Patent Publication Number: US-9407055-B2

Title: Methods of modulating microlasers at ultralow power levels, and systems thereof

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of U.S. patent application Ser. No. 13/764,620, filed on Feb. 11, 2013, which, in turn, claims priority to U.S. Provisional Application No. 61/605,462, filed on Mar. 1, 2012 and entitled “Methods of Modulating Microlasers at Ultralow Power Levels and Related Devices,” all of which are incorporated herein in their entirety by reference. 
    
    
     FIELD 
     The present teachings relate to microlasers. More specifically, the present disclosure relates to microlasers that can be incorporated into a variety of monitoring applications wherein low power consumption is desirable. 
     BACKGROUND 
       FIG. 1  shows a prior art microlaser system that includes a photovoltaic power source  105  for providing power via switch  115  to semiconductor microlaser  110  that is arranged in a forward-biased configuration. 
     When switch  115  is in an open condition, photovoltaic power source  105  is disconnected from microlaser  110 , thereby placing microlaser  110  in an off state. However, when switch  115  is closed (as a result of a switch activation signal provided via line  116 ), photovoltaic power source  105  provides a voltage in the range of 1.5-2V to drive microlaser  110  into an on state and generate output laser beam  111 . 
     The driving voltage (1.5-2V) required to turn on microlaser  110  is roughly ten times higher than what a single photo-voltaic cell can generate in an open circuit condition. Consequently, the prior art arrangement shown in  FIG. 1  necessitates a more complex and expensive photovoltaic power source  105  incorporating multiple photovoltaic cells in a tandem arrangement. Furthermore, the manner in which semiconductor microlaser  110  is operated proves inefficient in terms of power consumption. 
     SUMMARY 
     According to a first aspect of the present disclosure, a microlaser system includes an optical source, a microlaser, an actuator switch, and a photovoltaic power source. The microlaser, which includes a control element, is optically pumped by at least a portion of light emitted by the optical source. The actuator switch is configured to be activated by a triggering event. The photovoltaic power source is coupled in a series connection with the actuator switch and the control element, the series connection configured to connect the photovoltaic power source to the control element of the microlaser when the actuator switch is activated by the triggering event. 
     According to a second aspect of the present disclosure, a method of operation includes directing light upon a microlaser for optically pumping the microlaser; and detecting the occurrence of a triggering event based on a change in an optical output of the microlaser, the change in optical output occurring in response to connecting a photovoltaic power source to the microlaser only upon occurrence of the triggering event. 
     According to a third aspect of the present disclosure, a method of operation includes directing light upon a microlaser for optically pumping the microlaser; and connecting a photovoltaic power source to modify an operational condition of the microlaser only upon occurrence of a triggering event, the modified operational condition indicative of the occurrence of the triggering event. 
     Further aspects of the disclosure are shown in the specification, drawings and claims of the present application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more embodiments of the present disclosure and, together with the description of a few example embodiments, serve to explain the principles and implementations of the disclosure. The components in the drawings are not necessarily drawn to scale. Instead, emphasis is placed upon clearly illustrating various principles. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  shows a prior art microlaser system that includes a semiconductor microlaser arranged in a forward-biased configuration. 
         FIG. 2  shows a first embodiment of a microlaser system incorporating a temperature-based control element in accordance with the present disclosure. 
         FIG. 3  shows a second embodiment of a microlaser system incorporating a piezoelectric-based control element in accordance with the present disclosure. 
         FIG. 4  shows a third embodiment of a microlaser system incorporating a capacitor-based control element in accordance with the present disclosure. 
         FIG. 5  shows a fourth embodiment of a microlaser system incorporating a reverse biased diode-based control element in accordance with the present disclosure. 
         FIGS. 6A and 6B  show graphs depicting a time profile of intensity and wavelength of a pump light (an emitted beam) in accordance with the present disclosure. 
         FIGS. 7A and 7B  show graphs depicting a time profile of intensity and wavelength of a pump light (an emitted beam) in accordance with the present disclosure. 
         FIG. 8  shows some structural details of a microlaser incorporating a first embodiment of the capacitor-based control element in accordance with the present disclosure. 
         FIG. 9  shows some structural details of a microlaser incorporating a second embodiment of the capacitor-based control element in accordance with the present disclosure. 
