US20180083421A1 - Independent control of emission wavelength and output power of a semiconductor laser - Google Patents

Abstract

Methods for driving a tunable laser with integrated tuning elements are disclosed. The methods can include modulating the tuning current and laser injection current such that the laser emission wavelength and output power are independently controllable. In some examples, the tuning current and laser injection current are modulated simultaneously and a wider tuning range can result. In some examples, one or both of these currents is sinusoidally modulated. In some examples, a constant output power can be achieved while tuning the emission wavelength. In some examples, the output power and tuning can follow a linear relationship. In some examples, injection current and tuning element drive waveforms necessary to achieve targeted output power and tuning waveforms can be achieved through optimization based on goodness of fit values between the targeted and actual output power and tuning waveforms.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This application claims the benefit of U.S. Provisional Patent Application No. 62/129,607 filed on Mar. 6, 2015, the entire disclosure of which is herein incorporated by reference for all purposes.
FIELD
• This relates generally to methods for driving a semiconductor laser, and more particularly to independently controlling emission wavelength and output power of the laser.
BACKGROUND
• Semiconductor lasers may be useful for a range of applications, and in some instances, it may be desirable to have a tunable laser with narrow line widths and single frequency emission. These applications can benefit from tunable lasers with extremely narrow linewidths and single frequency emission.
• To achieve narrow linewidths and wide tuning range, external cavity laser (ECL) systems, Distributed Bragg Reflector lasers, or Distributed Feedback lasers with integrated tuning elements can be employed. However, the performance of these systems and lasers can be limited by unwanted mechanical moving parts, slow response times, narrow tuning ranges, and nonlinear distortions in the laser output.
SUMMARY
• This relates to methods for driving a tunable semiconductor laser with integrated tuning elements. The methods can include modulating the tuning current and laser injection current such that the laser emission wavelength and output power are independently controllable. In some examples, the tuning current and laser injection current are modulated simultaneously. In some examples, one of these currents or both is sinusoidally modulated. In some examples, a constant output power can be achieved while tuning the emission wavelength. In some examples, a larger tuning range can be achieved by examples of the disclosure compared to lasers whose tuning current and injection current are not modulated simultaneously. In some examples, the output power and tuning can follow a linear relationship. In some examples, injection current and tuning element drive waveforms necessary to achieve targeted output power and tuning waveforms can be achieved through optimization based on goodness of fit between the targeted and actual output power and tuning waveforms.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 illustrates an exemplary system in which examples of the disclosure can be implemented.
• FIG. 2A illustrates a drawing of an external cavity laser system according to examples of the disclosure.
• FIG. 2B illustrates a longitudinal view of an exemplary Distributed Bragg Reflector laser according to examples of the disclosure.
• FIG. 2C illustrates a longitudinal view of an exemplary Distributed Feedback Laser according to examples of the disclosure.
• FIG. 3 illustrates a cross-sectional view of a laser according to examples of the disclosure.
• FIG. 4A illustrates an exemplary laser with an integrated tuning element according to examples of the disclosure.
• FIG. 4B illustrates exemplary heat-related time delays for a laser with an integrated tuning element according to examples of the disclosure.
• FIG. 4C illustrates an exemplary plot of the optical output power in response to a square wave driven on the tuning element electrode, with asymmetry in rise and fall times due to the differences in heat-related time delays according to examples of the disclosure.
• FIG. 5A illustrates an exemplary relationship between laser injection current and laser output power according to examples of the disclosure.
• FIG. 5B illustrates an exemplary relationship between laser drive power and emission wavelength or wavenumber according to examples of the disclosure.
• FIG. 5C illustrates an exemplary relationship between injection current and laser voltage according to examples of the disclosure.
• FIG. 5D illustrates an exemplary relationship between the temperature of the active region and the thermal rollover power according to examples of the disclosure.
• FIG. 5E illustrates an exemplary relationship between tuning element power and laser output power according to examples of the disclosure.
• FIG. 5F illustrates an exemplary relationship between tuning element power and emission wavelength or wavenumber according to examples of the disclosure.
• FIG. 5G illustrates an exemplary relationship between tuning element drive current and tuning element voltage according to examples of the disclosure.
• FIG. 6 illustrates an exemplary setup for simultaneous modulation and dynamic adjustment of the emission wavelength and output power from a laser according to examples of the disclosure.
• FIG. 7A illustrates an exemplary sinusoidally modulated drive waveform applied to the tuning element and a constant drive waveform applied to the waveguide of a laser according to examples of the disclosure.
• FIG. 7B illustrates exemplary sinusoidally modulated drive waveforms applied to the electrodes of a laser according to examples of the disclosure.
• FIG. 7C illustrates exemplary sinusoidally varying wavelength shift and constant output power below a thermal rollover power from a laser according to examples of the disclosure.
• FIG. 7D illustrates exemplary sinusoidally varying wavelength shift and output power at or near a thermal rollover power from a laser according to examples of the disclosure.
• FIG. 7E illustrates exemplary sinusoidally varying wavelength shift and constant output power equal to a system requirement value according to examples of the disclosure.
