WO2017060793A1

WO2017060793A1 - A laser apparatus having an excitation source which comprises an array of controllable light emitters, and an associated method

Info

Publication number: WO2017060793A1
Application number: PCT/IB2016/055738
Authority: WO
Inventors: Ihar Anatolievich Litvin; Cobus JACOBS
Original assignee: Csir
Priority date: 2015-10-06
Filing date: 2016-09-26
Publication date: 2017-04-13

Abstract

A laser apparatus (100) has an excitation source (102, 202, 302) operable to excite a gain medium (114) provided in an optical cavity (112). The excitation source (102, 202, 302) comprises an array of individually-controllable light emitters (204, 304). The laser apparatus (100) includes a control module (120) operable to control each light emitter (204, 304) in the excitation source (102, 202, 302) independently of the other light emitters (204, 304) and in accordance with excitation profile criteria (122), thereby to excite the gain medium (114) in accordance with the excitation profile criteria (122). The profile of the gain medium (144) may thus be changed dynamically, electronically, and rapidly.

Description

A laser apparatus having an excitation source which comprises an of controllable light emitters, and an associated method

FIELD OF INVENTION

This invention relates broadly to lasers, and more specifically to a laser apparatus having an excitation source which comprises an array of individually- controllable light emitters, and to an associated method.

BACKGROUND OF INVENTION

A laser is operable to generate a laser beam having a particular beam profile. The particular beam profile generated by a laser depends on the configuration of the laser, e.g. the optical cavity, the gain medium, the optical elements at either end and within the optical cavity, etc. In order to sustain and amplify the laser beam, the gain medium has to be pumped or energised, usually by means of an excitation source (or a pump source).

Conventionally, the excitation source is a single (optionally fairly high power) light source such as a laser diode. However, the Applicant is also aware of the use of plural individual light sources to form a collective excitation source. The plural individual light sources can be, amongst others, in the form of a diode stack (also called diode laser stack, multi-bar module, or two-dimensional laser array) which contains a series of edge emitting diode bars, which are arranged in the form of a stack, or a vertical cavity surface emitting laser (VCSEL) which consists of a planar array of surface emitting diodes. In the case of the diode stack (as an example), the individual emitters are thus arranged in a two- dimensional array, with a first dimension provided by the individual emitters spaced along each bar in a first direction and a second dimension provided by the bars being stacked in a second direction (which is perpendicular to the first direction). http:/^'/www. rp-photon ics. com/diode bars, htm i (accessed 20 October 2014) discloses information about diode bars in general.

Each diode bar consists of plural edge emitters spaced evenly across the length of the bar. The most common arrangement is that of a vertical stack (but surface emitters arranged in an array format is also a possibly). Effectively this is a two-dimensional array of individual emitters that combine spatially to form a high power laser diode source. Such a stack can be fabricated by cleaving linear diode laser arrays (diode bars) from a wafer, attaching them to thin heat sinks, and stacking these assemblies so as to obtain a periodic array of diode bars and heat sinks. Depending on the application, a diode stack may be used with or without attached optics (collimation). Diode stacks can provide extremely high output powers of hundreds or thousands of watts, as used for pumping of high-power solid-state lasers, or used directly, e.g., for material processing. Devices incorporating diode stacks are available commercially from several international suppliers.

SUMMARY OF INVENTION

According to one aspect of the invention, there is provided a laser apparatus which includes an excitation source operable to excite a gain medium provided in an optical cavity, wherein: the excitation source comprises an array of controllable light emitters, at least some of which are controllable independently of other light emitters; and the laser apparatus includes a control module operable to control each light emitter or group of emitters in the excitation source independently of the other light emitters or groups of light emitters and in accordance with excitation profile criteria, thereby to excite the gain medium in accordance with the excitation profile criteria.

The excitation source may be a 1 D (one dimensional) array. The excitation source may comprise a single diode bar or a 1 D VCSEL.

The excitation source may be a 2D (two dimensional) array. The excitation source may comprise a stack of diode bars or a 2D VCSEL.

The excitation source may be an Individually Addressable Laser Diode Array

(IALDA). The Applicant notes that lALDAs as such are not new and are known in the graphic art/printing industry, e.g., patent nos. US6348358 and US20040260505. However, such lALDAs have never (to the best of the Applicant's knowledge) been used as part of a larger laser apparatus as the excitation source (or pump source).

