Patent Application
20170279246
None.
The present disclosure relates to laser systems and methods of operating laser systems. More particularly, the present disclosure relates to methods and systems for tuning laser output power.
Laser systems may incorporate collections of two or more laser diodes either as the direct source of the output laser radiation, or as a pump for a diode-pumped laser such as a fiber laser, a disk laser, a slab laser, a rod laser, a diode-pumped solid-state laser, a Raman laser, a Brillouin laser, an optical parametric laser, or an alkali-vapor laser.
Many laser applications require tunable laser output power from near zero power up to maximum power. In materials-processing applications, for example, low power levels may be required for alignment or pre-or post-treatment steps, whereas high power levels may be used for the actual processing steps such as cutting, welding, drilling, or scribing. As a second example, in a flexible machine intended for laser processing of a wide range of material types or thicknesses, certain applications may demand significantly lower power than others. To date, most lasers that are used in such applications provide such power tuning by varying the current applied to the laser diodes between zero and the current required for maximum power, equal currents being applied to each diode.
However, the laser diodes that generate the laser power may operate best within a high power range, therefore it may be preferable to operate the laser diodes within the high power range. In such situations it may not be possible or desirable to operate the laser diodes at individual powers spanning from low power to maximum power.
In a known power tuning method, the laser system is operated at constant, full power and a variable attenuator or modulator downstream is used to attenuate the output of the laser system. However, such attenuators or modulators may not be available or reliable at the power levels or operating wavelengths of interest, and, if available, may add significant cost to the system. Additionally, attenuation or modulation of full power to lower power levels achieved in this manner wastes energy.
Accordingly, there is a need for a low-cost, reliable technique to provide broad power tuning in an energy efficient manner.
A laser system and a power control method for controlling the laser system are provided. In one embodiment the laser system comprises diode banks configured to output laser beams, each of the diode banks including a laser diode, and the diode banks including a first diode bank and other diode banks; current controllers configured to receive current control signals corresponding to each of the diode banks and to enable current flows to the diode banks based thereon; and a control unit configured to receive an indication of a requested power and generate the current control signals based thereon, the current control signals including a first current control signal to control the first diode bank to output a first power, and other current control signals to control the other of the diode banks to output other powers, the first power being different than at least one of the other powers.
In variations of the present embodiment, the control unit is further configured to operate a diode bank of the diode banks only within a restricted power range, the restricted power range including a nominal power of one of the laser diodes. In one aspect of the present embodiment, the control unit is further configured to operate the laser diodes of the other of the diode banks in a wavelength-locked mode, wherein each of the laser diodes of the other of the diode banks are operated in a restricted power range of the laser diode including a nominal power of the laser diode, wherein the laser diodes lock reliably only in the restricted power range. In another aspect of the present embodiment, the control unit is further configured to operate the laser diodes of the other of the diode banks in a wavelength-controlled mode, wherein each of the laser diodes of the other of the diode banks are operated in a restricted power range of the laser diode including a nominal power of the laser diode, thusly generating a narrower emission band as compared to operating the laser diodes outside the restricted power range, the narrower emission band corresponding to the restricted power range. In a further aspect of the present embodiment, the control unit is further configured to operate the laser diodes of the other of the diode banks in a high-brightness mode, wherein each of the laser diodes of the other of the diode banks generate high-brightness output reliably only in a restricted power range of the laser diode including a nominal power of the laser diode, wherein operating in a high-brightness mode comprises operating only within said restricted power range. In a yet further aspect of the present embodiment, the control unit is further configured to operate the laser diodes of the other of the diode banks in a short-pulsed mode, wherein each of the laser diodes of the other of the diode banks pulse reliably within specified pulse parameters only in a restricted power range of the laser diode including a nominal power of the laser diode, wherein operating in a short-pulsed mode comprises operating only within said restricted power range. In a still further aspect of the present embodiment, wherein each of the laser diodes of the other of the diode banks operate with high electrical-to-optical efficiency within only in a restricted power range of the laser diode including a nominal power of the laser diode, the control unit is further configured to operate each of the laser diodes of the other of the diode banks only within said restricted power range.