         FIG. 10  shows some structural details of a microlaser incorporating the piezoelectric-based control element in accordance with the present disclosure. 
         FIG. 11  shows some structural details of a microlaser incorporating the temperature-based control element in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Throughout this description, embodiments and variations are described for the purpose of illustrating uses and implementations of the inventive concept. The illustrative description should be understood as presenting examples of the inventive concept, rather than as limiting the scope of the concept as disclosed herein. 
     The various embodiments described herein are generally directed at a microlaser system that may be used in various monitoring applications, including applications involving detection of one or more occurrences of an event. The microlaser system incorporates a microlaser which is optically pumped into operation based on photoluminescence by a light source (thereby avoiding a power penalty associated with using a separate power source, such as a photovoltaic cell). The microlaser emits a laser beam when pumped into operation by the light source. 
     The microlaser system further incorporates an event sensor circuit that provides a trigger signal to a switch. When a triggering event occurs, the switch is activated by the trigger signal, and a photovoltaic power source is coupled to a control element of the microlaser. When energized by the photovoltaic power source, the control element (which can be implemented in a variety of ways, some of which are described below) operates to cause a change in characteristic (wavelength, intensity etc.) of the emitted laser beam. This change in characteristic (which may be alternatively understood as modulation of an emitted laser beam) may be used to optically communicate the occurrence of the event. 
     Significantly, rather than using the photovoltaic power source to provide an electrical voltage/current (power) to drive the microlaser in the manner indicated in prior art  FIG. 1 , the photovoltaic power source is used to provide power for driving the control element instead. The power consumption of the control element is low enough to permit a relatively small photovoltaic power source to be used, thereby providing cost and efficiency benefits over prior art systems that use complex, higher capacity, and expensive photovoltaic power sources. 
     Four types of control elements and the corresponding changes in characteristics of the emitted laser beam are described below in accordance with the invention. In broad terms, the four types of control elements enable modulation of the emitted laser beam on the basis of temperature variation, piezoelectric deformation, capacitance variation, and electronic forward/reverse biasing of the microlaser. 
     To elaborate upon these aspects in more detail, attention is first drawn to  FIG. 2 , which shows a first embodiment of a microlaser system  200  based on varying the temperature of a microlaser whereby a wavelength shift and/or an intensity change is impressed upon an emitted beam of light. Microlaser system  200 , which can be used for detecting various types of event occurrences (triggering events) in a wide variety of applications, includes microlaser  210  that is optically pumped into operation based on photoluminescence by a light source  225 . 
     As can be understood, unlike prior art systems wherein a photovoltaic power source containing a significant number of photovoltaic cells is needed to drive a microlaser, in the embodiment shown in  FIG. 1 , photovoltaic power source  205  can be a low power source that generates low ‘open circuit voltage’ and current. More particularly, the open circuit voltage which is used to drive a control element (rather than the microlaser itself), is around 0.5 V, which is about 4 times smaller than what is required in prior art systems. 
     In the example embodiment shown in  FIG. 2 , the control element is a heater element, more particularly a micro-heater  230 . Other types of control elements will be described below using other embodiments. Micro-heater  230  is in contact with a surface of microlaser  210 , for example, a bottom surface of microlaser  210 , such that heat provided by micro-heater  230  affects the active layer (not shown) of microlaser  210  and consequently changes a wavelength and/or an intensity of emitted beam  211 . 
     A photovoltaic power source  205 , which is configured to receive light from light source  220 , convert the received light into electrical power, and provide the electrical power to micro-heater  230  when actuator switch  215  is in a closed (on) position. Actuator switch  215  is turned on/off (closed/open) on the basis of a trigger signal provided on line  216 . The trigger signal is derived from an event sensor circuit (not shown) that is selected on the basis of various applications. In one example implementation, event detection system  200  is partially or entirely embedded inside an animate object (such as a human being for example), and used to detect the occurrence of various biomedical events, such as for example, when an undesirable substance carried in the bloodstream of the animate object exceeds a threshold level. 
