Background Optoelectronic semiconductor components and devices, including both light emitting and light detecting components are at present very widely used in different kind of applications. One important group of optoelectronic semiconductor components consists of semiconductor lasers, i. e. diode lasers.

Generally speaking, laser light sources offer unique capabilities due to the features of their emitted light. One of these features is the emission spectrum, which in most cases is confined to a very narrow wavelength band whose center wavelength and bandwidth are well specified and particular to a given laser construction. This feature is essential in many laser applications. In some cases, especially in spectroscopic applications, it is also necessary or at least highly useful that the laser emission wavelength can be easily adjusted over a continuous wavelength range. A tunable laser is a laser device designed so that its main emission wavelength may be adjustable during operation.

The characteristic emission wavelengths of a laser device are determined by (i) the gain of the active lasing medium, where the light amplification by stimulated emission takes place, and by (ii) the optical properties of the resonator cavity surrounding said active lasing medium. In a tunable laser, one or both of these properties may change over time. In majority of conventional tunable laser designs, the

active medium has substantially fixed gain properties and the resonator cavity includes a tunable wavelength-sensitive component such as a grating, a prism or an etalon.

Tunable lasers are most widely employed in spectroscopic and telecommunication applications. These fields have been very active in research and development during the whole time period when practical laser instruments have been available. Over the last decade, the development of tunable lasers has been most active in the field of tunable diode laser devices. Being small, inexpensive and robust, diode lasers are gradually replacing established solutions such as dye or titanium-sapphire lasers. At present, diode lasers'available wavelength tuning ranges are somewhat more limited than with the aforementioned more traditional devices, which is an important limiting factor to their employment in many spectroscopical applications. The wavelength tuning ranges can be extended a limited amount by the use of nonlinear optical conversion techniques, like frequency doubling or mixing, known as such in the art especially when applied with the more traditional laser devices. However, unlike modern solid-state lasers, diode lasers cannot efficiently deliver extremely short and powerful pulses based on techniques like Q-switching or mode-locking, said techniques being well-established with the other laser types. For this reason, the efficiency of the nonlinear wavelength conversion remains very poor when applied with the continuous-wave diode laser emission, in order of 10-6 or less, so a continuous emission power in order of hundreds of milliwatts is generally required in order to effectively use nonlinear optical conversion with diode lasers.

Tunable diode lasers Semiconductor diode lasers are one of the few types of available laser devices where the spectral amplification of the active medium itself can be tuned. In a typical edge-emitting diode laser design, the active medium consists of a very thin (100-150 nm) semiconductor junction fabricated on the surface of a bulk compound semiconductor chip. The surface area of the junction is typically smaller than 500x1500 microns.

The primary emission spectrum of a diode laser is determined by the electron energy band distribution in this junction. It is well known that this energy band distribution is dependent on certain conditions of the this laser medium. The most important of these are the temperature, injection current density and mechanical strain prevailing in said medium. Tuning techniques based on the alteration of these conditions have been presented in literature. Of these, temperature tuning is maybe the most widespread in experimental systems. A typical value for the temperature wavelength tunability is 0.5 nm/K. The nominal characteristics of a diode laser, including emission wavelength and lifetime, are normally specified by the manufacturer at +25 °C. The practical limits for diode laser temperature are usually set by the laser lifetime at the high end, and by the properties of the cooling system at the low end. Obviously wavelength ranges of 5-10 nm can be relatively easily achieved by temperature tuning. The US patent 3,588, 253 presents an early temperature-tunable diode laser spectrometer.

In commercially available tunable diode laser devices, more conventional resonator tuning, for example based on a grating or prism, is usually employed instead of the active-medium tuning shortly described above. The reason for this is that the gain of the active medium of a diode laser is rather broad, typically several nanometers.

A much narrower linewidth is required in many applications, for example in high-resolution spectroscopy, and this demand can only be met by careful resonator design. External cavity diode laser (ECDL), where one part (reflector) of the cavity is a tunable grating, is presently a popular design of commercial tunable diode lasers. Several manufacturers offer this kind of systems, including Laser 2000 (UK) and Sacher Lasertechnik (Germany). One example of prior art type ECDL devices is presented in the US patent 5,319, 668. The available tuning range of ECDLs depends on the active media properties; under constant temperature and electrical current typical wavelength ranges are 5-15 nm. On the other hand, the diode lasers used in telecommunication applications employing wavelength division multiplexing (WDM) techniques, are usually temperature-tuned distributed feedback (DFB) lasers.