In one embodiment the power control method is provided for a laser system comprising laser diodes arranged in diode banks, each diode bank comprising at least one of the laser diodes and having a maximum power, and the method comprises: operating a first diode bank of the diode banks to output a first power; and concurrently operating other of the diode banks to output other powers, at least one of the other powers being different than the first power.
In variations of the present embodiment, the power control method further comprises operating the laser diodes of a diode bank of the diode banks in a restricted power range, the restricted power range including a nominal power of one of the laser diodes. In one aspect of the present embodiment, the method comprises operating the laser diodes of the other of the diode banks in a wavelength-locked mode, wherein each of the laser diodes of the other of the diode banks are operated in a restricted power range of the laser diode including a nominal power of the laser diode, wherein the laser diodes lock reliably only in the restricted power range. In another aspect of the present embodiment, the method comprises operating the laser diodes of the other of the diode banks in a wavelength-controlled mode, wherein each of the laser diodes of the other of the diode banks are operated in a restricted power range of the laser diode including a nominal power of the laser diode, thusly generating a narrower emission band as compared to operating the laser diodes outside the restricted power range, the narrower emission band corresponding to the restricted power range. In a further aspect of the present embodiment, the method comprises operating the laser diodes of the other of the diode banks in a high-brightness mode, wherein each of the laser diodes of the other of the diode banks generate high-brightness output reliably only in a restricted power range of the laser diode including a nominal power of the laser diode, wherein operating in a high-brightness mode comprises operating only within said restricted power range. In a yet further aspect of the present embodiment, the method comprises operating the laser diodes of the other of the diode banks in a short-pulsed mode, wherein each of the laser diodes of the other of the diode banks pulse reliably within specified pulse parameters only in a restricted power range of the laser diode including a nominal power of the laser diode, wherein operating in a short-pulsed mode comprises operating only within said restricted power range. In a still further aspect of the present embodiment, wherein each of the laser diodes of the other of the diode banks operate with high electrical-to-optical efficiency within only in a restricted power range of the laser diode including a nominal power of the laser diode, the method further comprises operating each of the laser diodes of the other of the diode banks only within said restricted power range.
Embodiments of the present disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which:
The embodiments described below are merely examples and are not intended to limit the invention to the precise forms disclosed. Instead, the embodiments were selected for description to enable one of ordinary skill in the art to practice the invention.
A laser system and a method of operating the laser system are provided herein. The laser system comprises laser diode banks. Each bank comprises at least one laser diode and may comprise multiple laser diodes connected in series. In one embodiment of the laser system, a control unit controls current flow individually to each of the banks to tune the output power of the laser system. In one variation, at least one of the banks is controlled to generate zero output power to reduce the output power of the laser system. In another variation, the banks are not individually tunable and are controlled in groups in which the group members each generate a predetermined output power (the output power is individually predetermined for each bank) to provide discrete power tuning. In a further variation, the banks are individually tuned over restricted output power ranges. In one example, one of the banks is power tunable over a broad output power range and other banks are power tunable over restricted output power ranges. In another example, one of the banks has a larger output power capacity than other banks. In a further example, one of the banks has a smaller output power capacity than other banks.
As used herein, the terms “turned on” and “turned off” refer to the output state of a diode bank. A diode bank may be turned on by application of a current to the diode bank which is greater than or equal to a threshold current necessary for lasing, and when turned on has an output power greater than zero. Conversely, a diode bank may be turned off even if a current is supplied to the diode bank if the current is less than the threshold current necessary for lasing, and when turned off has an output power substantially equal to zero.
As used herein, the term “tuning” refers to the regulation of the output power of a diode bank or a laser system. A “tunable” diode bank may be turned off or, when turned on, operated at an output power within its tuning range. The tuning range may be restricted or narrow. A “non-tunable” diode bank may be turned off or turned on, and when turned on may be operated to generate a predetermined, fixed, output power. The predetermined output power may be the nominal, optimal, maximum or any other predetermined output power. The laser system provided herein may be tunable by operation of non-tunable banks, tunable banks, and combinations thereof.
