Abstract:
A laser package comprising a semiconductor laser having an operating temperature range and a heater, wherein the heater is configured to heat the laser when the laser package is positioned in an environment having an ambient temperature which lies outside of the operating temperature range of the laser, so that the laser will remain within the operating temperature range.

Description:
REFERENCE TO PENDING PRIOR PATENT APPLICATION  
       [0001]     This patent application claims benefit of pending prior U.S. Provisional Patent Application Ser. No. 60/734,570, filed Nov. 8, 2005 by Peidong Wang et al. for UNCOOLED EXTERNAL CAVITY LASERS OPERATING OVER AN EXTENDED TEMPERATURE RANGE (Attorney&#39;s Docket No. AHURA-40 PROV), which patent application is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to lasers in general, and more particularly to semiconductor lasers.  
       BACKGROUND OF THE INVENTION  
       [0003]     Applications using Raman scattering to identify unknown materials are rapidly proliferating, e.g., in the areas of security and safety, biotechnology, biomedicine, industrial process control, pharmaceuticals, etc. This is largely due to the rich and detailed optical signatures made possible by Raman scattering.  
         [0004]     In a Raman analyzer, a laser is used to generate a stable and narrow linewidth light signal which functions as the Raman pump. The Raman pump is directed at the specimen, resulting in Raman scattering, which provides a unique Raman signature for the specimen. The specimen&#39;s Raman signature can then be used to determine the composition of the specimen.  
         [0005]     For portable Raman applications, small size and low electrical power consumption are essential. In this latter regard, it should be appreciated that the laser in a Raman analyzer typically accounts for the majority of the device&#39;s power consumption. Hence, in portable Raman units, the laser tends to dominate the battery lifetime of the unit.  
         [0006]     Semiconductor lasers are one of the most efficient lasers known. Semiconductor lasers can have wall-plug efficiencies of greater than 50%, which is rare for other types of lasers. However, the semiconductor lasers generally need to be wavelength stabilized (e.g., at 785 nm or other operating wavelengths, depending on the application).  
         [0007]     In Raman applications, the most common technique for stabilizing the laser&#39;s operating wavelength is to use a diffraction grating in an external cavity geometry. This arrangement can be used to easily and inexpensively stabilize the laser&#39;s operating wavelength to a few inverse centimeter (e.g., &lt;50 cm −1 ). However, such an arrangement tends to be relatively temperature sensitive, i.e., temperature changes can cause thermal expansion of various elements of the assembly, which can in turn detune the alignment and change the laser&#39;s operating wavelength and/or vary the laser linewidth. As a result, a thermo-electric cooler (TEC) is commonly used to stabilize the laser&#39;s operating temperature to within a couple of degrees Celsius. However, thermo-electric coolers themselves consume substantial amounts of power, making such an arrangement undesirable in portable applications where power consumption is an important consideration.  
         [0008]     As a result, in portable applications, it is common to use “uncooled lasers” (i.e., those lacking a thermo-electric cooler); however, these uncooled lasers are only operated over a relatively limited temperature range, in order to meet operating parameters of spectral purity and power consumption.  
         [0009]     Thus, there is a need for a low-power, uncooled laser which can provide a stable, narrow-linewidth signal, over a wider operating temperature range.  
       SUMMARY OF THE INVENTION  
       [0010]     This and other objects of the present invention are addressed by the provision and use of a novel uncooled external cavity laser capable of operating over an extended temperature range.  
         [0011]     In one form of the invention, there is provided a laser package comprising:  
         [0012]     a semiconductor laser having an operating temperature range; and  
         [0013]     a heater;  
         [0014]     wherein the heater is configured to heat the laser when the laser package is positioned in an environment having an ambient temperature which lies outside of the operating temperature range of the laser, so that the laser will remain within the operating temperature range.  
         [0015]     In another form of the invention, there is provided a laser package comprising:  
         [0016]     a semiconductor laser having an operating temperature range, wherein the laser has differing operating efficiencies within the operating temperature range; and  
         [0017]     a heater;  
         [0018]     wherein the heater is configured to heat the laser when the laser package is positioned in an environment having an ambient temperature which lies within the operating temperature range of the laser, but at a temperature which does not produce an optimal efficiency, so that the laser will operate at an increased efficiency.  
         [0019]     In still another form of the invention, there is provided a laser package comprising:  
         [0020]     a semiconductor laser having an operating temperature range, wherein the laser has differing operating efficiencies within the operating temperature range; and  
         [0021]     a heater;  
         [0022]     wherein the heater is configured to operate in at least one of the following modes: 
        (i) when the laser package is positioned in an environment having an ambient temperature which lies outside of the operating temperature range of the laser, the heater operates so as to heat the laser to within the operating temperature range of the laser; and     (ii) when the laser package is positioned in an environment having an ambient temperature which lies within the operating temperature range of the laser, but at a temperature which does not produce an optimal efficiency, the heater operates so as to heat the laser to a temperature at which the laser operates at an increased efficiency.        