The excitation source may be provided in an optical cavity as part of a laser resonator (or laser oscillator). In such case, the laser apparatus may be in the form of a laser resonator having an optical cavity with a longitudinal axis, an optical element at each end of the optical cavity, and the gain medium (or laser host medium) provided in the optical cavity, wherein the excitation source is operable to excite the gain medium. The laser resonator may be operable to generate a laser beam having a pre-defined fundamental mode and beam profile. The gain medium may be a solid-state laser crystal rod or slab, photonic crystal fibre, gas, etc.

Instead, the laser apparatus may be a laser amplifier (or oscillator-amplifier) system. In such case, the excitation source may be operable to excite the gain medium but may be provided outside the optical cavity, such that a laser beam which is coupled to the gain medium is amplified. The laser apparatus may include an optical transformation system (OTS). The excitation (pumping) of the gain medium may be achieved by direct array excitation of the gain medium (with no OTS) or by any convenient OTS. The OTS may be direct imaging with different magnification in horizontal and vertical axes, or another optical transformation, such as Fourier transform, etc.

By individually controlling every single emitter of the array, a 1 D or 2D intensity distribution of emitted light can be created. The control module may be operable to control the array in either a closed or open loop.

An application of controlling the output of the individual emitters in the array (with or without the OTS) may be to create a desired time and spatial intensity distribution in the gain medium. The required time and spatial intensity distribution of the laser output may depend on the individual task of such pumping or the application for which the laser apparatus is intended. Usefully, the control module may be configured to create an intensity distribution of the pump light in the gain medium which will fit a specified mode (mode-matching) of the laser cavity to achieve efficient coupling. This may dramatically increase the efficiency of such a laser and increase the beam quality of the output laser beam.

The excitation profile criteria may correspond to a fundamental mode of the laser. In such case, the control module may be configured to power or energise the individual light emitters of the excitation source in accordance with, or matched to, a beam profile of the fundamental mode of the laser.

The excitation profile criteria may correspond to a desired beam output of the laser. In such case, the control module may be configured to power or energise the individual light emitters of the excitation source in accordance with, or matched to, a beam profile of the desired beam output of the laser.

The excitation profile criteria may be dynamic or time-varying. The excitation profile criteria may thus dictate that the beam changes temporally over pulses or cycles. By way of development, the laser apparatus may be operable to be used for spatial controlling of the pumped intensity distribution of the laser to affect the instantaneous or average spatial beam profile of the output laser beam over time. The laser apparatus may allow control and playback of selected spatial intensity distributions of individual laser pulses in a pulsed laser

(or time-slices of a continuous wave beam) in order to create the required average intensity distribution for slow physical processes such as welding, cutting, polishing, drilling etc., or match the required intensity distribution of the physical processes as it evolves over time. For example, in a time cycle, the control module may be configured to direct the array to emit at a first spatial intensity si for a first time period ti and then at a second spatial intensity S2 for a second time period t2. Any material or substance to which the laser beams is applied and which is "slow" in comparison to the switching speed of the time periods ti and t2 may experience a time-averaged effective beam profile, (s_x x t-L + s₂ x t₂)/(s₁ + s₂). There may be more than two spatial intensities and time periods within a cycle. The array of light emitters may be switchable at frequencies in the order of a few (e.g., 5) Hz to 100s (e.g., 500) of MHz. This is typically a limitation of the electrical power supply rather than the diode array.

The laser apparatus may be used in conjunction with a spatial light modulator

(SLM), e.g., in conjunction with a setup similar to that described in WO2014064636 which is one of the Applicant's previous disclosures. The SLM may be used to shape the laser mode in the laser cavity. The disclosure of WO2014064636 may be used in conjunction with the present disclosure to shape simultaneously the gain profile in the gain medium and the laser mode to collectively optimise the laser or amplifier beam profile for a specific application. The control module may be operable to generate or receive a feedback signal and adjust the excitation source in accordance with the feedback signal. The control module may be operable to adjust the excitation source where the output deviates from the excitation profile criteria to compensate for the deviation.

The invention extends to a method of operating a laser apparatus, the method including: providing an excitation source which comprises an array of controllable light emitters, at least some of which are controllable independently of other light emitters; and controlling, by a control module, each light emitter or group of light emitters in the excitation source independently of the other light emitters or groups of light emitters and in accordance with excitation profile criteria, thereby to excite a gain medium in accordance with the excitation profile criteria.