In industrial materials processing applications, such as sheet-metal cutting and welding, it is desirable to maintain a constant energy per unit length of process. In these motion-control systems, the cutting/welding head must speed up and slow down depending on the shape of the cut/weld. Even in a straight-line cut, the head speed will follow a trapezoidal curve—accelerating up to some maximum cut speed and then decelerating at the end. To maintain a constant energy per unit length, the output power of the laser must be less than full power when the head is moving at less than full speed. For example, the average power could be proportional to head speed. A combination of pulsed operation and reduced instantaneous/peak power may be used to tune the output power in relation to cutting/welding head speed, for example to deliver an equal amount of energy with each pulse, while controlling the pulse rate in relation to the head speed. Thus, dynamic control of output power is desirable, while various control parameters may be available to tune output power as desired even when diode banks are operated at constant or restricted range output power.
One specific example of a laser system suitable for industrial materials processing applications according with the invention comprises multi-mode edge emitters on single-emitter chips, each producing up to about 10 W output power. In the present example, fourteen such single emitters are mounted in each package, wired in series, with an output of about 140 W coupled into one output fiber. Each bank comprises three diode packages, or modules, wired in series. With a typical diode drop of about 1.8 V per diode, each package has a total drop of about 25 V, and thus each bank operates at about 75 V. Five individually controllable banks wired in parallel may be used in the system, providing fifteen 140 W packages in total for a total available laser diode power of 2.1 kW. The fifteen output fibers may be coupled into one larger power delivery fiber leading to the workpiece, or into a pump input port of a fiber laser.
Example laser diodes suitable for use in the embodiments disclosed herein include any of various combinations of diode types and package types, such as edge emitters or vertical cavity surface-emitting lasers (VCSELs), single-transverse-mode or multi-mode. Diode chips may include one emitter per semiconductor chip (single-emitter chips), or multiple emitters per chip (e.g. diode bars, VCSEL arrays). The chips may be packaged with one or multiple single-emitter chips inside one package or one or multiple multi-emitter chips inside one package. The laser output from the diode package, or bank, may be delivered in an optical fiber or as a free-space beam.
To generate the laser output of the diode bank, the chip outputs are combined within the diode bank. Light emitted from a high power single emitter is typically highly asymmetric resulting in long and thin emitting apertures. The light beam emitted by such lasers has much higher brightness in its “fast axis” (perpendicular to the main pin junctions) than in its “slow axis” (parallel to the active layer). An optical fiber, generally, has a substantially circular or polygonal cross-section and a substantially symmetrical acceptance angle. To obtain the highest brightness, light beams from multiple single emitter diode lasers are coupled into a single fiber stacked in their fast axis direction. For example, an array of 3-10 individual laser emitters with a 100 micrometers (um) aperture width in the slow axis can be coupled into a fiber with a 105 um diameter and 0.15 NA (numerical aperture) by stacking individual laser beams in the fast axis direction.
Since the laser diode emission is typically polarized, polarization beam combining may be used to couple light emitted by two arrays of single emitters into a single fiber, thereby doubling the power and brightness of the output beam. One example of such laser beam combining incorporates both the spatial stacking of equally polarized laser beams with polarization multiplexing of stacked beams from two laser arrays. In one example, two rows of laser diodes are positioned on an upper level, and two rows of collimating lenses are positioned on a middle level. The light beams are collimated by the lenses and then reflected by two rows of vertically offset prism mirrors positioned on a lower level, to form two vertically stacked beams that are polarization combined using a polarization beam combiner (PBC) and a half wave plate.