Temperature tuning is useful even with ECDLs to extend their overall tuning range, but implementing a wide-range temperature control system is often deemed unpractical. High-precision diode laser devices are always equipped with a temperature control system for stable operation. However, these systems are typically designed for maintaining the laser temperature at a fixed point, to keep the system simple. Other practical reasons to avoid temperature tuning are: (i) significant increase of temperature may reduce component lifetime, interfere with the other device components, and consume extra power, (ii) significant decrease of temperature requires a relatively powerful cooling mechanism, due to the heat dissipation in the active medium.

At low temperatures, cooling may also introduce other side effects, like dew formation and freezing.

Another wavelength tuning option is the use of current tuning, which exploits the fact that the diode laser emission wavelength depends slightly on the laser injection current. It is not widely employed, because the laser wavelength and optical emission power cannot be adjusted independently. In practice, the existence of this feature is usually harmful and requires that high-precision instruments have precisely current-stabilized power supplies.

Temperature control of diode lasers Excess heat removal must be considered in all diode laser designs.

While the conversion efficiency of electric power to emitted light can be up to 50% with modern quantum-well diode lasers, all the remaining electric power will be dissipated as heat in the laser active medium.

Due to the small size of the active medium, even very small power levels will raise the temperature of an unmounted laser chip considerably. Because excessive temperature will reduce the lifetime of the device, practically all diode laser chips are mounted to a solid metal heat sink, which is typically considerably larger than the laser active medium itself. Also typically, the heat sink will act as one of the current-delivery electrodes to the diode laser.

Next, heat transfer from the heat sink to the surrounding environment must be considered. Simple cooling by conduction or natural convection is sufficient only at low power levels, in order of a few mW or below. As the continuous electric power consumption by present high-power diode lasers can be up to several watts per emitter, the heat transfer must often be enhanced by active cooling. The usual solution is forced gas or liquid coolant convection, in which case the heat sink must be designed accordingly with fins and/or flow channels.

Also, the device must be provided with an internal or external coolant circulation system, like fan or pump. The circulation system may be enhanced with a user-controlled or automated feedback mechanism that adjusts the cooling power according to the laser power level or environment conditions, usually by changing the coolant flow rate. The selection between gas or liquid coolant is dictated by the application; liquid coolants are very efficient due to their higher specific heat, but in most cases they require a closed-circuit flow system and separate heat exchangers, whereas air or other non-toxic gas coolant can often simply be released to the surroundings after passing the heat sink.

Another known way to reduce the operating temperature of a diode laser is to use a thermoelectric cooler. This well-know device is based on Peltier effect, where applying electric current over a particular bimetal junction will transfer heat over the junction, generating a temperature difference. These devices are widely applied in cooling of electronic components; typically, they are planar components with a heated and a cooled side. However, they do not help to dissipate any heat to the surroundings, so active cooling must always be applied to the heated side of the junction to ensure continuous operation. In short, these devices will only lower the component operating temperature with regard to the surroundings, but do not provide any increment in the total device cooling power, as will become more evident later in this text. Another useful property of the Peltier effect is that when reverse current is applied to the bimetal junction, it will act as heater. This way, a simple temperature control device can be built for a low-power emitter, like one presented in the US patent publication 2002/121,094.

Commercial thermoelectric cooler devices have two main specifications, which are maximum temperature difference across the junction (typ. 70 K) and maximal heat transfer rate (typ. less than 10 W/cm2) at the nominal junction current. Within the specified range, the cooling rate can be tuned precisely by the junction current. The heat transfer rate will scale approximately with the size of the cooler device, so high-rate coolers are large compared to typical diode laser chip size.

The parameters must be carefully interpreted when assessing the cooler performance in a given set-up. The maximum temperature difference is attained when the heat transfer from the cooled surface to the heated surface by the Peltier effect equals to the heat transfer to the opposite direction by conduction within the cooler device. At this state, no net heat transfer will be generated, so the cooling power of the device is zero. When net heat is transferred through the device, the actual temperature difference will be smaller. At the specified maximum heat transfer rate, the difference will fall to zero. Therefore, to maintain a temperature difference using a thermoelectric cooler, the specified power of the cooler must be larger than the actual heat transfer rate.