The method may include controlling the excitation source to power the light emitters dynamically in accordance with time-varying excitation profile criteria.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be further described, by way of example, with reference to the accompanying diagrammatic drawings.

In the drawings:

FIG. 1 shows a schematic view of a laser apparatus in accordance with the invention;

FIG. 2 shows a different schematic view of the laser apparatus of FIG. 1 ; FIG. 3 shows a schematic view of a 1 D embodiment of an excitation source of the laser apparatus of FIG. 1 ;

FIG. 4 shows a three-dimensional view of different configurations of 1 D and

2D excitation source / laser diode stack;

FIG. 5 shows a schematic view of a 2D embodiment of an excitation source of the laser apparatus of FIG. 1 ;

FIG. 6 shows a front schematic view of the 2D excitation source of FIG. 5;

FIG. 7 shows a schematic view of the excitation source of FIG. 5 and a corresponding spatial intensity profile;

FIG. 8 shows graphical views of example spatial intensity profiles of laser beams; and

FIG. 9 shows a schematic view of part of the laser apparatus 100 of FIG. 1 .

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENT

The following description of the invention is provided as an enabling teaching of the invention. Those skilled in the relevant art will recognise that many changes can be made to the embodiment described, while still attaining the beneficial results of the present invention. It will also be apparent that some of the desired benefits of the present invention can be attained by selecting some of the features of the present invention without utilising other features. Accordingly, those skilled in the art will recognise that modifications and adaptations to the present invention are possible and can even be desirable in certain circumstances, and are a part of the present invention. Thus, the following description is provided as illustrative of the principles of the present invention and not a limitation thereof.

FIG. 1 shows a schematic view of a laser apparatus 100 in accordance with the invention. The laser apparatus 100 has an excitation source 102 (see description below) operable to provide optical excitation energy to sustain and/or to amplify a laser beam 1 18. The excitation source 102 is directed via an optional OTS (Optical Transformation System) 104 which could be any appropriate OTS device, e.g., direct imaging with different magnification in horizontal and vertical coordinates or another optical transformation such as

Fourier transform.

The laser apparatus 100 in this example includes a laser resonator (LR) 1 10 (also known as a laser oscillator). The laser resonator 1 10 has a longitudinal axis and defines therein an optical cavity 1 12. A gain medium 1 14 is disposed within the optical cavity 1 12, optionally laterally offset. In this example, the gain medium 1 14 is a solid-state laser crystal (LC) but could be another practicable laser host medium. An optical element 1 16a, 1 16b is arranged at each end of the optical cavity 1 12.

(In a different embodiment (not illustrated), the laser apparatus may be a laser amplifier thus not having the laser resonator 1 10 but rather being directly coupled to the gain medium 1 14 optionally without the OTS 104.)

The excitation source 102 is communicatively coupled to a control module 120 via a communication link 124. The control module 120 has connected thereto a computer-readable medium on which excitation profile criteria 122 are stored. The control module 120 may be one or more microprocessors, controllers, digital signal processors (DSPs), or any other suitable computing device, resource, hardware, software, or embedded logic.

In FIG. 1 , the excitation source 102 is generically illustrated by means of a functional block. FIG. 2 is also a schematic view of the laser apparatus 100, but illustrates the excitation source 102 in more detail. Importantly, the excitation source 102 comprises a 2D array of individually-controllable light emitters (refer to FIGS 3-6 for more detail). The light emitters are arranged in a line or plane generally transverse to the longitudinal axis of the laser resonator 1 10. (A side pumped configuration may also be practicable but is not illustrated.) In FIG. 2, the excitation source has a 4x4 matrix:

711 112 113 114^'

121 122 123 124

131 132 133 134

.141 142 143 144.

The laser beam 1 18 has (or is intended to have) a required spatial intensity (RSI) which is the cross-sectional (e.g., radially-dependent or radially-varying) intensity profile 1 18a.

FIG. 3 illustrates a 1 D embodiment of the excitation source 202. In FIG. 3, the excitation source 202 is in the form of a single diode bar comprising a linear array of individually addressable light emitters 204 in the form of diodes. The excitation source 202 is a 1 D Individually Addressable Laser Diode Array (IALDA) with each light emitter 204 being independently controllable by the control module 120.

