Patent Application
20090028197
Mid Infrared (“MIR”) laser sources that produce a fixed wavelength beam can be used in many fields such as, in medical diagnostics, pollution monitoring, leak detection, analytical instruments, homeland security and industrial process control. For example, MIR laser sources can be used in the detection of certain molecules found in human breath that correlate to existing health problems such as asthma, kidney disorders and renal failure.
One type of fixed wavelength, MIR laser source is commonly referred to as Distributed Feedback (“DFB”) and includes a quantum cascade (“QC”) gain media, and a periodic grating either on the surface of the QC gain media or close to the active region of the QC gain media. The periodic grating on the QC gain media preferentially favors certain laser modes. This results in a MIR laser source having a fixed wavelength beam.
Commonly, the periodic grating is grown using the same methods used to grow the QC gain media. Unfortunately, none of the methods used to grow the periodic grating on the QC gain media are completely precise. For example, if it desired that the MIR laser source have a fixed wavelength of 9.25 microns, a large number of the MIR laser sources may have to be grown until one of these MIR laser sources has a fixed wavelength of 9.25 microns. Stated in another fashion, because of the imprecise growing methods, many of manufactured MIR laser sources will have a fixed wavelength that is different than the desired wavelength. Thus, the yield of this process is low and the process is inefficient and expensive.
It should be noted that DFB laser sources are tunable to a small degree by changing the temperature of the laser gain medium either by external temperature control or by variation of the input current to the QC gain media. Thus, while such laser devices are generally referred to as “fixed” wavelength, it is understood that only a relatively small variation of wavelength is nevertheless available (typically less than 1% of the peak wavelength) by means of temperature control. Unfortunately, this small tuning range is often not sufficient to achieve the desired fixed wavelength because of the imprecise manufacturing methods.
Embodiments of the present invention are directed towards a MIR laser source that produces a fixed frequency output beam that is within the MIR range. In one embodiment, the MIR laser source includes a QC gain media, and a wavelength dependent (“WD') reflector that is spaced apart from the QC gain media and that cooperates with the QC gain media to form an external cavity. By configuring a WD reflector to form an external cavity, control of the wavelength of the output beam may be gained. In this embodiment, the WD reflector is used to precisely tune a lasing wavelength of the external cavity, and the position of the WD reflector relative to the QC gain media is fixed to maintain the precise lasing wavelength of the external cavity. As a result thereof, in certain embodiments, each MIR laser source can be individually tuned with the WD reflector to have desired fixed frequency output beam that is within the MIR range. Stated in another fashion, with the individual tuning of the WD reflector, the manufacturing process used to make this MIR laser source may be very precise. Further, relatively large adjustments to the lasing wavelength can occur without adjusting the QC gain media. This results in a high production yield, and ultimately a relatively inexpensive MIR laser source.
Additionally, in certain embodiments, the MIR laser source is designed to be relatively small, portable, rugged and battery-powered. As a result thereof, the MIR laser source can be easily used in remote locations.
As used herein, to be classified as a MIR laser source, the output beam of the MIR laser source has a wavelength in the range of approximately 3-14 microns. Stated in another fashion, as used herein, the MIR range is approximately 3-14 microns. These MIR laser sources are particularly useful in absorption spectroscopy applications since many gases of interest have their fundamental vibrational modes in the MIR range and thus present strong, unique absorption signatures within the MIR range.
In one embodiment, the MIR laser source includes a rigid, one-piece mounting base that maintains the position of the WD reflector relative to the QC gain media. With this design, the MIR laser source is relatively rugged and relatively easy to manufacture.
Additionally, in some embodiments, the MIR laser source can include a cavity optical assembly positioned between the QC gain media and the WD reflector, that is also fixedly secured to the mounting base so that the mounting base maintains the relative position of the QC gain media, the cavity optical assembly, and the WD reflector. In this embodiment, the cavity optical assembly collimates and focuses the light that passes between the QC gain media and the WD reflector. In certain embodiments, the cavity optical assembly includes a relatively small lens to facilitate the small form factor of the MIR laser source disclosed herein.
Further, in some embodiments, the MIR laser source can include a temperature controller that is in thermal communication with the mounting base. In this embodiment, the temperature controller controls the temperature of the mounting base and the QC gain media. As a result of the integrated temperature controller, the MIR laser source can be used in remote locations away from external cooling sources.
Moreover, in one embodiment, the MIR laser source includes a battery that powers the QC gain media. With this design, the MIR laser source can be made to be extremely portable.