 
         [0025]     In still another form of the invention, there is provided a method for operating a semiconductor laser over an extended temperature range, comprising:  
         [0026]     providing a semiconductor laser having an operating temperature range, and providing a heater;  
         [0027]     operating the heater when the laser package is positioned in an environment having an ambient temperature which lies outside of the operating temperature range of the laser, so as to heat the laser to within the operating temperature range of the laser.  
         [0028]     In still another form of the invention, there is provided a method for operating a semiconductor laser over an extended temperature range, comprising:  
         [0029]     providing a semiconductor laser having an operating temperature range, wherein the laser has differing operating efficiencies within the operating temperature range, and providing a heater;  
         [0030]     operating the heater when the laser package is positioned in an environment having an ambient temperature which lies within the operating temperature range of the laser, but at a temperature which does not produce an optimal efficiency, so as to heat the laser to a temperature at which the laser operates at an increased efficiency. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]     These and other objects and features of the present invention will be more fully disclosed or rendered obvious by the following detailed description of the preferred embodiments of the invention, which are to be considered together with the accompanying drawings wherein like numbers refer to like parts, and further wherein:  
         [0032]      FIG. 1  is a schematic view showing uncooled laser temperature performance, plotting power versus current, for various laser operating temperatures;  
         [0033]      FIG. 2  is a schematic view showing uncooled laser temperature performance, plotting power versus platform start temperature;  
         [0034]      FIG. 3  is a schematic view showing uncooled laser temperature performance, plotting laser wavelength versus platform start temperature;  
         [0035]      FIG. 4  is a schematic top view of an external cavity laser (ECL) with a platinum (Pt) heater mounted on the top side of an aluminum nitride (AlN) platform;  
         [0036]      FIG. 5  is a schematic bottom view of a structure similar to that shown in  FIG. 4 , except that the Pt heater is mounted on the back side of the AlN platform; and  
         [0037]      FIG. 6  is a schematic view showing a 2 W Pt heater.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     Introduction  
       [0038]     This invention relates to the application of external cavity lasers in spectroscopy, particularly for Raman spectroscopy where spectral purity and stable laser power are desirable. The present invention provides uncooled laser operation over an extended temperature range (e.g., from −30° C. to 60° C.), with sufficient output power and without calibration of laser wavelength. Such laser systems are particularly useful for portable and field-ready Raman instruments.  
         [0039]     By way of example, the following Table 1 provides some key specifications that are generally necessary for 785 nm ECLs which are to be used in Raman spectroscopy:  
                                                     TABLE 1                       Parameter   Unit   Min   Typical   Max                                Power   mW   375               Temperature   ° C.   −30       60       Center Wavelength   nm   784.5       785.5       3 dB Linewidth   nm       0.12   0.18       Center wavelength stability   nm           0.03       (on/off cold start)       Center wavelength shift   nm       0.1   0.2       (−30° C. to +60° C.)       SMSR (laser-gain peak)   dB   20       Total power consumption   W           5                  
 
         [0040]     To this end, a semiconductor external cavity laser (ECL) laser structure was specially designed for uncooled operations at 785 nm at a temperature range of between −20° C. and 40° C. (with all other specifications as set out in the above Table 1). The performance of this semiconductor laser structure, including various modifications to permit use over an extended temperature range, will now be discussed.  
       Conventional Uncooled Laser Operation  
       [0041]      FIG. 1  shows the laser intensity (LI) curve (i.e., plotting power versus current) for various laser operating temperatures. In essence,  FIG. 1  shows that the LI curve generally rises with laser temperature—in other words, as the laser temperature increases, less current is required in order to initiate optical output, and a given level of current yields a higher level of optical output.  
         [0042]      FIGS. 2 and 3  show additional performance characteristics for an uncooled external cavity laser (ECL). More particularly,  FIG. 2  shows that the laser can meet all of the desired performance criteria while operating between −20° C. and 50° C. The operating current approaches (or equals) 1 A, with a bias of ˜2.2 V, across the operative temperature range. Therefore, the total power consumption of the laser is ˜2.2 W.  
         [0043]      FIG. 3  shows wavelength and linewidth changes in the laser output across the operative temperature range. By manipulating the thermal properties of the laser chip sub-mount, the laser lens and the wavelength-stabilizing diffraction grating, one can minimize changes in the laser&#39;s output wavelength which may be caused by mechanical deformations in the system components and/or by temperature changes in the system components. See, for example, pending prior U.S. patent application Ser. No. 11/119,076, filed Apr. 29, 2005 by Daryoosh Vakhshoori et al. for EXTERNAL CAVITY WAVELENGTH STABILIZED RAMAN LASERS INSENSITIVE TO TEMPERATURE AND/OR EXTERNAL MECHANICAL STRESSES, AND RAMAN ANALYZER UTILIZING THE SAME (Attorney&#39;s Docket No. AHURA-24), which patent application is hereby incorporated herein by reference.  
         [0044]     However, the convential, low-power, uncooled laser still needs to operate over a relatively limited temperature range in order to provide a stable, narrow-linewidth signal. Thus, there is the need for a new approach which will enable a low-power, uncooled laser to operate over a wider temperature range while still providing a stable, narrow-linewidth signal.  