• FIG. 7F illustrates exemplary non-sinusoidally modulated drive waveforms applied to the electrodes of the laser according to examples of the disclosure.
• FIG. 7G illustrates an exemplary linear ramp of the emission wavelength with a constant output power below a thermal rollover power from a laser according to examples of the disclosure.
• FIG. 7H illustrates an exemplary linear ramp of the emission wavelength and an output power at or near the thermal rollover power according to the examples of the disclosure.
• FIG. 7I illustrates exemplary variable wavelength shift and variable output power from a laser according to examples of the disclosure.
• FIGS. 8A-8B illustrate exemplary process flows for optimizing the injection current and tuning element current according to examples of the disclosure.
DETAILED DESCRIPTION
• In the following description of examples, reference is made to the accompanying drawings in which it is shown by way of illustration specific examples that can be practiced. It is to be understood that other examples can be used and structural changes can be made without departing from the scope of the various examples.
• Various techniques and process flow steps will be described in detail with reference to examples as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of one or more aspects and/or features described or referenced herein. It will be apparent, however, to one skilled in the art, that one or more aspects and/or features described or referenced herein may be practiced without some or all of these specific details. In other instances, well-known process steps and/or structures have not been described in detail in order to not obscure some of the aspects and/or features described or referenced herein.
• Further, although process steps or method steps can be described in a sequential order, such processes and methods can be configured to work in any suitable order. In other words, any sequence or order of steps that can be described in the disclosure does not, in and of itself, indicate a requirement that the steps be performed in that order. Further, some steps may be performed simultaneously despite being described or implied as occurring non-simultaneously (e.g., because one step is described after the other step). Moreover, the illustration of a process by its depiction in a drawing does not imply that the illustrated process is exclusive of other variations and modification thereto, does not imply that the illustrated process or any of its steps are necessary to one or more of the examples, and does not imply that the illustrated process is preferred.
• This disclosure relates to methods for driving a tunable laser with integrated tuning elements. The tuning current and laser injection current can be modulated such that the laser emission wavelength and output power are independently controllable. With independently controllable emission wavelength and output power, a wider tuning range (compared to a laser whose tuning current and injection current are not modulated simultaneously) can be achieved, and the laser can emit a substantially constant output power while the emission wavelength is tuned. Moreover, unwanted mechanical moving parts and compromise in the performance of the laser, such as slow response times and nonlinear distortions, can be avoided or reduced.
• Representative applications of methods and apparatus according to the present disclosure are described in this section. These examples are being provided solely to add context and aid in the understanding of the described examples. It will thus be apparent to one skilled in the art that the described examples may be practiced without some or all of the specific details. Other applications are possible, such that the following examples should not be taken as limiting.
• FIG. 1 illustrates an exemplary system in which examples of the disclosure can be implemented. Tracegas detection system110 can include adisplay112,laser114, anddetector116.Laser114 can be configurably operable using any of the methods and waveforms as will be disclosed.
• Tunable semiconductor lasers are needed for many applications, such as trace gas detection, environmental monitoring, biomedical diagnostics, telecommunications, and industrial process controls. These applications, in particular, can benefit from tunable lasers with narrow or extremely narrow linewidths and single frequency emission.
• Although typically used in large, bulky systems, tunable semiconductor lasers can have many uses in portable electronic devices. For example, wall-mountable tracegas detection system110 can be located in a user's garage and can be used to detect if the exhaust from an automobile parked in the garage exceeds a safe level. The wall-mountable tracegas detection system110 can provide a warning ondisplay112 and additionally transmit the information to a mobile telephone. In response, the mobile telephone can warn the user of the hazard and can prevent the user from entering the garage.
• One way to achieve a wide tuning range is by using an external cavity laser (ECL) system.FIG. 2A illustrates a drawing of an external cavity laser system according to examples of the disclosure. TheECL system208 can include again medium200,lens202, anddiffraction grating204. The emission wavelength of theECL system208 can be tuned by rotating thediffraction grating204. Although theECL system208 can achieve a wide tuning range, the system can include macroscopic mechanical moving parts and thus can have limited tuning speed, can require precision alignment, can be susceptible to mechanical vibrations, and can have a tendency to exhibit mode hops. The external cavity laser architecture can be impractical for many of the applications discussed earlier, especially when used in portable electronic devices.
• Another way to achieve a wide tuning range and narrow linewidths is by incorporating a periodic structure into the semiconductor laser. Two types of lasers with an incorporated period structure are Distributed Bragg Reflector (DBR) and Distributed Feedback (DFB) lasers.
• FIG. 2B illustrates a longitudinal view of an exemplary DBR laser according to examples of the disclosure.DBR laser210 can comprise one or more diffraction gratings located in agrating section212 that can be separate from thegain section214. In some examples,DBR laser210 can include grating sections, such asgrating section212, located at both ends of the device (not shown). The diffraction grating can be located on or above theactive region216 and can be employed to provide optical feedback to reflect light back into the cavity to form a resonator. The grating can be configured to reflect only narrow bands of wavelengths to produce a single longitudinal mode wavelength, and the period of the grating can be adjusted to achieve specific laser emission wavelengths.