One non-exclusive example of a suitable WD reflector is a diffraction grating. Other suitable examples are discussed in the description section.
Additionally, in some embodiments, the WD reflector can include a reflector retainer that retains the diffraction grating and that allows the diffraction grating to selectively move relative to the QC gain media, and a reflector lock that selectively locks the position of the diffraction gating relative to the QC gain media. For example, the element lock can include a threaded member that moves the diffraction grating relative to the QC gain media and that selectively locks the position of the diffraction grating relative to the QC gain media.
In another embodiment, the fixed frequency, MIR laser source includes a rigid, one piece mounting base, a QC gain media, a battery that is electrically connected to the QC gain media, a cavity optical assembly, and a WD reflector. In this embodiment, the QC gain media, the cavity optical assembly, and the WD reflector are spaced apart from each other and are fixedly secured to the mounting base in a fixed orientation. In this embodiment, the WD reflector is again used to precisely and individually tune the lasing wavelength of the external cavity. The resulting MIR laser source is relatively inexpensive to manufacture, relatively small, lightweight, portable, and rugged.
Embodiments of the present invention are also directed to methods for generating a fixed frequency, output beam that is within the MIR range. In one embodiment, the method includes the steps of providing a rigid mounting base, fixedly securing a QC gain media to the mounting base, positioning a WD reflector spaced apart from the QC gain media, tuning a lasing wavelength of the external cavity with the WD reflector, and fixedly securing the WD reflector to the mounting base.
The novel features of embodiments of the present invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description of certain embodiments, in which similar reference characters refer to similar parts, and in which:
As an overview, in certain embodiments, the WD feedback assembly 24 includes a wavelength dependent (“WD”) reflector 24A (illustrated in phantom) that cooperates with the QC gain media 14 to form an external cavity 28. Further, the WD reflector 24A can be tuned to adjust the lasing frequency of the external cavity 28 and the wavelength of the output beam 26, and the relative position of the WD feedback assembly 24 and QC gain media 14 can be fixedly maintained so that the laser source 10 produces a precise, fixed wavelength MIR output beam 26. Stated in another fashion, the feedback assembly 24 can be tuned (without changing the QC gain media 14) to achieve the desired wavelength of the output beam 26 within the MIR range. This results in a high production yield, and ultimately a relatively inexpensive to make MIR laser source 10.
In alternative, non-exclusive embodiments, the WD reflector 24A can be used to control the fixed wavelength of output beam 26 within the MIR range to within approximately 0.1, 0.01, 0.001, or 0.0001 microns. As a non-exclusive example, the WD reflector 24A can be adjusted so that the MIR laser source 10 has a fixed output beam 26 of (i) 9.125 microns, (ii) 9.126 microns, (iii) 9.127 microns, (iv) 9.128 microns, (v) 9.129 microns, (vi) 9.130 microns, or any other specific wavelength in the MIR range.
In certain embodiments, with the designs provided herein, the output beam 26 has a relatively narrow line width. In non-exclusive examples, the MIR laser source 10 can be designed so that the line width of the output beam 26 is less than approximately 5, 4, 3, 2, 1, 0.8, or 0.5 cm−1.
Further, in certain embodiments, because of the unique design disclosed herein, the MIR laser source 10 can be relatively stable, rugged, small, portable, and self contained source of MIR light.
A number of Figures include an orientation system that illustrates an X axis, a Y axis that is orthogonal to the X axis and a Z axis that is orthogonal to the X and Y axes. It should be noted that these axes can also be referred to as the first, second and third axes.
The source frame 12 supports at least some of the components of the laser source 10. In one embodiment, (i) the QC gain media 14, the cavity optical assembly 16, the output optical assembly 22, and the WD feedback assembly 24 are each fixedly secured, in a rigid arrangement to the source frame 12; and (ii) the source frame 12 maintains these components in precise mechanical alignment to achieve the desired wavelength of the output beam 26.
Additionally, in
The design of the source frame 12 can be varied to achieve the design requirements of the laser source 10. In
The mounting base 12A provides a rigid platform for fixedly mounting the QC gain media 14, the cavity optical assembly 16, the output optical assembly 22 and the WD feedback assembly 24. In
In certain embodiments, the mounting base 12A is made of rigid material that has a relatively high thermal conductivity. In one non-exclusive embodiment, the mounting base 12A has a thermal conductivity of at least approximately 170 watts/meter K. With this design, in addition to rigidly supporting the components of the MIR laser source 10, the mounting base 12A also readily transfers heat away from the QC gain media 14 to the temperature controller 19. For example, the mounting base 12A can be fabricated from a single, integral piece of copper, copper-tungsten or other material having a sufficiently high thermal conductivity. The one piece structure of the mounting base 12A maintains the fixed relationship of the components mounted thereto and contributes to the small size and portability of the MIR laser source 10.