In another example, first and second rows of laser diodes are disposed in a staggered arrangement with a lateral offset therebetween. The present example is described in additional detail in U.S. Pat. No. 8,427,749, issued Apr. 23, 2013, which is incorporated herein by reference in its entirety and further described with reference to diode bank 20 in
Referring now to
In a further embodiment of a laser system and a power control method for controlling the laser system, the laser system comprises diode banks configured to output laser beams, each of the diode banks including a laser diode, and the diode banks including a first diode bank and other diode banks. The laser system also comprises a control unit configured to receive an indication of a requested power and generate current control signals based thereon, the current control signals configured to control the first diode bank at a first power and the other diode banks at other powers, the first power being different than at least one of the other powers. To receive an indication of a requested power comprises receiving a digital or an analog signal, in any manner known in the art, which comprises a value of a desired output power. To generate the current control signals based thereon comprises selecting an amount of the requested power to be output by one or more of the diode banks. Several ways of selecting the diode banks and the power to be output by each of them are described below. In one example, control logic is programmed to select the least number of diode banks whose maximum powers exceed the requested power. Thus, if each bank has a maximum capacity of 120 watts, and 580 watts are required, the control logic selects 5 banks. If the banks are configured to operate at only one power level, e.g. maximum power, the control logic may operate all 5 banks to output 600 watts. If each of the banks may be operated in a restricted power range, e.g. tunable from 70% to 100% of their maximum power, the control logic may operate each of the 5 banks at 116 watts, thus meeting the requirement while loading each bank equally. If most of the banks are configured to operate at only one power level, and at least one bank is tunable over at least 50% of its maximum range, the control logic may operate 4 banks at maximum power, and operate the tunable bank at 100 watts, thus meeting the requirement. In some embodiments, when fewer than all the banks are operated, the selection reduces the imbalance in the life-time use, or on-time, of each of the banks by tracking usage and selecting banks each time the laser system is used which have less on-time than the others, thus enhancing the life of the laser system. Various logic routines may be included in the laser system which can be used depending on how the laser system is configured. A configuration file may be included in the control unit and modified when the modular units of the laser system are selected or when an application of the laser system is selected. The configuration file may stipulate tuning ranges for the diode banks, maximum powers, and any other configuration or tuning parameter described below.
In an embodiment of the power control method, the method comprises: operating a first diode bank of the diode banks to output a first power; and concurrently operating other of the diode banks to output other powers, at least one of the other powers being different than the first power. As described previously, the first diode bank may be tunable while the other diode banks, at least some of them, may be operable at a single power output. In another example, the first diode bank is tunable over a wide power range while at least some of the other diode banks are tunable over restricted ranges. In another example, the diode banks are operable at a single output power, but one of them has a smaller maximum power than the others, therefore improving tunability of the laser system using diode banks operable with a single output power or with restricted output power ranges. Variations of the embodiments summarized in the preceding paragraphs, and others, will now be described with reference to
Referring to
The beam combiner 182 may comprise optical elements arranged to combine the beams in various techniques known in the art. One technique comprises spatial beam combining. An example spatial beam combining technique using free-space optics to combine the beams was previously presented with reference to
The time for a source to achieve wavelength stability will depend on how closely it is operated to its nominal output power. For example, with a wavelength locked diode laser, the wavelength will lock more quickly if the laser diode is operated at 100% of its optimal/nominal power than if it were operated at 50% of its optimal/nominal output power. Thus the sources, e.g. emitters or laser diodes, in a system with n sources operated at 100% of their nominal output power will wavelength lock faster than the sources in a different system with 2*n sources operated at 50% of their nominal output power. All else being equal, the system with n sources will exhibit a shorter rise time of the combined output power. In systems where the laser is modulated, the rise time impact of slow wavelength locking can be the limiting factor in how rapidly the laser can be modulated. Thus limiting operating power to a range close to the optimal/nominal output power will result in higher possible modulation rates. In one embodiment, the diode banks tuned over a broad range, e.g. from 0% to 100% of their maximum power, achieve rise times greater (i.e. slower) than 80 μsec. The rise times may be reduced to 40 μsec or less, providing strong wavelength locking, an improvement of at least 50%, by tuning the diode banks in a restricted range comprising 70% to 100% of their maximum power. Depending on the modulation frequency, an improvement of 40 μsec may translate to a 1-5% increase in output power. More preferably the range may be further restricted to achieve rise times of 25 μsec or less while still retaining tuning capability. Of course the actual rise times and tuning ranges achievable with a particular diode bank will be affected by other variables, including the temperature of the laser system.
In some embodiments, the modulated laser system includes temperature control features to control the temperatures of the laser diodes. A diode bank may comprise a liquid cooled cold plate on which the diode lasers are mounted. A working liquid flows through the cold plate, generally at a predetermined constant flow rate and temperature sufficient to maintain a desired temperature of the laser diodes, e.g. 30° C., when they are operated to generate a continuous wave. When the modulation frequency or duty-cycle of the laser system is reduced, the amount of heat generated by the laser diodes decreases. If the predetermined flow rate and temperature is maintained, more heat will be extracted than is generated, bringing the temperature down below the desired temperature. Furthermore, when modulating the laser diodes below 100% duty-cycle it may be desirable to maintain a higher desired temperature, for example 40° C. An example temperature control feature comprises an electrically resistive element with a feedback sensor providing feedback to temperature control logic structured to maintain the temperature of the diode bank at a desired level. The resistive element may comprise a resistive layer, a ceramic resistor, or any other known electrically resistive element. The temperature control logic may comprise tables that correlate the modulation frequency to the amount of heating necessary to raise the temperature of the diode bank to a desired temperature, which may be variable as a function of modulation frequency. The correlations may be determined empirically or with an energy-balance model. Once a modulation frequency is set, a correlated amount of electrical current is provided to the resistive element to heat the cold plate.