• FIG. 2C illustrates a longitudinal view of an exemplary DFB laser according to examples of the disclosure.DFB laser220 can comprise a diffraction grating located ingrating section222, which can be integrated into or located on theactive region226. The structure and functionality of the diffraction grating of a DFB laser can be similar to that of a DBR laser. However, unlike DBR lasers where thegrating section212 is separate from thegain section214, the DFB laser can have a gain section that coincides with the grating section. Since the gain section of the DFB laser can be located in the same region as thegrating section222, DFB lasers can have shorter cavity lengths. Additionally, DFB lasers can be less susceptible to mode hops.
• FIG. 3 illustrates a cross-sectional view of a laser according to examples of the disclosure.Laser300 can comprise asubstrate310, cladding layers312, insulatingcladding318,active region314, andelectrodes316.Laser300 can be mounted to or touchingsubmount320.Laser300 can be driven by asource324 that supplies an injection current I_L, for example. To tune the emission wavelength oflaser300, the temperature of theactive region314 or the injection current I_Lcan be adjusted. In some examples, the temperature can be adjusted to coarsely tune the emission wavelength, and the injection current I_Lcan be adjusted for more precise tuning.
• To adjust the temperature of theactive region314, a thermo-electric cooler (TEC)322 can apply heat tosubmount320. The heat can pass through theelectrode316,substrate310, andcladding layer312 toactive region314, which can cause a change in the refractive index of the grating (located on or above active region314). The change in refractive index can lead to a change in the emission wavelength. Therefore, the emission wavelength oflaser300 can be tuned by changing the heat applied throughTEC322. However, there can be a few problems with tuning through applied heat. Heating the laser may require a lot of power fromTEC322. Requiring a lot of power may not be practical for certain devices, such as portable electronic devices, which have stringent power constraints due to portability and size requirements. Additionally, heating thelaser300 usingTEC322 can lead to a slow response resulting in slow tuning speeds, asTEC322 may need to cool and heat thelaser300,submount320, and heat sink (not shown). Accordingly, the time delay between applying the heat and achieving the targeted emission wavelength can be on the order of seconds, and many applications may need quicker adjustment of the laser emission wavelength.
• Another method of tuninglaser300 can be through the injection current I_L. Injection current I_Lcan be used to drive the laser throughelectrodes316. Similar to usingheater322, the injection current I_Lcan heat up theactive region314 resulting in a change in the emission wavelength. While tuning using the injection current may provide a finer adjustment in wavelength, the tuning range can be limited when using this method because the laser efficiency may decrease at the high injection currents necessary to substantially increase the temperature. For example, the tuning range can be on the order of about 0.2% from the nominal wavelength. For a 9 μm laser, this can lead to a tuning range of only 0.036 μm, which may be insufficient for applications such as spectroscopy. Furthermore, independent control of the laser output power and the emission wavelength may not be possible, since both the output power and emission wavelength can be influenced by the injection current.
• One way to overcome the problems encountered when tuning through a TEC or through the injection current can be to use integrated tuning elements.FIG. 4A illustrates an exemplary laser with an integrated tuning element according to examples of the disclosure.Laser450 can comprise asubstrate460, cladding layers462,insulated cladding463,active region464,electrodes456,466, and468, andresistive cladding461.Laser450 can comprise twoseparate structures480 and482.Structure480 can be a tuning element, andstructure482 can be a laser structure, for example. Asource474 can be connected to the top andbottom electrodes456 and468 of thelaser structure482 to provide an injection current I_Lto drive thelaser450. Tuning element orstructure480 can be an electrically contacted structure that is in close proximity to structure482.Source476 can be connected to thetop electrode466 to provide tuning current I_Tto drive the tuning element orstructure480. In one embodiment, trench478 forms an electrical resistor that generates heat when current is provided bysource476. WhileFIG. 4A showssources474 and476 as current sources, one skilled in the art would understand that any type of source can be connected. Examples of the disclosure can include, but are not limited to,source476 altering the modal refractive index of the laser waveguide through heating, plasma dispersion effects, electro-optic effects, or a combination.
• Source474 can drive the laser through injection current I_Lto achieve a targeted output power, andsource476 can adjust the temperature of theactive region464 through tuning current I_Tto achieve a targeted emission wavelength. However, adjusting the injection current I_Lcan affect the temperature of theactive region464, and adjusting the tuning current I_Lcan affect the gain of thelaser450. Since the injection current I_Land the tuning current I_Tare not entirely decoupled, adjusting the injection current I_Land adjusting the tuning current I_Tcan have limited flexibility in optimizing the emission wavelength without negatively affecting the optical losses or electrical properties of the laser.