FIG. 4 shows computer-generated images of a series of water-cooled diode bars 202a-202e. The smallest diode bar 202a has one diode bar while the largest 202e has 10 bars stacked vertically. One of the diode bars 202d has fast axis collimation lenses attached thereto. In this example, these diode bars 202a-202e are manufactured by DILAS Diodenlaser GmbH (https://www.rp- photonics.com/diode_stacks.html).

FIGS 5-6 illustrate a 2D embodiment of the excitation source 302 - which is also an IALDA - which will permit a greater degree of control of the spatial intensity profile 1 18a of the laser beam 1 18. In FIG. 5, the excitation source 302 has a stack of diode bars 303, with each diode bar 303 having a plurality of diodes 304. The diodes 304 in each bar 303 extend in a first direction while the stack extends in a second transverse direction, thus providing the 2D array. FIG. 6 shows a front view of the 2D IALDA 302 showing the stacked configuration of the diode bars 303 (corresponding to the excitation sources 102 of FIG. 2).

The diodes 304 of the 2D lALDAs 302 in FIGS 5-6 can function similarly (albeit without important technical differences) to pixels in an electronic display screen or, more specifically, in an electronic projector, having the ability to be controlled individually, not just in terms of a binary "fully on" or "fully off", but in terms of an intensity gradient (e.g. , 25% intensity, 60% intensity, etc.) which depends on the configuration of the control module 120 and excitation profile criteria 122.

Although the lALDAs 302 have been illustrated as having a relatively low resolution (10x8 in FIG. 5 and 4x4 in FIG. 6), this is merely for clarity of illustration. The resolution may well be higher than this and the Applicant envisages that as technology in this field develops, lALDAs may be constructed having resolutions in the VGA range (640x480) or higher. The Applicant notes (in August 2015) that current diode stacks are available up to about 50x10. 1 D IALDA bars are commercially available up to 100x1 . An individually addressable VCSEL array is commercially available in 8x8 array format.

VCSELs have good future scaling potential, with simpler mounting, cooling and electrical connections compared to edge emitting stacks.

FIG. 7 illustrates a basic, fairly low-resolution example of the selective activation of individual laser diodes 304 in the 2D IALDA 302. The centre four diodes and corner four diodes of off, or mostly off, while the eight remaining peripheral diodes are on, or mostly on, thereby to produce a doughnut shaped intensity profile 1 18a which is a low-resolution approximation of a Laguerre-Gaussian (LG) profile of the order LGki where kl = 10. Using higher resolution lALDAs can produce more accurate higher order intensity profiles 1 18b such as

Laguerre-Gaussian and Hermite-Gaussian as illustrated in FIG. 8. The spatial intensity profile 1 18a delivered by the excitation source 302 will dictate the profile of the excitation energy delivered to the gain medium 1 14 which will amplify the laser beam 1 18 accordingly.

FIG. 9 shows a schematic view of a portion of the laser apparatus 100 of FIG.

1 , including the excitation source 302 and the control module 120. The control module 120 may be a programmable computer or any other suitable computing device. In test setups, the control module 120 can be a laptop or personal computer which permits a high degree of configurability and testing. In more commercial implementations, the control module 120 can be in the form of circuitry including a microprocessor embedded into the IALDA 302 as a self- contained device.

The control module 120 is connected to the IALDA 302 via the communication link 124. In this example, the communication link 124 is a wired communication bus (comms bus 124, for brevity). The comms bus 124 renders each of the laser diodes 304 of the IALDA 302 individually addressable to direct the addressed laser diode 304 to produce an optical output of a specified intensity (in accordance with the excitation profile criteria 122). For example, the control module 120 may communicate in the format (x, y, i) with x being the x- coordinate, / being the y-coordinate, and /^' being the intensity.

The spatial intensity 1 18a of FIG. 9 is approximately Gaussian and is merely shown as an example. To achieve such a profile, central diodes of the IALDA 302 are driven by the control module 120 with a greater intensity output while the peripheral diodes are driven with a progressively decreasing intensity output.

