of the semiconductor laser is dithered over the input facet of the wavelength conversion device on a second dithering trace centered on the first alignment set point. The angular orientation of the adjustable optical component is adjusted about the first scanning axis to vary a position of the second dithering trace over the input facet of the wavelength conversion device on a second scan line as the wavelength of the output beam is modulated and the adjustable optical component is oscillated about the second scanning axis. A second alignment set point on the second scan line is determined based on the output intensity of the wavelength converted output beam as the position of the second dithering trace is varied over the second scan line, wherein the first alignment set point and the second alignment set point define a position where the output beam of the wavelength conversion device is aligned with the waveguide portion of the wavelength conversion device. The modulation of the wavelength of the output beam of the semiconductor laser is terminated an the output beam of the semiconductor laser on the waveguide portion of the wavelength conversion device using the first alignment set point and the second alignment set point. 
         [0009]    According to another embodiment shown and described herein, a start-up method for a projection system comprising a first light source, a second light source and a frequency converted light source comprising a semiconductor laser, a wavelength conversion device, and an adjustable optical component arranged to optically couple a output beam of the semiconductor laser with an input facet of the wavelength conversion device is disclosed. The start-up method generally includes supplying the first light source with a first gain current I 1  at a first modulated gain frequency F I1 , supplying the second light source with a second gain current I 2  at a second modulated frequency F 12  and supplying the frequency converted light source with a constant gain current I C . A combined output intensity of the first light source, the second light source and the frequency converted light source is monitored at a frame rate F 0  of the projection system. The output beam of the semiconductor laser is directed onto an input facet of the wavelength conversion device with the adjustable optical component and a wavelength of the output beam of the semiconductor laser is modulated over a wavelength alignment range corresponding to at least a portion of the phase-matching bandwidth of a waveguide portion of the wavelength conversion device. The adjustable optical component is oscillated over a first angular range about a first scanning axis such that the output beam of the semiconductor laser is dithered over the input facet of the wavelength conversion device on a first dithering trace, wherein the adjustable optical component is oscillated about the first scanning axis at a first oscillation frequency F 1 =(N+¼)*F 0 , where 2≦N≦10. The angular orientation of the adjustable optical component is adjusted about a second scanning axis to vary a position of the first dithering trace over the input facet of the wavelength conversion device on a first scan line. A first alignment set point on the first scan line is determined by filtering the combined output intensity to isolate a portion of the combined output intensity attributable to an output intensity of a wavelength converted output beam emitted from the wavelength conversion device as the position of the first dithering trace is varied over the first scan line. Thereafter, the adjustable optical component is oscillated over a second angular range about a second scanning axis such that the output beam of the semiconductor laser is dithered over the input facet of the wavelength conversion device on a second dithering trace centered on the first alignment set point, wherein the second angular range is less than the first angular range, and the adjustable optical component is oscillated about the second scanning axis at a second oscillation frequency F 2 =F 1 . The angular orientation of the adjustable optical component is adjusted about the first scanning axis to vary a position of the second dithering trace over the input facet of the wavelength conversion device on a second scan line. A second alignment set point on the second scan line is determined by filtering the combined output intensity to isolate a portion of the combined output intensity attributable to an output intensity of the wavelength converted output beam emitted from the wavelength conversion device as the position of the second dithering trace is varied over the second scan line, wherein the first alignment set point and the second alignment set point define a position where the output beam of the wavelength conversion device is aligned with the waveguide portion of the wavelength conversion device. The modulation of the wavelength of the output beam of the semiconductor laser and the output beam of the semiconductor laser is positioned on the waveguide portion of the wavelength conversion device using the first alignment set point and the second alignment set point. 
         [0010]    Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings. 
         [0011]    It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operations of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  schematically depicts one embodiment of an frequency converted light source shown and described herein; 
           [0013]      FIG. 2  schematically depicts a semiconductor laser for use in conjunction with one or more embodiments of the frequency converted light sources shown and described herein; 
           [0014]      FIGS. 3A and 3B  schematically depict a wavelength conversion device for use in conjunction with one or more embodiments of the frequency converted light sources shown and described herein; 
           [0015]      FIGS. 4A and 4B  schematically depict one embodiment of a start-up method for a frequency converted light source according to one or more embodiments shown and described herein; 
           [0016]      FIG. 5  schematically depicts a laser projector system for use in conjunction with one or more of the start-up methods described herein; 
           [0017]      FIGS. 6A and 6B  graphically depict the output intensity of a wavelength converted output beam as a function of time according to one embodiment of a start-up method for a frequency converted light source shown and described herein; and 
           [0018]      FIG. 7  graphically depicts the output intensity of a wavelength converted output beam as a function of time according to one embodiment of a start-up method for a frequency converted light source shown and described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Reference will now be made in detail to embodiments of start-up methods for frequency converted light sources, examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. One embodiment of a frequency converted light source for use in conjunction with the control methods described herein is shown in  FIG. 1 . The frequency converted light source generally comprises a semiconductor laser, adaptive optics, a wavelength conversion device and a package controller. The output of the semiconductor laser is optically coupled into the input facet of the wavelength conversion device with the adaptive optics. The package controller is electrically coupled to the semiconductor laser and the adaptive optics and configured to control the output of the semiconductor laser and the alignment of the semiconductor laser with the wavelength conversion devices. Frequency converted light sources and start-up methods for aligning the components of the frequency converted light sources will be further described herein. 