For this reason, available thermoelectric coolers are generally not well suited to accurate temperature control of small size high-power components, even if the specified cooling power equals or slightly exceeds the heat dissipation in the component.

To reach even lower temperatures via active cooling, a low- temperature coolant can be used. Compressed liquid gas, for example nitrogen or helium, is used frequently for this purpose. These solutions increase device cost, complexity and size, and decrease safety and reliability. Extremely low temperatures with a good heat transfer rate can be reached this way, but building a control system with a broad temperature range is again more difficult. Liquid-gas cooling is rarely employed in diode laser systems, with the exception of far-infrared emitting lead-salt diode lasers, which must be operated at very low temperature (for example, see US patent 4,684, 805).

Most present low-and medium-power diode laser systems that employ active cooling have fan-circulated air cooling, possibly with thermoelectric cooler-based control system for temperature stabilization in uncontrolled ambient conditions. At higher power levels, the use of active water cooling is dominant. To design a temperature- tunable diode laser device for an uncontrolled environment, one must arrange a more accurate wide-range temperature control system with heating and cooling functionality. For steady-state operation, this system must also remove the excess heat generated in the diode laser.

In conclusion, medium-to high-power diode laser devices require active cooling for stable operation. The use of forced air or water cooling is dominant, but these techniques cannot lower the temperature of the laser active medium below the temperature of the coolant flow. Thermoelectric coolers can be used to this effect, but their capabilities are limited to low-power devices.

Summary of the invention The main purpose of the present invention is to introduce a new method for the temperature control of optoelectronic semiconductor components, which method does not suffer from those limitations discussed above and being typical for prior art solutions. To attain this purpose, the method according to the invention is primarily charac- terized in what will be presented in the characterizing part of the independent claim 1.

It is also an aim of the invention to provide an apparatus implementing the aforementioned method. The apparatus according to the invention, in turn, is primarily characterized in what will be presented in the characterizing part of the independent claim 7.

The dependent claims describe further some preferred embodiments of the invention.

The basic gist of the current invention is the use of a vortex tube for the generation of a gas flow, which is further used for active cooling or heating of an optoelectronic semiconductor component. According to the Applicant's understanding, vortex tubes have not been previously considered for this purpose, where they can provide very significant benefits over most prior art solutions.

More specifically, the present invention provides novel means for diode laser wavelength tuning by adjusting the temperature of the lasing active medium using gaseous coolant with the distinction that the temperature of said gaseous coolant is controlled using a vortex tube.

In addition to wavelength tuning of a laser diode, the invention provides means for removal of excess dissipated heat from the laser device or other optoelectronic semiconductor component, and makes it possible to operate and tune such devices in environments where the ambient temperature and other relevant conditions are not controlled.

According to a preferred embodiment of the invention, a closed-loop feedback control system is provided to adjust the coolant gas temperature and flow rate as a function of the diode laser emitter temperature or the optical properties of the emitted laser radiation.

The preferred embodiments of the invention and their benefits will become more apparent to a person skilled in the art through the description and examples given herein below, and also through the appended claims.

Description of the Drawings In the following, the invention will be described in more detail with reference to-the appended drawings, in which Fig. 1a shows schematically in a cross sectional view a diode emitter mounted on a single primary gas-convection cooled open dissipating heat sink,

Fig. 1b shows schematically in a cross sectional view a diode emitter mounted on a primary heat sink that is further connected by a thermoelectric cooler to a secondary gas- convection cooled open dissipating heat sink, Fig. 1c shows schematically in a cross sectional side view a diode emitter mounted on a single primary gas-convection cooled closed dissipating heat sink, Fig. 2a shows schematically in a cross sectional view the mounting of a dual-soldered end block mounted edge-emitting laser chip to the primary heatsink, Fig. 2b shows schematically in a cross sectional view the mounting of a vertical-cavity surface emitting laser (VCSEL) chip to the primary heatsink, Fig. 3 shows schematically the cross section of a vortex tube, with the internal flow streamlines depicted as dashed arrow lines, Fig. 4 shows schematically major parts of a vortex-tube enhanced diode laser gas cooling system according to the invention, Fig. 5 shows a simplified diagram of a closed-loop control system for temperature tuning of a diode laser device, measuring either diode emitter or heat sink temperature, or the optical properties of the laser emission, Fig. 6a shows a schematical-view of an ECDL laser resonator, with a heat sink mounted laser emitter chip,

Fig. 6b, 6c present two wavelength graphs that demonstrate the operation of combined wavelength tuning by resonator and temperature gain, Fig. 7a shows a schematical view of particle imaging application in plasma spraying environment, where diode laser illumination is employed, and Fig. 7b shows a wavelength spectrum chart, including selected intense atomic argon spectral lines, and a demonstration of a temperature-tuning method for avoiding the interference of the spectral lines in the plasma spraying application.