Another example temperature control feature comprises flow control and temperature control logic structured to maintain the temperature of the diode bank at a desired level as a function of modulation frequency. Once a modulation frequency is set, the temperature control logic reduces the flow rate of the working liquid to increase the temperature of the cold plate, and vice versa. Flow may be reduced using a variable flow valve to divert some of the working liquid away from the cold plate. Flow may alternatively be reduced by controlling the duty-cycle or speed of a pump pumping the working liquid through the cold plate. In a further variation, the temperature of the working liquid may be increased at a system level by increasing the setpoint temperature of a chiller configured to cool the working liquid flowing to each of the diode banks.
In some embodiments, a feedback sensor 184 may be provided to sense a system parameter and transmit same via a feedback conductor 186 to the control unit 160. The control logic 162 comprises a closed-loop feedback portion to compare the value of a predicted system parameter to the sensed system parameter and adjust the current levels to compensate for the difference in accordance with feedback parameters, e.g. proportional, integral, and/or derivative parameters. Example system parameters include beam intensity, temperature, power, and current. Current can be measured at the output of each current controller.
The control unit 160 includes control logic structured to implement control methods described herein. The term “logic” as used herein includes software and/or firmware executing on one or more programmable processors, application-specific integrated circuits, field-programmable gate arrays, digital signal processors, hardwired logic, or combinations thereof. Therefore, in accordance with the embodiments, various logic may be implemented in any appropriate fashion and would remain in accordance with the embodiments herein disclosed. In one variation, the control unit 160 includes digital-to-analog converters (DAC) configured to output an analog control signal corresponding to a digital value. The control unit 160 also includes embedded Random Access Memory (RAM) look-up tables 164a-e depicting relationships between addresses and current levels. Each table corresponds to one DAC. Each address corresponds to an output power level. Thus, selection of an address identifies the corresponding current levels for every DAC for a given required power. In this way, the current profile of each individual bank (over the input, or required, power range) is infinitely flexible, enabling operation in accordance with any desired control algorithm or logic (including those described herein). The tables can be populated manually by a user, or automatically using a firmware and/or programmable logic based program. Since the refresh rate of the DACs is much faster than the inherent rise/fall time of the current regulator circuits, there is no need to distinguish or use different algorithms between continuous wave (CW) operation and pulsed/modulated operation, unless for optical reasons. The operating currents will simply “track” the requested power signal (i.e. follow their respective programmed profiles) to within the rise/fail capability of the regulator circuits. Stability may be increased by provision of a deadband on the requested power signal or the feedback signal to reduce current variation. The deadband may be user-programmable for added flexibility, thereby allowing a user to determine the amount of hysteresis. Tables may be re-written at any time, permitting a high degree of flexibility arid customization. The tables and parameters of the closed-loop feedback may be updated periodically to reduce instances of discontinuities in output power by the laser system. Discontinuities may occur when a diode bank is turned on or off, for example. The currents may be tuned to account for response times and other factors to adjust when the diode banks are switched on and off to reduce instances of discontinuities. The tables arid parameters of the closed-loop feedback may also be updated periodically to increase the power accuracy of the laser system. The power accuracy may be increased by reducing an error between the requested power and the output power of the laser system. The tables and parameters may change over time to account for aging of the laser diodes and other factors that reduce power accuracy,