         [0020]      FIG. 1  generally depicts one embodiment of a frequency converted light source  100  described herein. It should be understood that the solid lines and arrows indicate the electrical interconnectivity of various components of the frequency converted light source. These solid lines and arrows are also indicative of electrical signals propagated between the various components including, without limitation, electronic control signals, data signals and the like. Further, it should also be understood that the dashed arrows indicate light beams, such as infrared and/or visible light beams, emitted by the semiconductor laser and the wavelength conversion device. 
         [0021]    Referring initially to  FIG. 1 , the concepts of particular embodiments of the methods described herein may be conveniently illustrated with general reference to the frequency converted light source  100  which includes, for example, a semiconductor laser  110  optically coupled to a wavelength conversion device  120 . An output beam  119  of the semiconductor laser  110  is coupled into the waveguide portion of wavelength conversion device  120  using adaptive optics  140 . The wavelength conversion device  120  converts the output beam  119  (such as an infrared light beam) into higher harmonic waves and outputs a wavelength converted output beam  128 . This type of frequency converted light source is particularly useful in generating shorter wavelength laser beams from longer wavelength semiconductor lasers and can be used, for example, as a visible laser source for laser projection systems. 
         [0022]    One embodiment of a semiconductor laser  110  is schematically illustrated in  FIG. 2 . The semiconductor laser generally comprises a wavelength selective section  112 , a phase section  114 , and a gain section  116 . The wavelength selective section  112 , which may also be referred to as the distributed Bragg reflector or DBR section of the semiconductor laser  110 , typically comprises a first order or second order Bragg grating positioned outside the active region of the laser cavity. This section provides wavelength selection, as the grating acts as a mirror whose reflection coefficient depends on the wavelength. The gain section  116  of the semiconductor laser  110  provides the major optical gain of the laser and the phase section  114  creates an adjustable optical path length or phase shift between the gain material of the gain section  116  and the reflective material of the wavelength selective section  112 . The wavelength selective section  112  may be provided in a number of suitable alternative configurations that may or may not employ a Bragg grating. 
         [0023]    Respective control electrodes  113 ,  115 ,  117  are incorporated in the wavelength selective section  112 , the phase section  114 , the gain section  116 , or combinations thereof, and are merely illustrated schematically in  FIG. 2 . It is contemplated that the electrodes  113 ,  115 ,  117  may take a variety of forms. For example, the control electrodes  113 ,  115 ,  117  are illustrated in  FIG. 2  as respective electrode pairs but it is contemplated that single electrode elements in one or more of the sections  112 ,  114 ,  116  will also be suitable. The control electrodes  113 ,  115 ,  117  can be used to inject electrical current into the corresponding sections  112 ,  114 ,  116  of the semiconductor laser  110 . For example, in one embodiment, current injected in to the wavelength selective section  112  of the semiconductor laser  110  can be used to control the wavelength λ 1  of the output beam  119  emitted from the output facet  118  of the semiconductor laser  110  by altering the operating properties of the laser. The injected current may be used to control the temperature of the wavelength selective section  112  or the index of refraction of the wavelength selective section. Accordingly, by adjusting the amount of current injected into the wavelength selective section, the wavelength of the output beam  119  emitted by the semiconductor laser may be varied. Current injected into the phase section  114  or gain section  116  may be similarly used to control the output of the semiconductor laser  110 . 
         [0024]    Referring now to  FIGS. 3A-3B , an input facet  132  ( FIG. 3A ) and cross section ( FIG. 3B ) of one embodiment of a wavelength conversion device  120  is schematically depicted. The wavelength conversion device  120  comprises a bulk crystal material  122 , such as lithium niobate, with an embedded waveguide portion  126  formed from, for example, MgO-doped lithium niobate. The waveguide portion  126  extends between an input facet  132  and an output facet  133 . When the wavelength conversion device  120  is a PPLN crystal, the waveguide portion  126  of the PPLN crystal may have dimensions (e.g., height and width) on the order of 5 microns. In the embodiment of the wavelength conversion device  120  shown in  FIGS. 3A and 3B , the wavelength conversion device  120  comprises a waveguide portion  126  which is embedded in a low refractive index layer  130  disposed between two slabs of bulk crystal material  122 A,  122 B. Referring to  FIG. 3A , each slab of bulk crystal material  122 A,  122 B may be substantially rectangular or square in cross section. Typical cross sectional dimensions of the input facet  132  are on the order of 500-1500 microns. 
         [0025]    Referring to  FIG. 3B , when an output beam having a first wavelength λ 1  is directed into the waveguide portion  126  of the wavelength conversion device  120 , such as the output beam  119  of the semiconductor laser  110 , the output beam may be propagated through the waveguide portion  126  of the wavelength conversion device  120  where at least a portion of the output beam is converted to a second wavelength λ 2 . The wavelength conversion device  120  emits a wavelength converted output beam  128  from the output facet  133 . The wavelength converted output beam  128  may comprise converted wavelength light (e.g., light having a second wavelength λ 2 ) as well as unconverted light (e.g., light having the first wavelength λ 1 ). For example, in one embodiment, the output beam  119  produced by the semiconductor laser  110  and directed into the waveguide portion  126  of the wavelength conversion device  120  has a wavelength of about 1060 nm (e.g., output beam  119  is an infrared light beam). In this embodiment, the wavelength conversion device  120  converts at least a portion of the infrared light beam to visible light such that the waveguide portion  126  of the wavelength conversion device emits a wavelength converted output beam  128  a wavelength of about 530 nm (e.g., visible green light) in addition to light having a wavelength of about 1060 nm. 