Detailed description of some preferred embodiments of the invention It is to be understood that the drawings presented herein below are designed solely for purposes of illustration and thus, for example, not for showing the various components of the devices in their correct relative scale and/or shape. For the sake of clarity, the components and details which are not essential in order to explain the spirit of the invention have been omitted in the drawings.

Heat sink arrangements Referring to Figs 1 a-c, we first discuss some preferred heat sink cooling schemes of a diode laser emitter DL. In Figs 1a-c the incoming coolant flow F is indicated by black arrows and the outgoing, i. e. exhaust coolant flow is indicated by white arrows.

In Figs 1a-c the diode laser emitter chip 10 is mounted on the primary heat sink 11. The primary heat sink is significantly larger, both in at least one dimension and mass, than the emitter chip itself.

Alternatively, a multitude of emitters, either on a single chip or separate chips may be mounted on the primary heat sink 11. The laser chips may be attached to the primary heat sink by soldering, or by clamping from the opposite surface, as is known from related art.

With an edge-emitting diode laser, the mounting side is preferably the anode electrode of the laser diode, and the primary heat sink 11 acts also as the anode electrical contact for the laser chip. The cathode electrode of the laser chip is on the unmounted side, and the electrical contact to the cathode is provided by bonding a conductor wire or a foil 17 to this side. This wire or foil is connected to the laser power supply (not included in the drawing). An insulator block 18 prevents the electric short circuit between the laser electrodes.

If a thermoelectric cooler 12 (Peltier element) is included, as shown in Fig. 1 b, a secondary heat sink 13 is also present. Both primary 11 and secondary 13 heat sinks have a flat surface whose side approximately matches the size of the cooler 12 active surface. The primary heat sink 11 is attached on the cold side CS and the secondary heat sink 13 is attached on the hot side HS of the thermoelectric cooler 12. The heat sinks 11,13 are attached to the cooler device 12 by clamping, preferably with heat-conductive sealant paste applied to the contacting surfaces.

The heat sink that dissipates the heat to the coolant flow has a structural design that enhances the convective heat transfer. In case of an open disspating heat sink (Fig. 1a and Fig. 1b), this may comprise ridge-or rod-shaped fins 14 that protrude from one surface of the heat sink to the path of the main coolant flow F. If the heat sink design is closed (Fig. 1c), the dissipating heat sink has bored or otherwise machined internal channels or chambers 15 that confine the coolant flow F, the inner walls of such a channel or chamber may also contain heat transfer enhancing fins 16.

Diode laser mounting Referring to Figs 2a and 2b, some alternative diode laser emitter DL mounting solutions that are compatible with the current invention are presented. In Fig. 2a, a symmetrical structure comprising an edge- emitting diode laser 20, dual end-blocks 21 and an electrical insulator

sheet 22 are arranged to be soldered together. This structure is like the one described in the patent US 5,913, 108. Alternatively, the diode laser chip can be a vertical-cavity surface emitting laser chip 23, mounted on the primary heat sink on its non-emitting side as shown in Fig 2b.

According to the invention, any of the heat sink arrangements presented in Figs 1a-c may be applied with any of the diode laser mounting solutions presented in Figs 1a, 2a-b.

Vortex tube operation Vortex tube is a device that divides an incoming gas flow into heated and cooled output flows. Vortex tubes and their operation principle is known as such and described, for example, in the US patent 1,952, 281. It comprises a cylindrical elongated flow channel, where the incoming gas is directed through a narrow inlet through the tube wall, in tangential direction close to one end of the tube.

It is characteristic to the operation of the vortex tube that it does not require any external heat pump or other energy source in addition to the source of pressurized gas. No toxic agents, like ammonia or freon, need to be employed. Commercial vortex tube modules are manufactured by ITW Vortec (US) and Exair (US). They are available in a selection of sizes, with gas flow capability and total cooling power increasing with size. Vortex tubes are known to be used for cold gas flow generation in many applications, including cooling of enclosures (cf. Pat. US 6,401, 463) and sewing machine needles (cf. Pat. US 4,305, 339). However, according to the Applicant's best knowledge, vortex tubes have not been considered to be used to control the temperature of, or remove the dissipated heat from any optoelectronic components such as diode lasers.