• The laser can also include multiple heat-related time delays, as shown inFIG. 4B.Path490 and496 can be associated with heat induced by the tuning element current orsource476.Paths492 and494 can be associated with heating or cooling induced byTEC422. Heat alongpath490 can diffuse from the tuning element to theactive region464 in the laser structure, leading to a time delay that can be on the order of hundreds of μs. Heat alongpath496 can diffuse through insulatingcladding463,substrate460, andelectrode468 toTEC422. Heat alongpath494 can diffuse from the submount (not shown) andsubstrate460 to theactive region464 to heat the laser ridge, and heat alongpath492 can diffuse from the laser ridge to thesubstrate460 and submount to cool the laser ridge. The time delays frompaths492 and494 can be about 1-100 ms. Due to the large difference between the time delays ofpath490 andpath492/494, the use ofsource476 and474 along withTEC422 can lead to nonlinear distortions and long response times.FIG. 4C illustrates an exemplary plot of the output power response to a square wave modulation of thetuning element source476 due to the differences in heat-related time delays according to examples of the disclosure.
• FIGS. 5A-5C illustrate exemplary effects of altering the injection current applied to the laser according to examples of the disclosure.FIG. 5A illustrates an exemplary relationship between laser injection current and laser output power according to examples of the disclosure. For a constant submount temperature, the laser output power can be linearly dependent on the injection current above the laser threshold current. The slope efficiency of the laser can be determined based on this linear dependency. At high injection currents, this linear dependence can break down due to an increase in the temperature of the active region causing a decrease in internal quantum efficiency, which in turn can cause the output power to saturate (i.e., thermal rollover).
• FIG. 5B illustrates an exemplary relationship between laser drive power and emission wavelength or wavenumber according to examples of the disclosure. For a constant submount temperature, the emission wavelength or wavenumber can be linearly dependent on the laser drive power. As shown, the emission wavelength can redshift with increased drive power. As discussed above, a change in temperature of the active region can lead to a change in the refractive index, and therefore a change in the emission wavelength.
• FIG. 5C illustrates an exemplary relationship between injection current and laser voltage according to examples of the disclosure. The shift in emission wavelength that results from thermal tuning can be proportional to the power dissipation inside the laser, which can be related to the product of the voltage and current. Due to the relationship between injection current and laser voltage illustrated in the figure, the emission wavelength may not be directly proportional to the injection current.
• FIG. 5D illustrates an exemplary relationship between the temperature of the active region and the thermal rollover power according to examples of the disclosure. As discussed above, while increasing the injection current can lead to a higher output power, the temperature of the active region can increase. Above a certain injection current, the temperature of the active region can be so high that a reduction in the efficiency of the laser and a deviation from the linear dependency (or slope of the plot illustrated inFIG. 5A) can result. The output power can saturate to a maximum output power (i.e., thermal rollover power or where the derivative of power to injection current is zero) within the safe operating range of the laser. The maximum output power or thermal rollover power can depend on many factors including, but not limited to, the heatsink or substrate temperature, the tuning or heater current, device length, and device design. The thermal rollover power can decrease as the temperature of the active region increases, as shown in the figure. As a result, the thermal rollover power can vary for the same laser device depending on operating conditions.
• FIGS. 5E-5G illustrate exemplary effects of altering the tuning element of the laser according to examples of the disclosure.FIG. 5E illustrates an exemplary relationship between tuning element power and laser output power according to examples of the disclosure. For a constant laser injection current, the laser output power can be linearly dependent on the tuning element power. As shown in the figure, the laser output power can decrease as tuning power is increased.
• FIG. 5F illustrates an exemplary relationship between tuning element power and emission wavelength or wavenumber according to examples of the disclosure. For constant laser injection current, the emission wavelength can be nearly linear dependent on the tuning element drive power. As shown in the figure, the laser output power can decrease as tuning element drive power increases.
• FIG. 5G illustrates an exemplary relationship between tuning element drive current and tuning element voltage according to examples of the disclosure. The IV curve of the tuning element may become nonlinear at high tuning element currents. This nonlinear relationship can cause the laser output power and emission wavelength to diverge from having a known relationship (e.g., quadratic dependence) with respect to the tuning element current. Due to divergence from a known relationship and because of the coupling between the injection current and tuning element current with emission wavelength and output power, simultaneous optimization of the emission wavelength and output power can become a challenge. Additionally, the performance of the laser (e.g., tuning range and output power) can be limited.
• FIG. 6 illustrates a setup for simultaneous modulation and dynamic adjustment of the emission wavelength and output power of a laser according to examples of the disclosure. The setup can include alaser600.Laser600 can include, but is not limited to, a DFB or DBR semiconductor laser, such as a quantum cascade laser (QCL), interband cascade laser (ICL), or type-I laser. In some examples, the laser can operate in several wavelength ranges including the near- and mid-infrared.Laser600 can include twoseparate electrodes664 and666. One electrode, such aselectrode664, can be coupled to the tuning element (such asstructure480 illustrated inFIG. 4A). The other electrode, such aselectrode666, can be coupled to the laser structure (such asstructure482 illustrated inFIG. 4A).
• Two sources can be coupled to the electrodes to independently drive the electrodes. For example,source676 can be coupled toelectrode666 of the tuning element, andsource674 can be coupled toelectrode664 of the laser. Theoutput672 oflaser600 can be directed atlens630, which can reflect offmirror632.Beamsplitter634 can split the optical beam into twodifferent paths640 and642.Path640 can be directed atdetector644.Path642 can be directed atmirror636, which can reflect the light todetector646.Detector644 can measure the laser output power, anddetector646 can measure emission wavelength. In some examples, an etalon can be included inpath642 betweenmirror636 anddetector646. In some examples, an optical spectrometer can be used to measure emission wavelength. The detected signals fromdetectors644 and646 can be input to a computer orcontroller650.