         [0026]    In another embodiment, when an output beam having a first wavelength λ 1  is directed onto the input facet  132  of the wavelength conversion device, but not into the waveguide portion  126  of the wavelength conversion device  120  (e.g., the output beam is incident on the bulk crystal material  122  of the wavelength conversion device  120 ), the output beam is guided through the bulk crystal material  122  of the wavelength conversion device  120  and emitted from the output facet  133  without being converted to a second wavelength λ 2  due to the phenomenon of total internal reflection. For example, when the output beam  119  incident on the non-waveguide portion or bulk crystal material  122  of the wavelength conversion device  120  has a first wavelength λ 1  of 1060 nm (e.g., the output beam  119  is an infrared beam), the wavelength converted output beam  128  emitted from the output facet  133  of the wavelength conversion device will also have a wavelength of 1060 nm since little or no wavelength conversion occurs in the bulk crystal material  122 . 
         [0027]    Referring now to  FIG. 1 , one embodiment of a frequency converted light source  100  is depicted in which the semiconductor laser  110 , the wavelength conversion device  120  and the adaptive optics  140  are oriented in a folded configuration. More specifically, the output of the semiconductor laser  110  and the input of the wavelength conversion device  120  are positioned on different optical axes. The output beam  119  emitted by the semiconductor laser  110  is coupled into the waveguide portion of the wavelength conversion device  120  with adaptive optics  140 . Specifically, the output beam  119  is redirected from its initial pathway in order to order to facilitate coupling the output beam  119  into the waveguide portion of the wavelength conversion device  120 . Accordingly, in this embodiment, the adaptive optics  140  may comprise an adjustable optical component  144 , specifically an adjustable mirror, and a lens  142 . 
         [0028]    The lens  142  of the adaptive optics  140  may collimate and focus the output beam  119  emitted by the semiconductor laser  110  into the waveguide portion  126  of the wavelength conversion device  120 . 
         [0029]    The adjustable optical component  144  may be rotated about a first scanning axis substantially parallel to the x-axis depicted in  FIG. 1  and about a second scanning axis substantially parallel to the y-axis to introduce angular deviation in the output beam  119 . The adjustable optical component  144  may comprise a mirror portion and an actuator portion and the adjustable optical component  144  may be rotated about either scanning axis by adjusting the actuator portion of the adjustable optical component. 
         [0030]    For example, in one embodiment, the adjustable optical component  144  may comprise one or more movable micro-opto-electromechanical systems (MOEMS) or micro-electro-mechanical system (MEMS) operatively coupled to a mirror. The MEMS or MOEMS devices may be configured and arranged to vary the position of the output beam  119  on the input facet of the wavelength conversion device  120 . Use of MEMS or MOEMS devices enables adjustment of the output beam  119  to be done extremely rapidly over large ranges. For example, a MEMS mirror with a +/−1 degree mechanical deflection, when used in conjunction with a 3 mm focal length lens, may allow the output beam to be angularly displaced +/−100 μm on the input face of the wavelength conversion device  120 . The adjustment of the output beam may be done at frequencies on the order of 100 Hz to 20 kHz due to the fast response time of the MEMS or MOEMS device. 
         [0031]    In the frequency converted light source  100  illustrated in  FIG. 1 , the adjustable optical component  144  is a micro-opto-electromechanical mirror incorporated in a relatively compact, folded-path optical system. In the illustrated configuration, the adjustable optical component  144  is configured to fold the optical path such that the optical path initially passes through the lens  142  to reach the adjustable optical component  144  as a collimated or nearly collimated beam and subsequently returns through the same lens  142  to be focused on the wavelength conversion device  120 . This type of optical configuration is particularly applicable to wavelength converted laser sources where the cross-sectional size of the output beam generated by the semiconductor laser  110  is close to the size of the waveguide on the input face of the wavelength conversion device  120 , in which case a magnification close to one would yield optimum coupling in focusing the output beam on the input face of the wavelength conversion device  120 . For the purposes of defining and describing this embodiment of the frequency converted light source  100 , it is noted that reference herein to a “collimated or nearly collimated” beam is intended to cover any beam configuration where the degree of beam divergence or convergence is reduced, directing the beam towards a more collimated state. 
         [0032]    Still referring to  FIG. 1 , the frequency converted light source  100  may also comprise a filtering window  179  positioned proximate the output of the wavelength conversion device  120 . The filtering window prevents non-wavelength converted light (i.e., IR light in the embodiments shown and described herein) from being emitted from the frequency converted light source  100 . Accordingly, it will be understood that, once the wavelength converted output beam  128  exits the frequency converted light source  100 , the wavelength converted output beam  128  only contains wavelength converted light. 
         [0033]    The frequency converted light source  100  may also comprise a package controller  150 . The package controller  150  may comprise one or more micro-controllers or programmable logic controllers used to store and execute programmed instructions for operating the frequency converted light source  100 . Alternatively, the micro-controllers or programmable logic controllers may directly execute an instruction set. The package controller  150  may be electrically coupled to the semiconductor laser  110 , the adaptive optics  140  and an optical detector  170  (described further herein) and programmed to operate both the semiconductor laser  110  and the adaptive optics  140 . More specifically, in one embodiment, the package controller  150  may comprise drivers  152 ,  154  for controlling the adaptive optics and the wavelength selective section of the semiconductor laser, respectively. 