     Furthermore, in this example embodiment as well as other embodiments described herein, light sources  220  and  225  may be combined into a single light source, a laser for example. In the example biomedical implementation described above, such a laser may be also embedded into the animate object for providing light to photovoltaic power source  205  and microlaser  210 . The emitted beam  211  from microlaser  210  may be observed through suitable viewing ports provided in the animate object, or by using optical fiber to optically transport emitted beam  211  out of the animate object. 
     Micro-heater  230  may be implemented in a variety of ways. In one example implementation based on platinum/titanium (Pt/Ti) [3], micro-heater  230  can be selected to provide a localized temperature of around 150° C. when provided with 2.25 mW of driving power (0.75V×3 mA) from photovoltaic power system  205 . At a driving power of 1 mW (0.5V×2 mA), the localized temperature can reach 75° C. 
     Attention is next drawn to  FIG. 3 , which shows a second embodiment of an event detection system  300  based on piezoelectric deformation of a portion of microlaser  310 . The piezoelectric deformation results in a wavelength shift and/or an intensity change in an emitted beam of light. More particularly, in contrast to micro-heater  230  which is the control element in the first example embodiment, in this example embodiment, the control element is a piezoelectric layer  330  that may be fabricated as an integral layer inside microlaser  310 . Piezoelectric layer  330  is a p-i-n doped layer that can be actuated by application of a low voltage under a reverse biased condition. 
     In one example implementation, piezoelectric layer  330  is significantly deformed when photovoltaic power system  205  provides a few hundred millivolts. This mechanical deformation causes emitted beam  211  to undergo a change in wavelength—either an increase or a decrease in wavelength depending on the way microlaser  310  is fabricated. The electrical power provided by photovoltaic power system  205  for enabling this wavelength change is as low as a few nW because there is essentially no current flow through microlaser  310  that is configured to operate in a reverse biased state (by suitable polarity-based connections between microlaser  310  and photovoltaic power system  205 ). 
       FIG. 4  shows a third embodiment of an event detection system  400  based on varying a capacitive element  430  that is a part of microlaser  410 . In one example implementation, capacitive element  430  is a metallic membrane located above an active layer (described below in more detail using  FIG. 8 ) of microlaser  410 . The metallic membrane is deflected closer to a lasing disk upon application of a voltage provided by photovoltaic power system  205 . The deflection results in an increase in light scattering by the metallic membrane and/or absorption of light in the metallic membrane and switches off microlaser  410 . 
     In another example implementation, a thin insulating layer separates two micro-disks (described below in more detail using  FIG. 9 ) that operate as a lasing cavity. The separation distance between the two micro-disks can be changed by electrostatic deflection of one or both of these two micro-disks upon application of a voltage provided by photovoltaic power system  205 . The change in separation distance in microlaser  410  is intended as a means to alter a quality factor of the microlaser (in other word, Q-switching) and/or a resonant wavelength of the microlaser. As a result, wavelength/intensity of emitted beam  211  can be modulated in response to the triggering signal  212 . The power provided by photovoltaic power system  205  for enabling this action is quite low because there is essentially no current flow through microlaser  410  that is configured to operate in a reverse biased state by suitable polarity-based connections between microlaser  410  and photovoltaic power system  205 . 
       FIG. 5  shows a fourth embodiment of an event detection system  500  based on electronic reverse biasing of microlaser  510 , which may be interpreted as integrally incorporating a reverse-biased diode assembly. The electronic reverse biasing, which is carried out by suitable polarity-based connections between microlaser  510  and photovoltaic power system  205 , is operative to changing the carrier concentration within microlaser  510 . In this embodiment, light source  225  provides enough light to optically pump enough carriers within microlaser  510  so as to exceed a lasing threshold. An electrostatic field can be used to move carriers inside microlaser  510 . In this approach, the carrier depletion close to the active lasing material inside microlaser  510  results in turning off microlaser  510 . The resulting on-off binary nature of the emitted beam  211  can be used to carry digital information optically. The electrical power provided by photovoltaic power system  205  is quite low because there is essentially no current flow through microlaser  510  that is configured to operate in the reverse biased state. 