The cover 12B is secured to the mounting base 12A, and the cover 12B cooperates with the mounting base 12A to define a cover cavity 12C and form a housing that encloses and protects the QC gain media 14, the cavity optical assembly 16, the output optical assembly 22 and the WD feedback assembly 24. In
Additionally, the cover 12B can include a cover fastener (not shown) that secures the cover 12B to the mounting base 12A. For example, the cover fastener can include one or more threaded bolts, an adhesive, one or more clamps, solder, or a weld.
In one embodiment, the cover 12B is hermetically sealed to the mounting base 12A in an air tight manner. This allows the source frame 12 to provide a controlled environment around some of the components of the laser source 10. For example, the cover cavity 12C can be filled with a fluid such as nitrogen or an air/nitrogen mixture to keep out moisture and humidity; or the cover cavity 12C can be subjected to a vacuum.
In certain embodiments, the overall size of the source frame 12 is quite small so that the laser source 10 is very portable. For example, the source frame 12 can have dimensions of approximately 20 centimeters (height) by 20 centimeters (width) by 20 centimeters (length) (where length is taken along the propagation direction of the laser beam) or less, and more preferably, the source frame 12 has dimensions of approximately 3 centimeters (height) by 4 centimeters (width) by 5 centimeters (length). Still alternatively, the source frame 12 can have dimensions of less than approximately 10 millimeters (height) by 25 millimeters (width) by 30 millimeters.
The QC gain media 14 is a unipolar semiconductor laser that includes a series of energy steps built into the material matrix while the crystal is being grown. With this design, electrons transmitted through the QC gain media 14 emit one photon at each of the energy steps. In one embodiment, the QC gain media uses two different semiconductor materials such as InGaAs and AlInAs (grown on an InP or GaSb substrate for example) to form a series of potential wells and barriers for electron transitions. The thickness of these wells/barriers determines the wavelength characteristic of the QC gain media 14. Fabricating QC gain media of different thickness enables production of MIR laser having different output frequencies within the MIR range.
It should be noted that fine tuning of the output beam 26 may be achieved by controlling the temperature of the QC gain media 14, such as by changing the DC bias current. Such temperature tuning is relatively narrow and may be used to vary the wavelength by approximately 1-2 gigahertz/Kelvin which is typically less than 0.01% of the peak emission wavelength.
In the case of QC gain media 14, the “diode” has been replaced by a conduction band quantum well. Electrons are injected into the upper quantum well state and collected from the lower state using a superlattice structure. The upper and lower states are both within the conduction band. Replasing the diode with a single-carrier quantum well system means that the generated photon energy is no longer tied to the material bandgap. This removes the requirement for exotic new materials for each wavelength, and also removes Auger recombination as a problem issue in the active region. The superlattice and quantum well can be designed to provide lasing at almost any photon energy that is sufficiently below the conduction band quantum well barrier.
As used herein the term QC gain media 14 shall also include Interband Cascade Lasers (ICL). ICL lasers use a conduction-band to valence-band transition as in the traditional diode laser.
In
In one embodiment, the first facet 14A is coated with an anti-reflection (“AR”) coating 14D and the second facet 14B is coated with a reflective coating 14E. The AR coating 14D allows light directed from the QC gain media 14 at the first facet 14A to easily exit the QC gain media 14 and allows the light reflected from the WD feedback assembly 24 to easily enter the QC gain media 14. In contrast, the reflective coating 14E reflects at least some of the light that is directed at the second facet 14B from the QC gain media 14 back into the QC gain medium 14. In one non-exclusive embodiment, the AR coating 14D can have a reflectivity of less than approximately 2 percent, and the reflective coating 14E can have a reflectivity of between approximately 2-95 percent. In this embodiment, the reflective coating 14E acts as an output coupler for the external cavity 28.
In one embodiment, the QC gain media 14 is approximately 2 millimeters by 0.5 millimeters, by 90 microns. Further, the QC gain media 14 can include a media fastener 14F that fixedly secures the QC gain media 14 to the mounting base 12A. For example, the media fastener 14F can include one or more threaded bolts, an adhesive, one or more clamps, solder, or a weld.