         [0034]    The adaptive optics driver  152  may be coupled to the adaptive optics  140  with leads  156 ,  158  and supplies the adaptive optics  140  with x- and y-position control signals through the leads  156 ,  158 , respectively. The x- and y-position control signals facilitate positioning the adjustable optical component of the adaptive optics in the x- and y-directions which, in turn, facilitates positioning the output beam  119  of the semiconductor laser  110  on the input facet of the wavelength conversion device  120 . For example, when the adjustable optical component  144  of the adaptive optics  140  is an adjustable mirror, as shown in  FIG. 1 , the x- and y-position control signals may be used to rotate the adjustable mirror about the first scanning axis and the second scanning axis such that the output beam  119  of the semiconductor laser  110  is scanned over the input facet of the wavelength conversion device  120 . 
         [0035]    The wavelength selective section driver  154  may be coupled to the semiconductor laser  110  with lead  155 . The wavelength selective section driver  154  may supply the wavelength selective section  112  of the semiconductor laser  110  with wavelength control signal(s) which facilitates adjusting the wavelength λ 1  of the output beam  119  emitted from the output facet of the semiconductor laser  110 . 
         [0036]    The frequency converted light source  100  shown in  FIG. 1  may be coupled to a data source  160 , such as a programmable logic controller, which supplies the frequency converted light source  100  with an encoded data signal which may be representative of a video image, still image or the like. More specifically, the data source  160  may be coupled to the gain section of the semiconductor laser  110  via lead  162 . The data source  160  may control the lasing intensity of the semiconductor laser  110  such that the output of the frequency converted light source  100  forms an image when projected. To control the lasing intensity of the semiconductor laser  110 , the encoded data signal injects a gain current I into the gain section of the semiconductor laser  110 . Typically, the periodic frequency F DATA  of the gain current I is representative of the video image or still image of the encoded data signal such that, when the output of the frequency converted light source is projected, as modulated by the periodic frequency F DATA  of the gain current I, the projected image is the video image or still image of the encoded data signal. Typically, the periodic frequency F DATA  of the encoded data signal is about 60 Hz which generally corresponds to the video frame rate of a projected image. 
         [0037]    In order to measure the output intensity of the wavelength converted output beam  128  emitted from the frequency converted light source  100 , a beam splitter  180  may be positioned proximate the output of the wavelength conversion device  120 . The beam splitter  180  is used to redirect a portion of the wavelength converted output beam  128  emitted from the wavelength conversion device  120  into an optical detector  170  which is used to measure the intensity of the emitted visible wavelength converted output beam  128  and output an electrical signal proportional to the measured intensity. In the embodiments shown herein, the optical detector  170  may be coupled to the package controller with lead  172  such that the electrical signal proportional to the measured intensity of the wavelength converted output beam is provided to the controller. 
         [0038]    Startup methods for aligning the frequency converted light source  100  will now be discussed with specific reference to the frequency converted light source  100  shown in  FIG. 1  and  FIGS. 4A and 4B . 
         [0039]    Referring now to FIGS.  1  and  4 A- 4 B,  FIGS. 4A and 4B  schematically depict a start up method for a frequency converted light source  100 . In an initial step, the semiconductor laser  110  of the frequency converted light source  100  is powered on and a output beam  119  of the semiconductor laser  110  is directed on to the input facet  132  of the wavelength conversion device  120  with the adaptive optics  140 , as described above. In one embodiment, a constant gain current I C  is initially supplied to the semiconductor laser  110  to control the lasing intensity of the output beam  119 . However, it should be understood that the gain current I C  may be, in the alternative, be modulated, such as when the frequency of the gain current I is indicative of an encoded data signal (i.e., an image). 
         [0040]    With the output beam  119  of the semiconductor laser  110  directed on to the input facet  132 , a modulated wavelength control signal is supplied to the wavelength control section of the semiconductor laser to modulate the wavelength of the output beam  119  over a wavelength alignment range. In one embodiment, the wavelength alignment range generally corresponds to at least a portion of the phase-matching bandwidth of the waveguide portion  126  of the wavelength conversion device  120 . For example, in one embodiment, the phase matching bandwidth of the waveguide portion  126  has a full-width-half-max (FWHM) of 0.24 nm. In this embodiment, the wavelength of the output beam  119  of the semiconductor laser  110  may be varied over a wavelength alignment range which is 0.24 nm or more. In at least one embodiment of a start-up method described herein, the wavelength of the semiconductor laser  110  is varied continuously throughout the start-up method until first and second alignment set points are determined for the output beam  119  on the input facet of the wavelength conversion device  120 . In the embodiments described herein, the wavelength of the output beam is modulated at a wavelength modulation frequency F mod . In one embodiment, the wavelength modulation frequency may be from about 1 kHz to about 20 kHz. In another embodiment, the wavelength modulation frequency may be from about 1 kHz to about 10 kHz. In yet another embodiment, the wavelength modulation frequency may be from about 5 kHz to about 10 kHz. 
         [0041]    As will be described in more detail herein, the adjustable optical component  144  may be oscillated about a first scanning axis at a first oscillation frequency and about a second scanning axis at a second oscillation frequency. In general, the wavelength modulation frequency F mod  is greater than either the first oscillation frequency F 1  or the second oscillation frequency F 2 . In one particular embodiment, the modulation frequency F mod  is greater than 5 times the greater of F 1  and F 2 . For example, in one embodiment, the first oscillation frequency F 1  and the second oscillation frequency F 2  are on the order of about 400 Hz while the wavelength modulation frequency F mod  is on the order of about 2 kHz. In another embodiment, the first oscillation frequency F 1  and the second oscillation frequency F 2  are on the order of about 1 kHz while the wavelength modulation frequency F mod  is on the order of about 5 kHz. 