     In an alternative approach, in lieu of the reverse bias, a forward bias can be used to supply more carriers inside microlaser  510  so as to increase a laser gain. In this approach, light source  225  provides a stationary laser gain within microlaser  510  at a level close to/above a lasing threshold of the microlaser  510 , which results in stationary laser emission  211  whose intensity does not change in time. Intensity of the emitted beam  211  can be modulated (increased) in response to the forward biased current by the photovoltaic power system  205 . The electrical power provided by photovoltaic power system  205  for enabling this action is relatively low in comparison to prior art implementations. 
     Attention is next drawn to  FIG. 6A  which shows a train of optical pulses provided by pump light source  225  to wirelessly power any of the microlaser embodiments described above. The train of optical pulses can be characterized by using a variety of parameters such as, a pulse period, a pulse width, a duty cycle, and/or a repetition rate. One or more of these parameters may be varied in accordance with one or more respective applications. For example, in a biological application where it is undesirable to overheat biological samples, it may be preferable to use pulse widths in the range of tens of nanoseconds. However, in certain other applications this low pulse width may not be suitable and consequently, a larger pulse width may be selected. 
     As for duty cycles, microlasers can be operated at duty cycles of 1% to 10% when a pulse period is in the range of 0.1 is to a few μs. Some advanced microlasers having ultra-low thresholds may be operated continuously (100% duty cycle) without encountering thermal problems. Peak power to operate a microlaser is typically about 1 kW/cm 2 . About 10-20% of the pump light power  225  incident on some microlasers may be absorbed by the microlasers and used to create population inversion in the active layer. 
       FIG. 6A  further shows a graph of light intensity versus wavelength (in other words, ‘spectrum’), wherein wavelength λ p  represents the wavelength of the light provided by light source  225  to wirelessly power any of the microlaser embodiments described above. In certain embodiments, this wavelength may not only be used by light source  225  but by light source  220 . As indicated above, these two light sources may be implemented as a merged, single light source. 
       FIG. 6B  shows a train of emitted beam pulses  211  from the microlaser embodiments described above. The train of microlaser output pulses is synchronized with that of the pump laser  225 . The microlaser  211  is operated at a wavelength of λ e , which is longer than that of the pump light wavelength λ p . In the absence of a triggering ‘event’  212 , the peak output intensity and the emission wavelength λ e  do not change in time. 
       FIG. 7A  shows a train of optical pulses that is provided by light source  225  at a wavelength of λ p  to wirelessly power any of the microlaser embodiments described above.  FIG. 7A  further shows a graph of wavelength versus time. As can be understood, the wavelength of λ p  remains unchanged over time regardless of the presence of an event (which would close actuator switch  215  but could not affect the pump light source  225 ). 
     On the other hand,  FIG. 7B  illustrates a change in the time profile  705  of the event as a result of the occurrence of the event. The change in time profile  705  is characterized by a change in the intensity of emitted beam  211  in accordance with certain embodiments of the invention. In these and/or embodiments, a change in the wavelength (Δλ e &gt;0 or &lt;0) of emitted beam  211  may take place along with the intensity change as indicated in the graph of wavelength versus time. 
       FIG. 8  shows some structural details of a microlaser  410  incorporating a first embodiment of the capacitor-based control element in accordance with the present disclosure. In this embodiment, capacitive element  430  (shown in  FIG. 4 ) is a metallic membrane  830  separated by a gap  835  from lasing structure  825 , which includes an active layer  815 . Metallic membrane  830  is deflected closer to active layer  815  upon application of a voltage that is provided by photovoltaic power system  205  (shown in  FIG. 4 ). The deflection results in an increase in light scattering by the metallic membrane and/or absorption of light in the metallic membrane and switches off microlaser  410 . Layers  805  and  820  are configured as electrodes located on opposing sides of lasing structure  825  for application of the voltage by photovoltaic power system  205 . 
       FIG. 9  shows some structural details of a microlaser  410  incorporating a second embodiment of the capacitor-based control element in accordance with the present disclosure. In this embodiment, capacitive element  430  is formed of an insulating layer  30  separating two micro-disks  35  and  40  (having active layers  15  and  20  respectively). The separation distance between the two micro-disks can be changed by electrostatic deflection of one or both of these two micro-disks upon application of a voltage (provided by photovoltaic power system  205 ). The change in separation distance causes microlaser  410  to be detuned out of the gain region, thereby resulting in a change in wavelength/intensity of emitted beam  211 . 