Optionally, the control module 120 is operable to receive feedback, e.g., via a detected feedback signal 702, about the actual (or measured) spatial intensity distribution 1 18a. This can be useful for compensating for tolerances in individual diodes 304 of the IALDA 302. For example, if a particular diode 304 is underperforming compared with its expected (rated) output, this would lead to a dull spot. The power supplied to that diode 304 can be boosted to compensate for its underperformance to achieve, or at least get as close as possible to, the desired spatial intensity distribution 1 18a.

There may be a number of practical applications of the laser apparatus 100. In a more basic implementation, the control module 120 can direct the IALDA 302 to have a static (that is, time-constant) intensity distribution, thus producing a fixed spatial intensity distribution 1 18a. The spatial intensity distribution 1 18a is defined by the excitation profile criteria 122 stored in, or coupled with, the control module 120.

There may be a number of ways of selecting the desired static spatial intensity distribution 1 18a. It may be matched to the fundamental mode of the laser resonator 1 10 (that is, mode-matching). This can yield a very high-efficiency laser apparatus 100 because energy is not lost exciting "unused" or less used regions of the gain medium 1 14. This couples the excitation source 102, 302 efficiently with the gain medium 1 14. Also, the IALDA 302 has a high-power output (in contrast with external modulators which are typically limited in power handling capabilities). It may improve quality of the laser beam 1 18. It also reduces the overall heat load in the apparatus 100 (both at the IALDA 302 and the gain medium 1 14) and consequently reduces cooling requirements.

Instead of matching the excitation profile criteria 122 to the fundamental mode of the gain medium 1 14, the excitation profile criteria 122 can be calculated so as to shape, or assist in shaping, the laser beam 1 18. By having the gain medium 1 14 excited in accordance with the excitation profile criteria 122 will shape the laser beam 1 18 towards that profile. By way of explanation, the gain profile has a large effect on laser beam profile. Other factors include resonator optics and losses (such as intra-cavity spatial filters). Again this is where an SLM (e.g., as disclosed in WO2014064636) may be used in conjunction to further accomplish laser beam shaping.

In a more advanced implementation, the excitation profile criteria 122 can be dynamic (that is, time-varying). The excitation profile criteria 122 can be manually varied, e.g., based on a user input received from an operator, or automatically varied, e.g. as part of a pulse cycle.

The control module 120 may include a user interface, e.g., a computer screen and input device (not illustrated), via which a user input from the operator of the laser apparatus 100 can be received. The operator may be able to select a desired spatial intensity distribution 1 18a. Instead, or in addition, the operator may be able to select a desired beam profile of the laser beam 1 18 and the control module 120 can then automatically calculate the excitation profile criteria 122 required to realise the desired beam profile, and adjust the IALDA

302 output accordingly. This may permit real-time and high speed control of the gain in the gain medium 1 14 and permit flexible spatial intensity control of the gain.

The control module 120 may be configured to vary the spatial intensity distribution 1 18a automatically and very rapidly, in the order of Hz to 100s of MHz. This can be readily achieved with electronic control of the IALDA 302. The control module 120 can affect the instantaneous or average spatial beam profile 1 18a of the output laser beam 1 18 over time. The laser apparatus 100 allows the control and playback of selected spatial intensity distributions of individual laser pulses in a pulsed laser (or time-slices of a continuous wave beam) in order to create the required average intensity distribution for slow physical processes such as welding, cutting, polishing, drilling etc., or match the required intensity distribution of the physical process as it evolves over time. Overall, depending on the particular implementation of the laser apparatus 100, it may have one or more of the following advantages in comparison to conventional pumping:

• Real time and high speed control of the gain in the gain medium 1 14 (e.g., laser crystal);

• Flexible spatial intensity 1 18a control of the gain in the gain medium 1 14 and consequently of the output laser beam;

• High resolution control of the gain in the gain medium 1 14 and consequently of the output laser beam;

• Individual pulse spatial intensity distribution control for a pulsed laser, or temporal control of the evolution of the intensity distribution of a CW (Continuous Wave) laser;

• Compactness of the laser apparatus 100 (in contrast to the use of external modulators to achieve the same effect);

• High power of emitted light (in contrast with external modulators which are typically limited in power handling capabilities);

• Improvement of the efficiency of the laser apparatus 100 and laser amplifiers (which is an alternative implementation - not illustrated - of the present inventive principles); and

• Improvement of the beam quality of output beam 1 18 (if required).