• Thecontroller650 can monitor deviations between the detected signals and the targeted signals or response waveform. Based on the deviations, thecontroller650 can generate drivewaveforms613 and615. Drivewaveforms613 and615 can be new waveforms or adjusted waveforms used to minimize or reduce the deviations. Digital-to-analog converters (DACs)652 and654 can be coupled tocontroller650 andsources674 and676 to convert the signals or waveforms generated bycontroller650 from digital to analog signals.
• FIGS. 7A-7B and 7F illustrate exemplary drive waveforms applied to the electrodes of a laser according to examples of the disclosure.FIGS. 7C-7E, 7G-7I illustrate exemplary output waveforms of the laser according to examples of the disclosure. As shown inFIG. 7A, the tuning current702 can be sinusoidally modulated, while the laser injection current704 can be constant. In some examples, both the tuning current702 and laser injection current704 can be sinusoidally modulated (as shown inFIG. 7B). In some examples, the laser injection current704 can be proportional to the square root of the sine of the modulating tuning current702.
• Exemplary resulting output waveforms are shown inFIG. 7C, where thewavelength shift706 of the laser can vary sinusoidally. Additionally, the laser can emit a constant or substantiallyconstant output power708. This example shows that a substantially constant output power can be achieved while tuning the wavelength. With the injection current greater than the threshold current, the output power can be less than the thermal rollover power for a given tuning range or range of wavelength shift. As a result, the output power can be varied (increased or decreased) to achieve a desired tuning range. Achieving a constant output power while tuning the wavelength can be useful for applications such as wavelength modulation spectroscopy.
• FIG. 7D illustrates exemplary waveforms where the laser can be operated such that the output power is close to the thermal rollover power according to examples of the disclosure. The laser can be operated with an output power where the output power to current slope begins to deviate from a linear relationship (i.e., close to the thermal rollover power). In some examples, the laser can be operated such that the output power is substantially equal to the thermal rollover power, leading to a decreased tuning range compared to when theoutput power708 is substantially less than thethermal rollover power710. In some examples, the laser can be operated with an output power between deviation from the linear relationship and the thermal rollover power (i.e., power where the derivative of the output power to current is zero). Examples of the disclosure include adjusting the operating conditions of the laser such that derivative of the output power to current is zero given the active region temperature. Operating the laser such that theoutput power708 is substantially equal or close to thethermal rollover power710 can be useful for applications such as wavelength modulation spectroscopy measurements that are normalized to a reference detector.
• In some examples, a larger tuning range can be achieved by decreasing the output power. In some examples, the laser can be operated such that the output power is can be based on the requirements needed for the system or application in which the laser is employed. As illustrated inFIG. 7E, by operating the laser with conditions such that the systemoptical power requirement711 is met but the requirements are not substantially exceeded, a larger tuning range or wavelength shift can be achieved compared to the tuning range when operating the laser above system requirements. For example, trace gas detection system (discussed above and illustrated inFIG. 1) located in a user's garage can be configured such that a detected carbon monoxide level less than 400 ppm is within a safe level. The system requirement can be defined by the operating conditions which 400 ppm of carbon monoxide is detected. The laser associated with the trace gas detection system (such aslaser114 illustrated inFIG. 1) can then operate at a value such that the system requirement is met, 400 ppm of carbon monoxide is detected, and a wider tuning range is achieved (than operating the laser such that the system requirements are exceeded). In some examples, the value can be equal to system operation at 1-10% above system requirements.
• In some examples, the amount of phase that differs between tuning current702, laser injection current704, andwavelength shift706 can depend on the frequency of modulation. In some examples, the phase difference between the laser injection current704 and thewavelength shift706 can be 0° or 180°. Although the disclosure includes thermal heating as a tuning mechanism, examples of the disclosure can include, and are not limited to, other tuning mechanisms such as carrier concentration modulation.
• FIG. 7F illustrates exemplary non-sinusoidally modulated drive waveforms applied to the electrodes of the laser according to examples of the disclosure. As shown in the figure, the tuning current712 can be modulated, but unlike the tuning current702 ofFIG. 7A, tuning current712 can be non-sinusoidal in order to correct for nonlinearities and thermal time constants, such as the nonlinearities and thermal time constants discussed above. Additionally, laser injection current714 can be non-sinusoidal. The results can be a sinusoidally varying output wavelength and an output power that is constant.
• FIG. 7G illustrates an exemplary linear ramp of the emission wavelength with a constant output power below a thermal rollover power from a laser according to examples of the disclosure. This example also shows that the emission wavelength can be tuned without affecting the output power of the laser. Linearly tuning the laser while achieving a constant output power that is less than thethermal rollover power730 can be useful for applications such as direct absorption spectroscopy.