         [0042]    Referring to  FIG. 4A , as the wavelength of the output beam  119  is modulated, a control signal is supplied to the adjustable optical component  144  to maximize the displacement of the adjustable optical component  144  about one axis in one direction (i.e., either clockwise or counter clockwise). For example, when the adjustable optical component is a MEMS mirror, as described herein, a y-position control signal may be supplied to the MEMS mirror which, in turn, rotates the adjustable optical component to its maximum displacement about a second scanning axis which, in the embodiment shown in  FIG. 4A , is parallel to the y-axis. In one embodiment, when the adjustable optical component  144  is at its maximum displacement, the output beam  119  of the semiconductor laser  110  is positioned at or near an edge or the input facet  132  of the wavelength conversion device  120 . 
         [0043]    With the adjustable optical component at its maximum displacement in one direction, a modulated x-position control signal is supplied to the adjustable optical component  144  with the adaptive optics driver  152  of the package controller  150  causing the adjustable optical component  144  to oscillate about the first scanning axis which, in the embodiment shown in  FIGS. 1 and 4A , is parallel to the x-axis. The adjustable optical component is oscillated about the first scanning axis at a first oscillation frequency F 1 , as described above. In the embodiments described herein, the x-position control signal may be a ramp function, a triangle function, a sinusoidal function or any other waveform or function suitable for oscillating the adjustable optical component  144  about the first scanning axis. In one particular embodiment, the first oscillation frequency F 1  is equal to (N+¼)*F 0 , where 2≦N≦10 and F 0  is a video frame rate of a projection system in which the frequency converted light source is used. In general, the x-position control signal causes the adjustable optical component  144  to oscillate over a first angular range which, in one embodiment, corresponds to the maximum range of angular displacement of the adjustable optical component about the first scanning axis. 
         [0044]    Oscillating the adjustable optical component  144  about the first scanning axis causes the output beam  119  to traverse or dither over the input facet  132  on a first dithering trace  202 , which, in the embodiment shown, is generally parallel with the y-axis. As the adjustable optical component is oscillated about the first scanning axis, the angular orientation of the adjustable optical component is adjusted about the second scanning axis such that the position of the first dithering trace  202  is varied over the input facet  132  of the wavelength conversion device  120  on a first scan line  204 . In the embodiments described herein, the position of the first dithering trace is varied by a first amount which is less than ten times a dimension of the waveguide portion of the wavelength conversion device in a direction of the first scan line. For example, in one embodiment, the first amount is on the order of about 10 microns. 
         [0045]    As the position of the first dithering trace  202  is varied over the input facet  132  on the first scan line  204 , the intensity of the wavelength converted output beam  128  is monitored with the optical detector  170 . As noted hereinabove, the filtering window  179  filters the output of the frequency converted light source  100  such that the only light which reaches the optical detector  170  is wavelength converted light. Accordingly, the optical detector  170  only produces an output signal when the output beam  119  is positioned on the waveguide portion  126  of the wavelength conversion device  120 . 
         [0046]    Referring now to  FIG. 6A , a plot of time vs. output intensity (i.e., power) of the wavelength converted output beam  128  is graphically depicted as the first dithering trace is scanned over the waveguide portion  126  of the wavelength conversion device on the first scan line  204 . In the embodiments described herein, the output intensity of the wavelength converted output beam  128  fluctuates as the position of the first dithering trace  202  is varied. This is because the position of the output beam  119  on the waveguide portion  126  of the wavelength conversion device does not always coincide with the wavelength of the output beam  119  which yields a wavelength converted output beam  128  (i.e., both position and wavelength are simultaneously fluctuating). This leads to the “spiky” or “pulsing” character of the time dependent signal as shown in  FIG. 6A . In either case, the presence of an intermittent increase in intensity is indicative of the output beam  119  being positioned on the waveguide at a particular location along the first scan line  204 . Accordingly it should be understood that the period of oscillation of the adjustable optical component about the first scanning axis and the modulation period of the wavelength of the semiconductor laser should be significantly faster than the period between consecutive steps of the first dithering trace  202  on the first scan line  204  to insure that, if the output beam  119  is positioned on the waveguide portion  126  during the scan, the output intensity of the wavelength converted output beam  128  will be observed with the optical detector  170 . 
         [0047]    In the embodiments described herein, the output signal from the optical detector may be rapidly sampled such that spikes in the output intensity of the wavelength converted output beam  128  are resolved, and the value of the highest spikes are recorded by the package controller  150 . Alternatively, the output signal from the optical detector may be integrated or averaged and the integrated or averaged value recorded by the package controller  150  over a measurement time period which is greater than an oscillation period of the adjustable optical component about a scanning axis. In embodiments where the output signal of the optical detector  170  is rapidly sampled, the sampling frequency is selected to prevent aliasing (i.e. the sampling frequency is much greater than the oscillation frequencies of either the adjustable optical component or the wavelength of the semiconductor laser). In embodiments where the first oscillation frequency F 1  is equal to (N+¼)*F 0 , the output signal of the optical detector may be filtered at a frequency 2*F 1  and a maximum value of the filtered output signal is utilized to determine the first alignment set point. Such a technique may be utilized in embodiments where the wavelength converted output beam  128  is combined with the output beams of other light sources to form an image, such as in a projector system. 
         [0048]    The package controller  150  processes the output signal of the optical detector  170  and, based on the output signal, determines the location along the first scan line  204  where the output beam  119  is aligned with the waveguide portion  126  of the wavelength conversion device  120  and stores this location (and the corresponding angular orientation of the adjustable optical component  144  about the first scanning axis) as the first alignment set point  206 . Accordingly, it should be understood that the first alignment set point generally corresponds to the position of the output beam  119  on the input facet of the wavelength conversion device  120  where the output intensity of the wavelength converted output beam  128  is maximized. 