A typical vortex tube implementation has two tube connectors: first, for incoming gas and second for the cooled flow output. The heated flow is released to surroundings via an exhaust port. The exhaust port also

includes an adjustable valve, which controls the gas flow division between heated and cooled flows. The actual performance of a vortex tube will depend on operating parameters described below. In general, temperature differences greater than 50 K can be achieved between the incoming and cooled gas flows.

The use of a vortex tube allows for more flexibility in the design of a cooling system for optoelectronic components compared to prior art solutions. Vortex tube provides high cooling power, which can be easily controlled. If necessary, the vortex tube can also be easily bypassed or removed in conditions where the enhanced cooling is not required.

Also notably, the vortex tube can also act as a supply of heated gas, if required. Vortex tube enhanced cooling according to the invention is especially useful in optoelectronic devices intended for industrial environments, because in such environments pressurized gas is usually readily available and simple and rugged cooling system with high cooling power is in many cases of primary importance.

Referring now to Fig. 3, the operation of the vortex tube VT is described in more detail. The pressurized and therefore compressed gas is inserted to the main tube 30 via the tangential inlet 31. The main tube geometry is designed so that a rapid outer vortex OW is generated in the main tube close to the inner wall. This vortex extends through the entire main tube length between the gas inlet and the far end of the tube 32, where a narrow annular orifice 33 is provided. Part of the gas flow is allowed to exit through said annular orifice 33. The flow division between the escaping and remaining gas is adjusted by the control valve 34 that adjusts the width of the aforementioned annular orifice 33.

The remaining gas shall flow back in the central zone of the tube, to the end where the incoming gas inlet is located. The swirling angular. velocity of this inner vortex IW is greatly reduced from that of the outer vortex. The return flow will exit via a small circular opening 35 in the center of the tube end wall, close to the incoming gas inlet.

The above described flow geometry will lead to heat transfer between the inner IW and outer OW vortices, with the result that the gas flowing out from the annular orifice 33 will be hotter, and the gas flowing out from the circular orifice 35 will be colder, than the incoming gas flow 31. The actual temperature difference, and the respective hot and cold gas flow rates, will depend on the width of the orifice 33. The ratio of the cold gas flow rate to the incoming gas flow rate is referred hereafter as the cold ratio. The temperature difference will increase, and the cold gas flow decrease respectively with decreasing cold ratio. On the other hand, the total gas cooling power of the vortex tube depends mostly on the incoming gas pressure. This combination of parameters allows for flexible adjustments of the cooling system performance under given requirements of operation temperature and power dissipation of that device which requires cooling.

The vortex tube VT input gas should naturally be dry and free from particulate impurities to provide the most efficient cooling and to avoid the clogging of the vortex tube and the other flow channels.

Referring to Fig. 4, a more complete diode laser cooling system design according to the invention is now presented. The compressed air or other coolant gas is provided by a compressor 40 or other suitable source of pressurized gas and delivered to the vortex tube VT. The control valve 42 will reduce the pressure to a suitable level, determined by the specifications of the vortex tube VT and the required cooling power. The hot gas flow produced by the vortex tube VT is exhausted to the surroundings via the exhaust port 43 and the cold gas flow 44 is directed to the dissipating heat sink. A muffler device 45 may be present at the exhaust port to reduce the audial noise of the vortex tube operation.

After passing the dissipating heat sink 11 of the optoelectronic device DL, the coolant gas may be allowed to dissipate into the enclosure containing the optoelectronic device DL (diode laser 10), or it may be directed outside said enclosure via an exhaust channel, as required by the particular device structure and application.