• In some examples, the laser can be operated such that the output power waveform has the same shape as the thermal rollover power, as illustrated inFIG. 7H.FIG. 7H illustrates an exemplary linear ramp of the emission wavelength with an output power close to the thermal rollover power according to the examples of the disclosure. The laser can be operated at conditions such that theoutput power728 is substantially equal to thethermal rollover power730 while linearly changing thewavelength shift726. Such a linear change in emission wavelength can be useful for applications such as direct absorption spectroscopy with normalization to a reference detector.
• FIG. 7I illustrates exemplary variable wavelength shift and variable output power over time from a laser according to examples of the disclosure. As shown in the figure, bothoutput power748 and wavelength shift746 can vary sinusoidally. In some examples,output power748 and wavelength shift746 can both vary at non-harmonically related frequencies. Simultaneous sinusoidal modulation of both output power and emission wavelength can be useful for applications such as wavelength modulation spectroscopy.
• One skilled in the art would understand that the exemplary figures described above can represent targeted waveforms. In some examples, the resulting output waveforms can differ from the targeted waveforms due to nonlinearities (not shown).
• FIGS. 8A-8B illustrate exemplary process flows for optimizing the injection current and tuning element current according to examples of the disclosure.Process800 can begin with dividing a time interval into a series of sample points (step802). In some examples,process800 can be performed by a processor or controller (such ascontroller650 ofFIG. 6). In some examples, the time interval can be based on a repetition period of the desired power and wavelength response waveforms. The controller can determine the target output power and emission wavelength at each sample point (step804) and from this, can calculate the expected photodiode signals (step806). The expected photodiode signals can be based on factors such as the response curves of the photodiode, the length of the etalon, the optical properties of the etalon, the response of the spectrometer, or any combination.
• The controller can set the tuning element current and laser drive current to initial values (step808). In some examples, the tuning element can be set to zero, and the laser drive current can be set to a constant value above threshold (i.e., value that the laser is turned on). Alternatively, the laser can be driven using pre-determined waveforms. In some examples, the pre-determined waveforms can be based on previous characterization data from the laser. The pre-determined waveforms can include, but are not limited to, light IV curves for different injection currents and tuning element currents and step responses of injection current perturbations. The controller or a signal acquisition system can detect the output signals and digitize over the time interval (step810). In some examples, the time interval can be one time interval. In some examples, the output signals can be signals measured from the detectors, such asdetectors644 and646 ofFIG. 6.
• From the output signals or waveforms, the goodness of fit can be calculated (step812). The goodness of fit (GOF) can be a figure of merit to quantify the difference between the targeted and measured laser outputs or waveforms, including the output powers and emission wavelengths. For example, the GOF can be defined as:
• 
  GOF=Σ_n=1^Pw₁×(W(t)−W_T(t))²+w₂×(P(t)−P_T(t))²  (1)
• where W(t) is the measured wavelength at time t, W_T(t) is the targeted wavelength at time t, P(t) is the measured output power at time t, P_T(t) is the targeted output power at time t, w₁and w₂are weighted values, and P is the number of sample points in the time interval.
• The controller can optimize the drive injection current or tuning element current or both (step814).Process850 ofFIG. 8B illustrates an exemplary flow diagram of optimizing the drive injection current or tuning element current. The controller can begin with the first sample point in the time interval (step852). The controller can change the laser injection current or tuning element current or both in a first direction (step854), and then detect the output waveform (step856) and calculate the first GOF (step856) from the change in the first direction. The controller can also change the laser injection current or tuning element current or both in a second direction (step860), and then detect the output waveform (step862) and calculate the second GOF (step864) from the change in the second direction. In some examples, the first direction can be opposite the second direction. For example, the first direction can be one step size higher in value and the second direction can be one step size lower in value than the injection current set instep808 ofFIG. 8A. Instep868, the first GOF and second GOF can be compared. Based on which GOF has the lowest value (compared to the other GOF) or deviation from a pre-determined or targeted GOF, the drive waveforms can be adjusted.
• The controller can determine if all of the sample points have been tested (step870). If not, the controller can move to the next sample point (step872) and repeat the adjustment process for that next sample point. If all sample points have been tested, the drive waveforms can be stored in memory (step874).
• In some examples, processes800 and850 can be repeated until a certain GOF value is achieved. In some examples, processes800 and850 can run continuously or at incremental periods while the laser is operating to actively correct for any drift in the source properties over time.
• One or more of the functions described above can be performed, for example, by firmware stored in memory and executed by a processor or controller (such ascontroller650 ofFIG. 6). The firmware can also be stored and/or transported within any non-transitory computer-readable storage medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. In the context of this document, a “non-transitory computer-readable storage medium” can be any medium (excluding a signal) that can contain or store the program for use by or in connection with the instruction execution system, apparatus, or device. The non-transitory computer readable storage medium can include, but is not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, a portable computer diskette (magnetic), a random access memory (RAM) (magnetic), a read-only memory (ROM) (magnetic), an erasable programmable read-only memory (EPROM) (magnetic), a portable optical disc such as a CD, CD-R, CD-RW, DVD, DVD-R, or DVD-RW, or flash memory such as compact flash cards, secured digital cards, USB memory devices, memory sticks and the like. In the context of this document, a “transport medium” can be any medium that can communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The transport readable medium can include, but is not limited to, an electronic, magnetic, optical, electromagnetic, or infrared wired or wireless propagation medium.