       Operation in Extended Temperature Range (E.g., from −30° C. to 60° C.)  
     (i) Heater to Enable Operation Across a Lower Temperature Range  
       [0045]     In a first form of the present invention, the operating temperature range of the system is extended in the lower temperature range by incorporating a heater into the laser package. This heater is not designed to stabilize the operating temperature of the laser in an absolute sense—as a result, the heater-enhanced laser system is still considered to be an “uncooled laser”. However, the addition of the heater to the laser package permits the system to be used in ambient environments which fall outside the laser&#39;s operating temperature range. In accordance with the present invention, when the ambient temperature falls below the laser&#39;s operating temperature range, the heater is used to heat the laser package so as to keep the laser within the laser&#39;s operating temperature range.  
         [0046]     Furthermore, by providing a heater in the laser package, the temperature of the laser can be increased as desired. This can be useful even when the temperature of the laser falls within the laser&#39;s operating temperature range. In this form of the invention, the heater is used so as to operate the laser with greater efficiency (i.e., in the context of  FIG. 2 , the heater is operated so as to shift operation of the laser from one LI curve to another LI curve). This can be done even where the laser is in an ambient environment which falls within the laser&#39;s operating temperature range (but where the laser is at a temperature which is less than optimally efficient).  
         [0047]     Thus, by using a heater to increase the laser&#39;s operating temperature range, power consumption can be reduced while keeping the desired spectral purity.  
         [0048]     In addition to the foregoing, the heater can be incorporated into the platform so as to ensure wavelength insensitivity over the temperature range. The laser gain peak should also be adjusted by +3 nm, e.g., from 783 nm to 786 nm as measured at 20° C. The heater material can be nichrome, platinum or other materials.  
         [0049]     By adjusting the gain peak, adequate performance of the laser can be expected from −10° C. to 60° C. Thus, for an ambient temperatures of below −10° C., the heater should be turned on so as to maintain a minimum temperature of −10° C. on the platform. Furthermore, for ambient temperatures of between −10° C. to 60° C., the heater can be turned on so as to ensure optimal operation.  
         [0050]     It can be informative to estimate the power consumption at the most extreme ambient temperature condition (i.e., −30° C.). Assuming a 2 W platinum (Pt) heater design, the power consumption needed to maintain a change in temperature (ΔT) of 20° C. is only 2 W. The initial time required to heat the platform from −30° C. to −10° C. (˜70 Joules) is approximately 30 seconds. This is computed by assuming a copper tungsten (CuW) package base (0.5 mm thick) and an aluminum nitride (AlN) platform of 2 cm wide by 2.4 cm long by 0.26 cm high. The thermal conductivity of both CuW and AlN is 200 W/m.K. The specific heat of AlN is ˜800 J/kg.K. The total power consumption at −30° C. is therefore 4.2 W, i.e., 2.2 W (as computed above for an unconventional unheated laser)+2 W (for the heater) for a total of 4.2 W. The power consumption remains the same (i.e., 2.2 W) at 60° C. (assuming the heater is not be operated to shift the laser from one LI curve to another LI curve).  
         [0051]     Various designs for incorporating a heater on the platform are shown in  FIGS. 4-6 .  FIG. 4  is a top view of an external cavity laser (ECL) with an aluminum nitride (AlN) platform. Note the heater, the temperature sensor and on-board logic circuitry for controlling operation of the heater.  FIG. 5  is the same platform, but with a back-mounted platinum (Pt) heater.  FIG. 6  is a 2 W platinum (Pt) heater design.  
         [0052]     If desired, a more powerful heater can be used to reduce the initial set-up time, i.e., the time required to achieve the desired change in temperature (ΔT).  
         [0053]     In another embodiment of the present invention, a local heater may be placed adjacent to the laser chip, (or directly on the laser chip) so as to further reduce the power consumption of the laser package. By way of example, for a p-down, die-bonded laser chip, the heater can be lithographically added on the back metal contacts of the laser chip. This design can be advantageous, since it effectively maintains the current footprint of the laser chip. Where the heater is placed directly on the laser chip, 1 W local heater is adequate to change the laser chip temperature by 20° C. The estimated wavelength shift is ˜−0.1 nm over the 20° C. temperature change (ΔT). However, some wavelength calibration may be necessary for laser operation at the lower temperatures of −30° C. to −10° C.  
       (ii) Semi-Uncooled Laser Operation  
       [0054]     In this new design, a thermo-electric cooler (TEC) is incorporated in the laser package just as is done with a conventional, “cooled” laser package. However, the TEC is only operated to maintain a ΔT=0° C. throughout the operating range of the laser. For a typical TEC the maximum power consumption is &lt;0.5 W at T=50° C. for a 2.2 W heat load. This results in total power consumption of &lt;2.7 W. However, in the low temperature range of −30° C. to −10° C., the TEC should act as a heater. The estimated heat power consumption at −30° C. is ˜2 W, with a total power consumption of &lt;4.2 W.  
       Further Constructions  
       [0055]     It is to be understood that the present invention is by no means limited to the particular constructions herein disclosed and/or shown in the drawings, but also comprises any modifications or equivalents within the scope of the invention.