A suitable QC gain media 14 can be purchased from Alpes Lasers, located in Switzerland, Maxion Technologies, Inc., located in Maryland, US and Hamamatsu located in Japan.
The QC gain media 14 generates a relatively strong output IR beam and also generates quite a bit of heat. Accordingly, the temperature controller 19 can be an important component that is needed to remove the heat, thereby permitting long lived operation of the laser source 10.
The cavity optical assembly 16 is positioned between the QC gain media 14 and the WD feedback assembly 24 along the lasing axis 14C (along the X axis in Figures), and collimates and focuses the light that passes between these components. For example, the cavity optical assembly 16 can include one or more lens 16A (one is illustrated in phantom), a lens mount 16B and a lens fastener 16C.
For example, the lens 16A can be an aspherical lens having an optical axis that is aligned with the lasing axis 14C. In one embodiment, to achieve the desired small size and portability, the lens 16A has a relatively small diameter. In alternative, non-exclusive embodiments, the lens 16A has a diameter of less than approximately 5 or 10 millimeters, and a focal length of approximately 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 mm and any fractional values thereof. The lens 16A can comprise materials selected from the group of Ge, ZnSe, ZnS Si, CaF, BaF or chalcogenide glass. However, other materials may also be utilized. The lens 16A may be made using a diamond turning or molding technique. The lens 16A can be designed to have a relatively large numerical aperture (NA). For example, the lens 16A can have a numerical aperture of at least approximately 0.6, 0.7, or 0.8. The NA may be approximated by the lens diameter divided by twice the focal length. Thus, for example, a lens diameter of 5 mm having a NA of 0.8 would have a focal length of approximately 3.1 mm.
The lens mount 16B fixedly and rigidly retains the lens 16A. For example, the lens mount 16B can be a rigid bracket. The lens fastener 16C fixedly and rigidly secures the lens mount 16B and the lens 16A to the mounting base 12A. For example, the lens fastener 16C can include one or more threaded bolts, an adhesive, one or more clamps, solder, or a weld.
The power source 18 provides electrical power for the QC gain media 14, the laser electronic controller 20, and the temperature controller 30. In
The temperature controller 19 can be used to control the temperature of the QC gain media 14, the mounting base 12A, and/or one or more of the other components of the laser assembly 12. Further, the temperature controller 19 can be used to maintain the relative position of the QC gain media 14, the optical assemblies 16, 22 and the WD feedback assembly 24.
In one embodiment, the temperature controller 19 includes a thermoelectric cooler 19A and a temperature sensor (not shown). The thermoelectric cooler 19A may be controlled to effect cooling or heating depending on the polarity of the drive current thereto. In
Alternatively, an intermediate plate may be attached between the thermoelectric cooler 19A and the mounting base 12A.
In certain embodiments, the temperature controller 19 can be used to change the temperature of the laser source 10 to affect a relatively small wavelength tuning range on the order 0.01% or less.
The temperature sensor provides temperature information that can be used to control the operation of the thermoelectric cooler 19A so that the thermoelectric cooler 19A can maintain the desired temperature of the MIR laser source 10. The temperature sensor 10 may comprise an encapsulated integrated circuit with a thermistor as the temperature sensor active component. The temperature sensor can be positioned on the mounting base 12A near the QC gain media 14 and can be used to monitor the temperature of the QC gain media 14.
Additionally, or alternatively, the source frame 12 can be mounted to a heat sink (not shown) inside a larger housing (not shown) which may also contain additional equipment including cooling fans and vents to further remove the heat generated by the operation of the laser source 10.
The laser electronic controller 20 controls the operation of the MIR laser source 10 including the electrical power that is directed to the QC gain media 14 and the temperature controller 19. For example, the laser electronic controller 20 can include a processor that controls QC gain media 14 by controlling the electron injection current. In
The output optical assembly 22 is positioned between the QC gain media 14 and the window 12D in line with the lasing axis 14C; and the output optical assembly 22 collimates and focuses the light that exits the second facet 14B of the QC gain media 14. For example, the output optical assembly 22 can include one or more lens 22A (one is illustrated in phantom), a lens mount 22B and a lens fastener 22C.
The lens 22A can be somewhat similar in design to the lens 16A of the cavity optical assembly 16. The lens mount 22B fixedly and rigidly retains the lens 22A. For example, the lens mount 22B can be a rigid bracket. The lens fastener 22C fixedly and rigidly secures the lens mount 22B and the lens 22A to the mounting base 12A. For example, the lens fastener 22C can include one or more threaded bolts, an adhesive, one or more clamps, solder, or a weld.