• In some examples, a method of driving a laser having a tuning element and a waveguide is disclosed. The method may comprise: applying a first waveform to the tuning element of the laser; applying a second waveform to the waveguide of the laser; modulating the first waveform simultaneously with modulating the second waveform, wherein an emission wavelength and an output power of the laser are independently controllable. Additionally or alternatively to one or more examples disclosed above, in other examples, the first waveform is different from the second waveform. Additionally or alternatively to one or more examples disclosed above, in other examples, the method further comprises modulating first and second waveforms at non-harmonic frequencies. Additionally or alternatively to one or more examples disclosed above, in other examples, at least one of the first waveform and the second waveform is sinusoidal. Additionally or alternatively to one or more examples disclosed above, in other examples, at least one of the first waveform and the second waveform is non-sinusoidal. Additionally or alternatively to one or more examples disclosed above, in other examples, the method further comprises modulating the first and second waveforms to vary the emission wavelength sinusoidally. Additionally or alternatively to one or more examples disclosed above, in other examples, the method further comprises modulating the first and second waveforms to maintain the output power at a constant value. Additionally or alternatively to one or more examples disclosed above, in other examples, wherein the laser is included in a system with a requirement the method further comprises modulating the second waveform such that the system is operating at the requirement. Additionally or alternatively to one or more examples disclosed above, in other examples, the method further comprises modulating the second waveform such that a derivative of the output power to an injection current is zero. Additionally or alternatively to one or more examples disclosed above, in other examples, the method further comprises modulating the first and second waveforms such that the output power varies at least in part sinusoidally. Additionally or alternatively to one or more examples disclosed above, in other examples, the second waveform includes a 0° or 180° phase shift from the emission wavelength. Additionally or alternatively to one or more examples disclosed above, in other examples, the method further comprises modulating the first and second waveforms such that the emission wavelength is constant or varies linearly. Additionally or alternatively to one or more examples disclosed above, in other examples, the method further comprises modulating the first and second waveforms such that a derivative of the output power to injection current is zero. Additionally or alternatively to one or more examples disclosed above, in other examples, the method further comprises modulating the first and second waveforms such that the emission wavelength varies linearly and the output power is constant. Additionally or alternatively to one or more examples disclosed above, in other examples, the method further comprises modulating the first and second waveforms such that the emission wavelength is constant and the output power varies sinusoidally. Additionally or alternatively to one or more examples disclosed above, in other examples, the method further comprises: detecting the emission wavelength; detecting the output power; determining a first difference between the detected emission wavelength and a targeted wavelength; determining a second difference between the output power and a targeted power; and adjusting at least one of the first and second waveforms to reduce at least one of the first difference and the second difference.
• In some examples, a non-transitory computer readable storage medium is disclosed. The non-transitory computer readable storage medium may store one or more programs, the one or more programs comprising instructions, which when executed by a device, cause the device to perform a method comprising: applying a first waveform to a first electrode of a laser, wherein the first electrode is coupled to a tuning element of the laser; applying a second waveform to a second electrode of the laser, wherein the second electrode is coupled to a waveguide of the laser; and modulating the first waveform simultaneously with modulating the second waveform.
• In some examples, a device is disclosed. The device may comprise: a laser including a first electrode and a second electrode, wherein the first electrode is driven by a first waveform and the second electrode is driven by a second waveform; and logic configured to modulate the first and second waveforms simultaneously, wherein at least one of an emission wavelength and an output power of the laser are based on the simultaneous modulation of the first and second waveforms. Additionally or alternatively to one or more examples disclosed above, in other examples, a tuning range of the laser is at least 0.2% of a center wavelength of the laser. Additionally or alternatively to one or more examples disclosed above, in other examples, the laser is a Distributed Feedback (DFB) or a Distributed Bragg Reflector (DBR) laser. Additionally or alternatively to one or more examples disclosed above, in other examples, the laser is a quantum cascade laser (QCL), interband cascade laser (ICL), or type-I laser. Additionally or alternatively to one or more examples disclosed above, in other examples, the first electrode forms a portion of a tuning element and the second electrode forms a portion of a laser structure, the tuning element thermally coupled to the laser structure.
• Although the disclosed examples have been fully described with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. Such changes and modifications are to be understood as being included within the scope of the disclosed examples as defined by the appended claims.