Claims

1. A laser apparatus which includes an excitation source operable to excite a gain medium provided in an optical cavity, wherein: the excitation source comprises an array of controllable light emitters, at least some of which are controllable independently of other light emitters; and the laser apparatus includes a control module operable to control each light emitter or group of emitters in the excitation source independently of the other light emitters or groups of light emitters and in accordance with excitation profile criteria, thereby to excite the gain medium in accordance with the excitation profile criteria.

2. The laser apparatus as claimed in claim 1 , in which the excitation source is a 1 D (one dimensional) array.

3. The laser apparatus as claimed in claim 2, in which the excitation source comprises a single diode bar or a 1 D VCSEL (Vertical Cavity Surface Emitting Laser).

4. The laser apparatus as claimed in claim 1 , in which the excitation source is a 2D (two dimensional) array.

5. The laser apparatus as claimed in claim 4, in which the excitation source comprises a stack of diode bars or a 2D VCSEL (Vertical Cavity Surface Emitting Laser).

6. The laser apparatus as claimed in any one of claims 1 -5, in which the excitation source is an Individually Addressable Laser Diode Array (IALDA).

7. The laser apparatus as claimed in any one of claims 1 -6, in which: the excitation source is provided in the optical cavity as part of a laser resonator; and the laser resonator is operable to generate a laser beam having a pre-defined fundamental mode and beam profile.

8. The laser apparatus as claimed in any one of claims 1 -6, which is in the form of a laser amplifier and in which the excitation source is operable to excite the gain medium but is provided outside the optical cavity, such that a laser beam which is coupled to the gain medium is amplified.

9. The laser apparatus as claimed in any one of claims 1 -8, which includes an optical transformation system (OTS).

10. The laser apparatus as claimed in any one of claims 1 -9, in which the control module is configured to create an intensity distribution in the gain medium by controlling the excitation source, the intensity distribution fitting a specified mode of the optical cavity.

11. The laser apparatus as claimed in any one of claims 1 -10, in which: the excitation profile criteria correspond to a fundamental mode of the laser; and the control module is configured to power or energise the individual light emitters of the excitation source in accordance with, or matched to, a beam profile of the fundamental mode of the laser.

The laser apparatus as claimed in any one of claims 1 -10, in which: the excitation profile criteria correspond to a desired beam output of the laser; and the control module is configured to power or energise the individual light emitters of the excitation source in accordance with, or matched to, a beam profile of the desired beam output of the laser.

13. The laser apparatus as claimed in any one of claims 1 -12, in which the excitation profile criteria are dynamic or time-varying.

14. The laser apparatus as claimed in claim 13, in which the excitation profile criteria dictate that the beam changes temporally over pulses or cycles.

15. The laser apparatus as claimed in any one of claims 13-14, which is operable to control spatially of the pumped intensity distribution of the laser to affect the instantaneous or average spatial beam profile of the output laser beam over time.

16. The laser apparatus as claimed in any one of claims 13-15, which allows control and playback of selected spatial intensity distributions of individual laser pulses in a pulsed laser in order to create a required average intensity distribution.

The laser apparatus as claimed in any one of claims 1 -16, in which the array of light emitters is switchable at frequencies in the order of 5 Hz to 500 MHz.

18. The laser apparatus as claimed in any one of claims 1 -17, which is used in conjunction with a SLM (Spatial Light Modulator).

19. The laser apparatus as claimed in claim 18, in which the SLM is operable to shape the laser mode in the optical cavity.

20. The laser apparatus as claimed in claim 19, which is configured simultaneously to shape the gain profile in the gain medium and the laser mode, collectively to optimise the laser or amplifier beam profile.

21. The laser apparatus as claimed in any one of claims 1 -20, in which the control module is operable to generate or receive a feedback signal and to adjust the excitation source in accordance with the feedback signal.

22. A method of operating a laser apparatus, the method including: providing an excitation source which comprises an array of controllable light emitters, at least some of which are controllable independently of other light emitters; and controlling, by a control module, each light emitter or group of light emitters in the excitation source independently of the other light emitters or groups of light emitters and in accordance with excitation profile criteria, thereby to excite a gain medium in accordance with the excitation profile criteria.

23. The method as claimed in claim 22, which includes controlling the excitation source to power the light emitters dynamically in accordance with time-varying excitation profile criteria.