         [0049]    In the embodiments described herein, stepping the first dithering trace  202  over a first scan line  204  as the output beam  119  traverses over the first dithering trace  202  effectively produces a one-dimensional scan of the output beam  119  over the input facet  132  of the wavelength conversion device  120 . This one-dimensional scan can be performed very rapidly. For example, in one embodiment, only 1 msec of data acquisition time is necessary for each position of the first dithering trace  202  on the first scan line  204 . If the first scan line has a length of +/−50 microns and the first dithering trace  202  is positioned in 1 micron increments, then only 100 msec of time is needed to complete the one dimensional scan. 
         [0050]    Referring now to  FIGS. 1 and 4B , after the first alignment set point is determined with the package controller  150 , the oscillation of the adjustable optical component about the first scanning axis is terminated. Thereafter, the semiconductor laser  110  and the adjustable optical component  144  are operated to determine a second alignment set point of the output beam  119  on the waveguide portion  126  of the wavelength conversion device  120  in a manner similar to that used to the determine the first alignment set point along the first scan line. For example, the wavelength of the wavelength converted output beam  128  is modulated as described above. Thereafter, a control signal is supplied to the adjustable optical component  144  to maximize the displacement of the adjustable optical component  144  about a first scanning axis. For example, when the adjustable optical component is a MEMS mirror, as described herein, an x-position control signal may be supplied to the MEMS mirror which, in turn, rotates the adjustable optical component to its maximum displacement about a first scanning axis which, in the embodiment shown in FIGS.  1  and  4 A- 4 B, is parallel to the x-axis. In one embodiment, when the adjustable optical component  144  is at its maximum displacement, the output beam  119  of the semiconductor laser  110  is positioned at or near an edge of the input facet  132  of the wavelength conversion device  120 . 
         [0051]    With the adjustable optical component at its maximum displacement in one direction, a modulated y-position control signal is supplied to the adjustable optical component  144  with the adaptive optics driver  152  of the package controller  150  causing the adjustable optical component  144  to oscillate about the second scanning axis which, in the embodiment shown in FIGS.  1  and  4 A- 4 B, is parallel to the y-axis. The adjustable optical component is oscillated about the second scanning axis at a second oscillation frequency F 2 . In at least one embodiment described herein, the wavelength modulation frequency F mod  is greater than the second oscillation frequency F 2 . In another embodiment, the second oscillation frequency F 2  is equal to the first oscillation frequency F 1 . In the embodiments described herein, the y-position control signal may be a ramp function, a triangle function, a sinusoidal function or any other waveform or function suitable for oscillating the adjustable optical component  144  about the second scanning axis. In one particular embodiment, the first oscillation frequency F 2  is equal to (N+¼)*F 0 , where 2≦N≦10 and F 0  is a video frame rate of a projection system in which the frequency converted light source is used. The y-position control signal causes the adjustable optical component  144  to oscillate over a second angular range. In one embodiment, the second angular range is less than the first angular range. For example, in one embodiment, the second angular range may be less than 50% of the first angular range. In another embodiment, the second angular range may be less than 20% of the first angular range or even less than 10% of the first angular range. In another embodiment, the second angular range is less than the maximum range of angular displacement of the adjustable optical component about the second scanning axis. In another embodiment (not shown), the second angular range corresponds to the maximum range of angular displacement of the adjustable optical component about the second scanning axis. 
         [0052]    Oscillating the adjustable optical component  144  about the second scanning axis causes the output beam  119  to traverse over the input facet  132  on a second dithering trace  214 , which, in the embodiment shown, is generally parallel with the x-axis. In the embodiment shown in  FIG. 4B , the second dithering trace  214  is parallel with the first scan line  204  (shown in  FIG. 4A ) and centered on the first alignment set point  206 . In the embodiment shown in  FIG. 4B , the second angular range is less than the maximum displacement of the adjustable optical component about the second scanning axis which, in turn, decreases the linear dimension over which the output beam  119  is traversed over the input facet  132  of the wavelength conversion device on the second dithering trace  214 . In this embodiment, the linear dimension over which the output beam  119  is traversed over the input facet  132  on the second dithering trace  212  is less than the linear dimension over which the output beam  119  is traversed over the input facet  132  on the first dithering trace  202  thereby reducing the overall search time required and speeding up the alignment process. 
         [0053]    As the adjustable optical component is oscillated about the second scanning axis, the angular orientation of the adjustable optical component is adjusted about the first scanning axis such that the position of the second dithering trace  214  is varied over the input facet  132  of the wavelength conversion device  120  on a second scan line  212 . In the embodiments described herein, the position of the second dithering trace is varied by a second amount which is less than ten times a dimension of the waveguide portion of the wavelength conversion device in a direction of the second scan line. For example, in one embodiment, the second amount is on the order of about 10 microns. Stepping the second dithering trace  214  over a second scan line  212  as the output beam  119  traverses over the second dithering trace  214  produces a one-dimensional scan of the output beam  119  over the input facet of the wavelength conversion device  120 . This one-dimensional scan can be performed very rapidly, as described above. 
         [0054]    As the position of the second dithering trace  214  is varied over the input facet  132  on the second scan line  212 , the intensity of the wavelength converted output beam  128  is monitored with the optical detector  170 . 