Claims (22)

1. A method of driving a laser having a tuning element and a waveguide, the method comprising:

applying a first waveform to the tuning element of the laser;

applying a second waveform to the waveguide of the laser; and

modulating the first waveform simultaneously with modulating the second waveform,

wherein an emission wavelength and an output power of the laser are independently controllable.

2. The method ofclaim 1, wherein the first waveform is different from the second waveform.

3. The method ofclaim 1, further comprising modulating first and second waveforms at non-harmonic frequencies.

4. The method ofclaim 1, wherein at least one of the first waveform and the second waveform is sinusoidal.

5. (canceled)

6. The method ofclaim 1, further comprising modulating the first and second waveforms such that the emission wavelength is varied sinusoidally.

7. The method ofclaim 6, further comprising:

modulating the first and second waveforms such that the output power is maintained at a constant value.

8. The method ofclaim 6, wherein the laser is included in a system with an optical power requirement, the method further comprising:

modulating the first and second waveforms such that the system is operating at the optical power requirement.

9. The method ofclaim 6, further comprising:

modulating the first and second waveforms such that a derivative of the output power to an injection current is zero.

10. The method ofclaim 6, further comprising modulating the first and second waveforms such that the output power varies at least in part sinusoidally.

11. The method ofclaim 1, wherein the second waveform includes a 0° or 180° phase shift from the emission wavelength.

12. The method ofclaim 1, further comprising modulating the first and second waveforms such that the emission wavelength is constant or varies linearly.

13. The method ofclaim 1, further comprising modulating the first and second waveforms such that a derivative of the output power to an injection current is zero.

14. The method ofclaim 1, further comprising modulating the first and second waveforms such that the emission wavelength varies linearly and the output power is constant.

15. The method ofclaim 1, further comprising modulating the first and second waveforms such that the emission wavelength is constant and the output power varies sinusoidally.

16. The method ofclaim 1, further comprising:

detecting the emission wavelength;

detecting the output power;

determining a first difference between the detected emission wavelength and a targeted wavelength;

determining a second difference between the detected output power and a targeted output power; and

adjusting at least one of the first and second waveforms such that one or more of the first difference and the second difference are reduced.

17. (canceled)

18. A device comprising:

a laser including a first electrode and a second electrode, wherein the first electrode is driven by a first waveform and the second electrode is driven by a second waveform; and

logic configured to:

modulate the first and second waveforms simultaneously,

wherein an emission wavelength and an output power of the laser are independently controllable, and

wherein at least one of the emission wavelength and the output power of the laser are based on the simultaneous modulation of the first and second waveforms.

19. The device ofclaim 18, wherein a tuning range of the laser is at least 0.2% from a nominal wavelength of the laser.

20. The device ofclaim 18, wherein the laser is a Distributed Feedback (DFB) or a Distributed Bragg Reflector (DBR) laser.

21. The device ofclaim 18, wherein the laser is a quantum cascade laser (QCL), interband cascade laser (ICL), or type-I laser.

22. The device ofclaim 18, wherein the first electrode forms a portion of a tuning element and the second electrode forms a portion of a laser structure, the tuning element thermally coupled to the laser structure.

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