In accordance with one embodiment of a tuning method, a requested power level is selected, for example by a user or a machine comprising the laser system. Control logic 162 receives an indication of the requested power via signal conductor 155 and reads corresponding current levels from the tables 164a-e. Control logic 162 then communicates the current levels to the DACs, which output analog current signals to the current controllers 176, which in turn regulate power from power supply 152, 172a-e into currents having the desired levels for the diode banks 178. The power of the diode banks may thus be regulated, or tuned. In some embodiments, the power is tuned to operate the diode banks only within restricted power ranges. It should be understood that 100% of a maximum operating power corresponds to greater than 100% of the nominal power of a diode bank, therefore a 70-100% range of the maximum power includes the nominal power. Example restricted power ranges of a diode bank include a range comprising 50-100% of the maximum power, a range comprising greater than 70% of the nominal power, a range comprising 90-110% of the nominal power, and any other range compatible with a given diode bank and operating method. The range of power within which a diode bank may be tuned is referred to as the “accessible power” and powers outside the range are referred to as “inaccessible power”. Depending on the structure of the laser system, the requested power may be accessible, i.e. the laser system can be tuned to supply the requested power, or inaccessible, i.e. the laser system can only supply more or less than the requested power because there are no combinations of diode banks and restricted power ranges that can supply the requested power. As discuss below with reference to
In one variation of the present embodiment, at least some of the laser diodes are operated with zero tuning range, i.e. operated to generate a predetermined output power when turned on and zero output power when turned off. In the case in which N=5 banks, and all are non-tunable, the accessible power levels would be 20%, 40%, 60%, 80%, and 100%. Nonetheless, this degree of coarse power tunability is sufficient for many applications. Table 1 depicts an example of coarse, or discrete, tunability using the tables 164a-e in a configuration in which all the diode banks have the same power capacity. The power levels for each of the diode banks are depicted as a percentage of capacity for illustration purposes.
In a further aspect of the present variation of the present embodiment, the control logic is configured to optimize diode lifetime at the system level by periodically changing the order in which the banks turn on, which works to equalize or reduce imbalance of on-times between the banks, preventing any particular bank from accumulating more on-time than the others. “Periodically” could mean a time-based period or an event (e.g. on every power-up cycle). In one variation, the tables 164a-e include additional addresses and currents, which are configured to cycle which of the banks is operated for different requested powers. For example, control logic 162 can be configured with an address programmed to operate modular unit 170b, instead of operating modular unit 170a, when 20% output power is desired. Indexing logic can be provided to index the addresses after each start-up of the laser system, for example by incrementing the address of the given power level by the number of modular units.
In another variation of the present embodiment, one of the banks has a lower nominal power, or output capacity, than the others. In one example, the bank with the lowest nominal power has half the nominal power of the remaining banks. Thus, with N=5 banks, the accessible power levels vary by the capacity of the smallest capacity bank, as shown in Table 2. The tables 164a-e may be used to program the tuning algorithm. It should also be understood that any program or algorithm or logic, in any form thereof, may be used to program operation of the diode banks in accordance with the present disclosure.
In accordance with another embodiment of the tuning method, at least some of the diode banks 178 are operated in restricted power ranges. Each diode bank may be turned off or operated at an individualized power level within the restricted power range of the diode bank. Each bank preferably comprises a number of diodes wired in series and controlled by one current controller, thus ensuring that all the diodes in the bank are always driven with the same amount of current. The banks are preferably wired in parallel to one another and driven by one or more direct-current (DC) power supplies.
Operating the diodes of a diode bank within a restricted power range is particularly important for some specific conditions of diode operation or some specific diode designs. Wavelength-locked operation is one example. Here, the issue is that the spectral gain peak of the laser diode gain material typically has a strong temperature dependence; for GaAs-based laser diodes in the 800-1000 nm range, for example, the gain peak has a temperature coefficient of about 0.3 nm/° C. In typical high-power operation of such a laser diode, the temperature of the diode chip rises on the order of 30-40° C., thus causing a shift in the wavelength of the gain peak of about 9-12 nm. In low-power operation, for example less than 10% of maximum power, on the other hand, the temperature rises less than about 3° C., and the gain peak shifts only about 1 nm or less. In a non-locked laser diode, this gain shift is not a problem; the laser diode typically lases at approximately the gain peak under all circumstances, and so the output wavelength simply varies with power from, for example, about 930 nm at low power to about 940 nm at high power, for a laser diode that is specified for 940 nm operation at high power. Such variation is not a problem in a large number of applications.