         [0055]    Referring now to  FIG. 6B , a plot of time (x-axis) vs. output intensity (i.e., power) of the wavelength converted output beam  128  is graphically depicted as the second dithering trace is scanned over the waveguide portion  126  of the wavelength conversion device on the second scan line  212 . As described above, the output intensity of the wavelength converted output beam  128  fluctuates as the position of the second dithering trace  214  is varied. As with the period of oscillation of the adjustable optical component about the first scanning axis and the modulation period of the wavelength of the semiconductor laser, the period of oscillation of the adjustable optical component should be significantly faster than the period between consecutive steps of the second dithering trace  214  on the second scan line  212  to insure that, if the output beam  119  is positioned on the waveguide portion  126  during the scan, the output intensity of the wavelength converted output beam  128  will be observed with the optical detector  170 . 
         [0056]    The package controller  150  processes the output signal of the optical detector  170  and, based on the output signal, determines the location along the second scan line  212  where the output beam  119  is aligned with the waveguide portion  126  of the wavelength conversion device  120  and stores this location (and the corresponding angular orientation of the adjustable optical component  144  about the second scanning axis) as the second alignment set point  216 . Accordingly, it should be understood that the second alignment set point generally corresponds to a position of the output beam  119  on the input facet of the wavelength conversion device  120  where the output intensity of the wavelength converted output beam  128  is maximized. As noted hereinabove with respect to the first alignment set point, in one embodiment, the second alignment set point may be determined within 100 msec of initiating the search of the second alignment set point. 
         [0057]    After the first alignment set point and the second alignment set point are determined by the package controller  150 , oscillation of the wavelength of the output beam  119  is terminated. Thereafter, the first alignment set point and the second alignment set point are utilized to position the output beam  119  on the waveguide portion  126  of the wavelength conversion device  120 . 
         [0058]    In one embodiment, after the output beam  119  is positioned on the waveguide portion  126  of the wavelength conversion device  120  with the first alignment set point and the second alignment set point, the wavelength of the output beam  119  is varied over the wavelength alignment range as the output intensity of the wavelength converted output beam  128  is monitored with the optical detector  170 , as described above. In this embodiment, the package controller  150  determines an alignment wavelength at which the intensity of the wavelength converted output beam  128  is a maximum and, thereafter, operates the semiconductor laser such that the wavelength of the output beam  119  is the alignment wavelength. Because the wavelength of the semiconductor laser can be rapidly tuned over the wavelength alignment range (i.e., faster than 1 msec per step), and the output signal of the optical detector can be quickly sampled, the alignment wavelength can be rapidly determined. For example, covering a 2 nm wavelength alignment range with 0.05 nm steps yields (40 steps*1 msec)=40 msec of sweep time to determine the alignment wavelength. 
         [0059]    After the output beam  119  is positioned on the waveguide portion  126  of the wavelength conversion device  120  with the first alignment set point and the second alignment set point, closed-loop feedback control of the frequency converted light source  100  is initiated. For example, in one embodiment, the closed-loop control utilizes a 3-dimensional control loop (i.e., 2 spatial dimensions and a wavelength dimension) to maintain the alignment of the semiconductor laser with the wavelength conversion device. 
         [0060]    Referring to  FIG. 7 , which depicts a plot of time (x-axis) vs. the output power of the frequency converted light source (y-axis), it should be understood that, in at least one embodiment, the time elapsed from the initiation of the start-up method to initiation of closed loop control may be less than 2 seconds or even less than 1 second. In another embodiment, the elapsed time may be less than 650 msec or even less than 250 msec. Accordingly, it will be understood that the alignment method described herein can achieve alignment of the semiconductor laser with the wavelength conversion device more rapidly than prior art raster scanning techniques which may take as long as 100 seconds or longer. 
         [0061]    In the embodiments described herein, the wavelength of the semiconductor laser is modulated throughout the start-up routine at least until both the first alignment set point and the second alignment set point have been determined. However, it should be understood that, in other embodiments, the wavelength of the semiconductor laser may be maintained at a constant value during the start-up routine. 
         [0062]    In embodiments where 3D control space (i.e., x, y and wavelength) are searched, the method described herein reduces the overall search time and the number of search points needed to obtain alignment between the semiconductor laser and the wavelength conversion device. For example, when the frequency converted light source is utilized in a projector sample, the projector sample may only sample the output power of the frequency converted light source at the frame rate of the projector. Since a typical frame rate for a projector is 1/60 Hz (˜17 msec), only 60 independent samples of the frequency converted light source are taken every second. If a conventional raster search of the input facet of the wavelength conversion device is performed in 3D space, approximately 20×50×50=40,000 points are searched. At this search resolution and 17 msec sampling, the raster scan would take 667 seconds. To reduce the time one is forced to either reduce the volume of space, searched or reduce the resolution at which the search is performed. Both of these modifications raise the risk that the alignment location may be completely missed during the search. However with the start-up methods described herein, only 50+50+20=120 points are needed to determine the first alignment set point, the second alignment set point, and the alignment wavelength. At a search resolution or sampling rate of 17 msec, the start up method described herein takes 2 seconds or less with no reduction in search volume or search resolution. Although not all systems may be restricted to sampling the frequency converted light source at once per video frame, the benefits of the methods described herein (i.e., decreased search time and reduced sampling rates, etc.) may be taken advantage of in other systems regardless of the sampling rate. 