Referring to Fig. 5, a possible embodiment of a closed-loop temperature diode laser tuning system is now described. A heat-sink mounted diode laser emitter module DL is energized by a programmable power supply PS. One or more probes P monitor at least one operating parameter of the diode laser. The monitored parameters may be the temperature of a heat sink or the laser emitter, or an optical parameter related to the laser emission. One or several of these or other parameters may be monitored in a given implementation. The signal or signals related to said parameter or parameters is/are transmitted to a control system CS, which has been programmed or otherwise arranged to operate with a goal to tune the laser emission to a desired wavelength. Depending on the particular implementation, the control system CS may independently adjust the input gas pressure GP or the cold ratio CR of the vortex tube VT, or the operating current of the thermoelectric cooling device TC, or the laser current LC. These parameters effectively change the diode laser emitter temperature and optical emission properties, which is reflected as a difference seen in the monitored parameter P signal. This cycle is repeated until the laser emission achieves the desired wavelength. The control system may also be programmed to change the tuning of the laser over time in order to scan the emission wavelength in a desired manner.

The temperature tuning system may be used in combination with another tuning mechanism, for example together with external-cavity tuning or current tuning. These different tuning mechanisms preferably employ a single integrated control system common to all of them.

Preferably, the diode laser emitter 10 and the heat sink 11,13 are placed in an enclosure that provides thermal isolation from the environment. Suitably, the vortex tube VT is placed within the same enclosure with the diode laser emitter and the heat sink.

Preferably, the feedback control system operates by adjusting the vortex tube 30 incoming gas 31 pressure GP and/or the vortex tube VT

cold ratio CR; in case where a thermoelectric 12 cooler is employed, also the operating current of the cooler 12 may be adjusted.

Examples of some advantageous applications Finally, some advantageous applications that benefit from the use of vortex tube VT cooling of semiconductor optoelectronic components DL are discussed.

As already noted, the modern precision-tunable diode lasers are exclusively implemented with ECDL technology. Referring to Fig. 6a, a typical ECDL cavity structure is schematically presented, comprising a diode laser emitter chip 60, a heat sink 61, an optical diffraction grating 62 and a tuning mechanism 63. The laser resonator is formed between the grating 62 and the rear facet RF of the diode laser emitter 60 on the opposite side of the chip. The tuning mechanism changes the tilt angle of the grating 63, effectively changing the gain parameters of the laser resonator.

Referring to Fig. 6b, a combined temperature and resonator tuning mechanism of an ECDL device is presented. At a given active medium temperature T1, the laser medium has a constant wavelength gain function W1. In a given tuning position, the resonator has a wavelength gain function R1, the resonator and medium gain functions being independent on each other. The laser emission wavelength profile EP1 of the laser is determined by the product of the respective medium and resonator gain functions. Typically, the range of the resonator tuning R is determined by the cavity design, and exceeds the range of the laser medium gain.

When the laser active medium is cooled down to temperature T2 (see Fig. 6c) by a cooling mechanism according to invention, the laser medium gain function will be shifted by a wavelength WS that has a nearly linear dependence on temperature at values close to the room temperature, resulting in a new medium gain function W2. If the resonator is designed so that R extends to overlap W2, it can now be

tuned to position R2, resulting in a new laser emission profile EP2 that would be unavailable without the temperature tuning functionality provided by the invention.

The rate of temperature tuning depends primarily on the cooling power of the temperature control system and the combined thermal mass of the laser chip and the heat sinks. With proper design, the temperature tuning rate can be in order of 10 K/S, which, in terms of wavelength, would equal a few nanometers per second. This is far greater than the wavelength tuning rate of a typical prior art type high-precision resonator tuning mechanism.

Another application that greatly benefits from tunable diode lasers is imaging of rapid events under unstable or continuous high intensity background illumination. Examples of events like this are explosions, sparking and thermal spraying processes.

In general, diode lasers can deliver high-intensity light pulses with nanosecond-scale timing resolution, which is rapid and accurate enough to capture any kind of mechanical event. A CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor) camera provides a relatively cost-effective device for scientific or industrial imaging, but at present these cameras have relatively long minimum exposure times, typically 1 ms or more, which allows an excessive amount of background light to reach the detector.

Therefore, even if a very short stroboscopic pulse can be generated using a pulsed diode laser, the camera still unnecessary integrates background light also outside the pulse duration. In order to block this background light from reaching the detector during the exposure, optical filtering can be used. The narrow emission spectrum of the diode laser allows the use of a narrow-band interference filter for this purpose, which will reduce the total amount of the background light by orders of magnitude in most cases.

We will now discuss a practical example of an imaging application where the use of vortex tube driven temperature tuning of a diode laser

will prove highly useful. Referring to Fig. 7a, we present a system for particle detection in plasma spraying environment. Plasma spraying is a well-know thermal spraying method for manufacturing various kinds of special coatings, for example thermal barrier, corrosion resistant or low friction coatings.