However, in other applications such as narrow-line pumping of diode pumped solid state (DPSS) lasers and wavelength-division multiplexing of direct-laser diodes, wavelength control on the order of 1-2 nm or better may be required, and in such cases a dispersive element such as a grating is typically added to the laser-diode cavity to enforce lasing at the desired fixed wavelength. A grating may be written on the chip directly, as in the case of distributed-feedback (DFB) lasers and distributed-Bragg-reflector (DBR) lasers; or it may be external to the chip, as in the case of volume Bragg gratings (VBG's), fiber Bragg gratings (FBG's), or bulk gratings such as transmission gratings. Such gratings have a much lower temperature coefficient than GaAs and therefore typically provide wavelength control of 1-2 nm or less. However, the gain peak of the diode must still be accurately matched to the desired lasing wavelength as defined by the grating; if this is the case by design at high power, then at low power, the gain peak will be displaced from the lasing wavelength by on the order of 10 nm due to temperature variation. In GaAs lasers, the gain peak is itself on the order of 10-20 nm wide, so this shift is very significant relative to the width of the gain peak. If the laser is required to remain locked at low power, then in order for the grating to “pull” the lasing wavelength from the gain peak to the grating wavelength, it is necessary to use a higher grating strength than would otherwise have been necessary. This higher grating strength diverts more of the laser power back into the laser for locking rather than providing it as useful output power, resulting in lower output power and lower efficiency at high power than would have been necessary were it not necessary to ensure locking at low power. Thus, there is a tradeoff between locking range (and thus operating power range) and output power/efficiency. If the allowable operating power of the diode is restricted to, for example, 70% to 100% of maximum power, then the gain peak will tune by about 30% *10 nm=3 nm, which is small relative to the width of the gain peak and therefore would have little impact to the grating strength needed for reliable locking. Using multiple independently current controlled diode banks, each operated within a restricted output power range or turned off, can provide a wide range of total output power from the laser system with reduced wavelength shift due to temperature variation, thus generating a narrower emission band as compared to operating the laser diodes outside the restricted output power range to generate the same total output power.
A related example is wavelength-controlled (but non-locked) operation of a laser diode. Here, a diode is allowed to operate at its natural wavelength as determined by the gain peak, with no wavelength-locking mechanism, so that the output wavelength may vary by, for example, on the order of 9-12 nm for GaAs diodes as the power is tuned from zero to maximum power. Restricting the operating power range of the laser diode to a range of, for example, 70% 100% of maximum, reduces the output wavelength variation to about 3 nm. While this degree of wavelength control is not as tight as that enabled by wavelength locking, it is nonetheless a useful improvement for certain pumping applications, for example pumping Yb:glass, and for coarse wavelength division multiplexing in direct diode lasers. Using multiple independently current controlled diode banks, each operated within a restricted output power range or turned off, can provide more consistent thermal lens strength from the laser system over a wider range of total system output power than is possible from a similar laser system where each diode banks is operated to generate the same output power over the same (wider) range of total system output power.
Similarly, some diodes may be optimized for high-brightness operation near specific operating currents. Again the basis for this optimization is thermal in nature; a laser diode typically generates a lateral thermal gradient in high-power operation, which causes thermal lensing. A laser diode can be designed with a lateral chip design that produces a particular lateral mode quality at a particular thermal lens strength, corresponding to a specific output power level. However, at low power, the thermal lens strength drops to near zero, so a different mode quality will be observed. If the diode is optimized for a desirable mode quality at high power, then it may have worse, or unacceptable, mode quality at low power, for example leading to poor coupling efficiency into optical fiber in fiber-coupled laser diode systems. Thus, it can be advantageous in such laser diodes to restrict the operating power range to a subset of the full range, for example restricting the power range to 70% to 100% of maximum power, 70% to 100% of nominal power, or any other desirable range.
Short-pulsed laser diodes are another example of diodes that can benefit from operating within a restricted power range. Q-switching, gain switching, and mode-locking, for example, are techniques that typically generate nanosecond or shorter pulses from laser diodes by using the characteristics of the laser rate equations and optical nonlinearities. These effects depend directly on the operating power and also possibly on the chip temperature. Therefore it may not be possible or reliable to operate such diodes at powers outside of a restricted power range.