         [0063]    Referring to  FIG. 5 , the start-up method may be utilized in a projection system  500  comprising a first light source  502 , a second light source  504 , and a frequency converted light source  100 . The frequency converted light source  100  may comprise a semiconductor laser  110  optically coupled to a wavelength conversion device  120  as described herein. The output beams of the first light source  502 , the second light source  504  and the frequency converted light source  100  may be combined with optical components  506  to project a composite output beam  508 . Each of the first light source  502 , the second light source  504  and the frequency converted light source  100  may be coupled to a system controller  510  which is utilized to control the lasing intensity of each light source and thereby project a color image with the light sources. 
         [0064]    In one embodiment, the start-up methods described herein may be utilized in conjunction with the projection system  500 . In this embodiment, the first light source  502  is supplied with a first gain current I 1  at a first modulated gain frequency F I1  and the second light source  504  is supplied with a second gain current I 2  at a second modulated frequency F I2 . The frequency converted light source  100  is supplied with a constant gain current I c . A combined output intensity of the first light source  502 , the second light source  504  and the frequency converted light source  100  is monitored with an optical detector  170  at a frame rate F 0  of the projection system by the system controller  510 . 
         [0065]    As described herein above with respect to FIGS.  1  and  4 A- 4 B, the frequency converted light source  100  may be operated such that the output beam of the semiconductor laser is directed onto an input facet of the wavelength conversion device with the adjustable optical component and a wavelength of the output beam of the semiconductor laser is modulated over a wavelength alignment range corresponding to at least a portion of the phase-matching bandwidth of a waveguide portion of the wavelength conversion device. The adjustable optical component is oscillated over a first angular range about a first scanning axis such that the output beam of the semiconductor laser is dithered over the input facet of the wavelength conversion device on a first dithering trace. In this embodiment, the adjustable optical component is oscillated about the first scanning axis at a first oscillation frequency F 1 =(N+¼)*F 0 , where 2≦N≦10. The angular orientation of the adjustable optical component is adjusted about a second scanning axis to vary a position of the first dithering trace over the input facet of the wavelength conversion device on a first scan line. A first alignment set point on the first scan line is determined by filtering the combined output intensity at a frequency of 2*F 1  with a zero-phase delay filter to isolate a portion of the combined output intensity attributable to an output intensity of a wavelength converted output beam emitted from the wavelength conversion device as the position of the first dithering trace is varied over the first scan line. 
         [0066]    Thereafter, the adjustable optical component is oscillated over a second angular range about a second scanning axis such that the output beam of the semiconductor laser is dithered over the input facet of the wavelength conversion device on a second dithering trace centered on the first alignment set point. The second angular range is less than the first angular range, and the adjustable optical component is oscillated about the second scanning axis at a second oscillation frequency F 2 =F 1 . The angular orientation of the adjustable optical component is adjusted about the first scanning axis to vary a position of the second dithering trace over the input facet of the wavelength conversion device on a second scan line. 
         [0067]    A second alignment set point on the second scan line is determined by filtering the combined output intensity at a frequency 2*F 1  with a zero-phase delay filter to isolate a portion of the combined output intensity attributable to an output intensity of the wavelength converted output beam emitted from the wavelength conversion device as the position of the second dithering trace is varied over the second scan line. The first alignment set point and the second alignment set point define a position where the output beam of the wavelength conversion device is aligned with the waveguide portion of the wavelength conversion device. The modulation of the wavelength of the output beam of the semiconductor laser and the output beam of the semiconductor laser is positioned on the waveguide portion of the wavelength conversion device using the first alignment set point and the second alignment set point. 
         [0068]    In an alternative embodiment, each of the first light source  502 , the second light source  504  and the wavelength converted light source  100  may be supplied with a modulated gain current, such as when the projector system is being used to project an image during the start-up routine. In this embodiment, the system controller  510  may monitor the gain current supplied to the frequency converted light source  100  and, when the gain current falls below a predetermined threshold level, the position of the first dithering trace or the position of the second dithering trace is held constant on the input facet of the wavelength conversion device until the gain current for the frequency converted light source  100  returns to a level above the predetermined threshold level which, in turn, reduces the possibility that the output beam will traverse the waveguide portion of the wavelength conversion device when the intensity of the wavelength converted output beam is low or zero. 
         [0069]    For the purposes of defining and describing the present invention, it is noted that reference herein to values that are “on the order of” a specified magnitude should be taken to encompass any value that does not vary from the specified magnitude by one or more orders of magnitude. It is also noted that one or more of the following claims recites a controller “programmed to” execute one or more recited acts. For the purposes of defining the present invention, it is noted that this phrase is introduced in the claims as an open-ended transitional phrase and should be interpreted in like manner as the more commonly used open-ended preamble term “comprising.” In addition, it is noted that recitations herein of a component of the present invention, such as a controller being “programmed” to embody a particular property, function in a particular manner, etc., are structural recitations, as opposed to recitations of intended use. More specifically, the references herein to the manner in which a component is “programmed” denotes an existing physical condition of the component and, as such, is to be taken as a definite recitation of the structural characteristics of the component. 
         [0070]    It is noted that terms like “preferably,” “commonly,” and “typically,” when utilized herein, are not intended to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the present invention. Further, it is noted that reference to a value, parameter, or variable being a “function of” another value, parameter, or variable should not be taken to mean that the value, parameter, or variable is a function of one and only one value, parameter, or variable. 
         [0071]    For the purposes of describing and defining the present invention it is noted that the term “substantially” is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The term “substantially” is also utilized herein to represent the degree by which a quantitative representation. e.g., “substantially above zero,” varies from a stated reference, e.g., “zero,” and should be interpreted to require that the quantitative representation varies from the stated reference by a readily discernable amount.