The WD feedback assembly 24 reflects the light back to the QC gain media 14 along the lasing axis 14C, and is used to precisely adjust the lasing frequency of the external cavity 28 and the wavelength of the output beam 26. Stated in another fashion, the WD feedback assembly 24 is used to feed back to the QC gain media 14 a relatively narrow band wavelength which is then amplified in the QC gain media 14. In this manner, the output beam 26 may be tuned and set to a desired fixed wavelength with the WD feedback assembly 24 without adjusting the QC gain media 14. Thus, in the external cavity 28 arrangements disclosed herein, the WD feedback assembly 24 dictates what wavelength will experience the most gain and thus dominate the wavelength of the output beam 26.
As provided above, the WD feedback assembly 24 includes the WD reflector 24A that has wavelength dependent reflectivity. The design of the WD reflector 24A and the WD feedback assembly 24 can vary pursuant to the teachings provided herein. Non-exclusive examples of a suitable WD reflector 24A includes a diffraction grating, a MEMS grating, prism pairs, a thin film filter stack with a reflector, an acoustic optic modulator, or an electro-optic modulator. A more complete discussion of these types of WD reflectors 24A can be found in the Tunable Laser Handbook, Academic Press, Inc., Copyright 1995, chapter 8, Pages 349-435, Paul Zorabedian.
The WD reflector 24A is spaced apart from the QC gain media 14 and cooperates with the QC gain media 14 to form the external cavity 28. In this embodiment, the external cavity 28 extends from the output coupler (reflective coating 14E) to the WD reflector 24A. The term external cavity 28 is utilized to designate the WD reflector 24A positioned outside of the QC gain media 14. In
The type of adjustment done to the WD reflector 24A to adjust the lasing frequency of the external cavity 28 and the wavelength of the output beam 26 will vary according to the type of WD reflector 24A. For example, if the WD reflector 24A is a diffraction grating, rotation of the diffraction grating relative to the lasing axis 14C and the QC gain media 14 adjusts the lasing wavelength and the wavelength of the output beam 26. There are many different ways to precisely rotate and fix the position of the diffraction grating. Only a few, non-exclusive examples are illustrated and discussed in detail herein.
Additionally, as illustrated in
In the embodiment illustrated in
In this embodiment, changing the incidence angle 230 serves to preferentially select a single wavelength which is the first order diffracted light from the reflector surface 224D. This light is diffracted back onto the same path as the incident beam 232 to thereby tune the external cavity 28 to the diffracted wavelength. The diffracted laser light is received by the QC gain media 14 to provide stimulated laser emission thereby resonating the QC gain media 14 with the grating selected wavelength.
The reflector retainer 224B (i) retains the WD reflector 224A, (ii) allows for rotation of the reflector surface 224D relative to the incident beam 232, and the QC gain media 14, and (iii) secures the WD reflector 224A to the mounting base 12A (illustrated in
In one embodiment, the retainer base 224E is generally rectangular plate shaped and is fixedly secured to the mounting base 12A with the feedback fastener 24B (illustrated in
In
The reflector lock 224C can be used to selectively move the retainer plate 224F and the WD reflector 224A relative to the other components of the laser source 10 and to selectively inhibit relative movement between the WD reflector 224A and the QC gain media 14. In
Referring to
Further, in
In this embodiment, during manufacturing of the laser source 310, the various components, except the cover 312B and the WD feedback assembly 324, are fixedly attached to the mounting base 312A. However, the WD feedback assembly 324 is positioned adjacent to the curved region 338. Next, the laser source 310 is turned on, and a measurement device 336 is used to precisely measure the wavelength of the output beam 326. Subsequently, the WD feedback assembly 324 is moved along the curved region 338 until the laser source 310 generates the desired fixed wavelength output beam 326. Next, the WD feedback assembly 324 is fixed to the mounting base 312A. Finally, the cover 312B can be secured to the mounting base 312A.
In
In certain embodiments, the curved region 338 has a center of curvature at the intersection of the lasing axis and the grating face.
It should be noted that the processes used to the manufacture the laser sources disclosed herein can be performed manually or in an automated assembly process. For example, the laser sources disclosed herein can be tuned with a robot, a human or a combination thereof.
While the particular laser sources as shown and disclosed herein is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.