       FIG. 10  shows some structural details of a microlaser  310  incorporating the piezoelectric-based control element in accordance with the present disclosure. Microlaser  310  includes a pair of electrodes  50  and  55  that are located on opposite surfaces of layered structure  60 , which includes multiple p-i-n layers. An intrinsic layer  57  serves as both an active layer and a piezoelectric layer. Under a reverse bias condition, the voltage applied via electrodes  50  and  55  induces mechanical stress in intrinsic layer  57  based on the piezoelectric nature of this layer. The mechanical stress causes the brim of layered structure  60  to deform, which in turn changes the emission wavelength (λ e ) of the emitted beam  211  (shown in  FIG. 3 ). The change in wavelength is typically of the order of a few nanometers. 
       FIG. 11  shows some structural details of a microlaser  210  incorporating the temperature-based control element in accordance with the present disclosure. Microlaser  210  includes a metal pedestal  85  that is in contact with a microlaser layer  80 . In some example implementations, metal pedestal  85  is made of one or more metals such as gold and platinum. Such metals operate as very good thermal as well as electrical conductors. Heat generated by heater  230  (shown in  FIG. 2 ), which may be located inside metal pedestal  85  or external to metal pedestal  85 , is efficiently delivered to microlaser layer  80  via metal pedestal  85 . In one embodiment, where emitted beam  211  operates at a wavelength of 1.3 μm, microlaser layer  80  may incorporate an indium gallium arsenide phosphate (InGaAsP) compound. The diameter of microlaser layer  80  can be in a range of 3 to 5 μm, which results in single mode lasing operation. 
     It is generally known that a refractive index (n) of indium phosphate (InP), which is widely used as a backbone material for building a microlaser emitting at near infrared wavelengths, is a function of temperature (T) with dn/dT=2×10 −4 . [4]. For prior art microlaser designs emitting at near infrared wavelengths, this thermal coefficient can be translated into dλ e /dT˜0.1 nm/K, where λ e  is the emission wavelength of the laser[5], which implies that, if ΔT (temperature change)=50 K, one should observe about Δλ e =5 nm red-shift in the wavelength of emitted beam  211 . Therefore, as shown in  FIG. 6B , one would observe a wavelength increase of a few nanometers. 
     Most microlasers undergo noticeable degradation in output power upon an increase in the temperature of operation because the gain of a microlaser tends to decrease as temperature increases. In accordance with the disclosure, the modified operational conditions of a microlaser, more particularly the changes in the emission wavelength and/or the changes in the intensity of emitted beam  211 , can be used to detect an occurrence of an event. 
     Emitted beam  211  may be transported via a wide variety of transmission media, such as for example, free space optics and optical fiber media, and may be used for providing various types of information depending on the type of application in which the various embodiments described herein are used. For example, in a first application, the information may be a digital bit stream that indicates occurrence or non-occurrence of an event, or carries digital communication data. In a second application, the information pertains to voltage levels and/or current levels present in various elements of a system (for example, on line  216 , or on the lines that couple photovoltaic power source  205  to the microlaser). 
     Furthermore, the microlaser systems described herein can be used in a side variety of environments, such as in an in-vivo environment described above. In another implementation, the system may be integrated into certain structures, such as for example, a wing of an airplane made of a composite material. A suitable transparent epoxy window may be used in such an implementation for propagating emitted beam  211 . 
     All patents and publications mentioned in the specification may be indicative of the levels of skill of those skilled in the art to which the disclosure pertains. All references cited in this disclosure are incorporated by reference to the same extent as if each reference had been incorporated by reference in its entirety individually. 
     It is to be understood that the disclosure is not limited to particular methods or systems, which can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. The term “plurality” includes two or more referents unless the content clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosure pertains. 
     The examples set forth above are provided to give those of ordinary skill in the art a complete disclosure and description of how to make and use the embodiments described herein, and are not intended to limit the scope of what the inventors regard as their disclosure. Modifications of the above-described modes for carrying out the disclosure may be used by persons of skill in the relevant arts, and are intended to be within the scope of the following claims. 
     A number of embodiments of the disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the present disclosure. Accordingly, other embodiments are within the scope of the following claims. 
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