One last example of a motivation for operating diodes within a restricted power range would be with diodes that are optimized for high efficiency. Certain laser diodes are designed for optimum electrical-to-optical conversion efficiency at or near a specified operating power. Therefore, in order for the system to achieve optimum overall efficiency, it is preferred for these diodes not to operate outside of a restricted power range.
Thus, in these applications (wavelength locking, high brightness operation, short-pulsed, and high efficiency) it has been shown that it can be advantageous to operate the individual laser diode in a restricted power range. Other such applications or situations are expected to exist as well. Obtaining wide power tuning of a system that includes many such laser diodes is thus obtained advantageously by turning off some of the diodes and operating some of the diodes within their restricted power range.
Embodiments of a restricted power range tuning method will now be described with reference to
In one variation of the present embodiment, each of the diode banks has the same nominal power and the diodes in each bank may be operated in the same restricted range, denoted as X% of maximum power. The present variation will now be described with reference to
Some power levels may be inaccessible when the power ranges do not overlap. An appropriate response may be programmed in the control unit in the event that an inaccessible power level is requested by a machine application or a user. Example appropriate responses include providing the nearest accessible power level (which may be higher or lower than the requested power level), providing the next higher accessible power level, (c) providing the next lower accessible power level, (d) providing zero power, and/or providing a warning or an error message.
When power ranges overlap, multiple numbers of banks can be used to provide the requested power level. In the above example, 75% output power could be provided using either four or five banks. Various criteria may be used to determine how many banks to use. In one form of the present embodiment, a transition power level is determined to transition from a lower number of banks to a higher number of banks. For example, four banks could be used from 60% to 75% power, and five banks could be used above 75% power. Control logic in the control unit may be structured to operate in this manner in machine applications where the power will not be continuously tuned or modulated.
At 208 the method continues by determining if the requested output power is above the transition power level. If the requested power is above the transition power level, the method continues at 210 by operating M+1 banks. Otherwise the method continues at 220 by operating M banks. Of course, there may be multiple overlapping power ranges and transition power levels. In such case, the highest transition power level below (less than) the requested output power may be selected.
After 210, while operating M+1 banks, the method continues at 212, by determining if the requested power exceeds the minimum power of M+1 banks. If the requested output power exceeds the minimum output power of M+1 banks, the system continues operation with M+1 banks. Otherwise, the method continues at 220, by ceasing operation of one bank to operate with M banks. In this manner operation remains at M+1 banks even when the requested power is less than the transition power level, thereby delaying or preventing switching off of one bank, and reducing on and off instances of a diode bank.
After 220, while operating M banks, the method continues at 222 by determining if the requested power exceeds the maximum power of M banks. If the requested power exceeds the maximum power of M banks, the method continues at 210 by operating with M+1 banks. In this manner operation remains at M banks even when the requested power is greater than the transition power level, thereby delaying or preventing switching on an additional bank.
In an alternative aspect of the embodiment disclosed with reference to
In another variation of the tuning method, at least one of the banks is designed to provide full power tuning. For example, the full-power tuning bank may be power tuned from 0-100% power. With the banks having equal maximum powers and the other banks having restricted or zero power tuning range, full-system power tuning from 0-100% is possible. In this case, with N=5 banks, of which four are non-tunable and one is fully tunable, powers from 0 to 20% can be generated using only the tunable bank; from 20% to 40% can be generated using one non-tunable bank plus the tunable bank; from 40% to 60% can be generated using two non-tunable banks plus the tunable bank; and so on. Of course, a bank with less than full-power tuning in combination with non-tunable banks may also provide sufficient tuning capability.
In another variation of the tuning method, the powers of the diode banks are tuned so that they are equal whenever possible, to promote uniform aging of the laser diodes. Thus, if the requested power is 480 watts and each of the five diode banks has a maximum power of 200 watts, but is tunable from 140 to 200 watts, the requested power is provided by tuning three banks to output 160 watts each instead of, for example, tuning one bank to output 200 watts and two banks to output 140 watts each. The order in which the banks are turned on may be rotated, for example by initially turning on banks 1, 2 and 3, and in another instance turning on banks 3, 4 and 5, such that over time each laser diode is utilized the same amount of time or turned on the same number of times.
While this invention has been described as having an exemplary design, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-210491 | Oct 2014 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US15/54713 | 10/8/2015 | WO | 00 |