A typical plasma spraying system shown in Fig. 7a comprises a particle feeder 70, which provides a solid particle-laden gas flow. Said flow is mixed to a stream of hot plasma provided by a plasma torch 71, forming a plasma spray PS. The gas temperature in a plasma spray like this is typically 3000 K or maybe also significantly more, whereas the particle temperature is somewhat lower. To detect the particles, a diode laser 72 emits a short laser pulse LB that intersects the spray in a given position. The laser light will partially scatter from the particles, and a camera detector 73 will collect the scattered light, forming an image of the particles within the plasma spray. Typically, the diode laser 72 and the camera detector 73 are arranged normal to the travelling direction of the plasma spray PS. An optical narrowband filter 74 arranged in front of the camera 73 will pass the laser light, but block the very intense plasma gas background radiation, which would otherwise saturate the detector.

However, the use of optical filtering to block the background is not effective, if the background radiation is very intense at the exact laser illumination wavelength. The plasma stream typically comprises high amount of argon gas. In this case, the plasma radiation spectrum contains several high-intensity atomic Ar spectral lines, with the strongest of the lines at near infra-red parts of the electromagnetic spectrum. Unfortunately, these lines coincidently overlap with the range of the presently available emission wavelengths for the highest power GaAIAs diode laser devices, around 800 nm. This kind of lasers typically have a simple multi-mode resonator producing a relatively broad laser emission profile, with a bandwidth of at least 3-4 nm. With this kind of lasers, temperature tuning is the only viable wavelength tuning option.

The exact emission center wavelength of a diode laser chip will depend on the detailed composition of the laser active material; the composition can be controlled to some extent within the laser chip fabrication process. However, it is costly to build diode lasers with precise emission wavelength requirements and even in that case, the chip must be operated in strictly regulated conditions. For these reasons, it is hard to completely avoid overlap of the laser emission line with the spectral lines. Wavelength tuning can be employed to drive the laser emission wavelength to a value where there is no overlap with the background atomic lines, in this example atomic Ar lines, regardless of the nominal laser emitter wavelength.

Turning now attention to Fig. 7b, we present this procedure in argon plasma thermal spraying. The five prominent Ar spectral lines Ar795- Ar811 are strongly present in the background radiation of the plasma spray PS. The laser emission band DL1 of a commercially available diode laser is presented at the nominal center wavelength of 808 nm, with a strong overlap with the lines Ar810 and Ar811. It is viable to design an optical interference filter 74 so that the filter passband OF does not overlap with any of the aforementioned background lines.

However, its overlap with the untuned,"standard"laser line will also be minimal, leading to a weak scattering signal that may be undetectable by the camera 73. If the laser emission band is tuned to the position DL2, the overlap with the filter passband will be strong and the signal detected by the camera 73 will be elevated to a detectable level.

Vortex tube powered temperature tuning according to the present invention is the preferable approach to the above described application, because the environmental conditions in thermal spraying are very harsh. The plasma spray is a strong heat source that provides a large radiative and convective heat load to its environment, where both the laser diode device and camera device need to be operated.

The high cooling power provided by the vortex tube VT can be used to cool the diode laser components DL and enclosure in addition to the laser tuning function.

While the invention has been shown and described above with respect to selected embodiments of optoelectronic devices, mainly laser diode components, it should be understood that these embodiments are only examples and that a person skilled in the art could construct other embodiments utilizing technical details other than those specifically disclosed herein while still remaining within the spirit and scope of the present invention. It should therefore be understood that various omissions and substitutions and changes in the form and detail of the apparatuses illustrated, as well as in the operation of the same, may be made by those skilled in the art without departing from the spirit of the invention. It is the intention, therefore, to restrict the invention only in the manner indicated by the scope of the claims appended hereto.

Therefore, the wavelength tunability of a laser diode can also be used in other laser applications than those related directly to plasma spraying and Ar spectral lines. The wavelength tunability may also be used for other purposes than avoiding unwanted overlap with interfering spectral lines or other spectral features. For example, the wavelength of an semiconductor emitter may be tuned to coincide with other active of passive optical components of an optical system, or just to keep the emission wavelength constant in otherwise varying environments. The temperature control or cooling system based on the use of a vortex tube may also be used with optical detectors, for example with semiconductor camera detectors.