LASER PULSES ON DEMAND

Field of the invention

The present invention belongs to the field of laser devices for generation, multiplication, modulation or altering repetition frequencies with the help of stimulated emitted light waves, more precisely to the field of constructional details of laser devices and laser devices for controlling intensity, frequency, length, polarization or direction of emitted rays. The present inventions is an improved gain switched fiber laser, which enables changes of repetition frequency in a large range and at the same time maintains a constant peak power and duration of generated laser pulses.

Background of the invention and the technical problem

Current-day laser-based processing systems and methods exhibit a need for adaptable laser sources, which are able to generate laser pulses on demand with a controlled power and duration. This increases the adaptability of the system, while it at the same time decreases the number of components and the complexity of a scanning head, thus enabling work at higher frequencies in the case of processing systems. One possible example of a fast scanning head is realised with a polygonal scanner, which has an advantage of very high movement speed of the laser beam along the workpiece, wherein the movement speed cannot be adjusted as quickly as required. An arbitrary pattern produced by the laser beam can be achieved only by changing the delay between laser pulses. Another example is a resonant scanner, which has a non-linear response causing nonlinear relation between laser beam position and time. To write an arbitrary pattern an adaptable laser source is again required in this case. Pulsed laser sources are known for their good operation at constant repetition frequency, while a change in the latter usually leads to deviation in amplitude and width of the generated laser pulse, which is not acceptable for the majority of applications. Commonly used solutions for ensuring pulses on demand are usually complex and comprise additional components, such as output light modulators, additional seed diodes, etc... These components increase the overall price of the laser and in some cases are not suitable for lasers with output power of several 100 W.

One of the methods for generating laser pulses with a duration around a few tens of nanoseconds is gain switching. This is a method, where a laser pulse represents the first relaxation oscillation when pulse pumping the laser medium. Such lasers, except pump modules, do not need any additional components for achieving pulsed operation and represents a well-founded basis for an industrial laser able to generate pulses on demand. Gain switching of active fiber lasers has been described for active fibers doped with neodymium (Zenteno et al., Opt. Lett. 14(13), 671-673). This article discloses a connection between duration of the pump pulse and the generated laser pulse via a period of relaxation oscillations. In addition, it has been shown that for generation of short pulses (duration around a few tens of nanoseconds) a short oscillator is needed, which consequently decreases the output power of the laser. This problem is solved with an additional amplification stage, but this further increases the dependency of laser pulses on repetition frequency. The increased dependency in this case is a result of larger differences in the pumped upper laser level along the active fiber, which forms the oscillator and the amplifier. In pulsed pumping the oscillator makes up for the losses incurred by spontaneous emission between laser pulses sooner than the amplifier.

The technical problem, which is solved by the present invention, is a design of a compact laser system based on gain switching without any additional optical elements, such as for example an acoustic-optical modulator, along the laser chain. The compact laser system being able to generate pulses at arbitrary times, wherein the peak power and duration of the so generated pulses remain constant. At the same time, it is the aim of the invention to maintain robustness and compact design with low complexity of manufacture. The invention has to be suitable for integration into different laser devices and has to enable simple use in various laser procedures, where pulses on demand are required. Among these, laser micro-structuring methods have to be mentioned, where fast scanners with laser pulses produce an arbitrary pattern with the aim of material removal or change in material structure and/or colour. Robust lasers for generation of pulses on demand also have an added value in the field of distance measurement, in medicine for excitation of photo sensitive structures and elsewhere.

State of the art

Generating pulses with pulsed pumping of the active medium, so called gain switching, is well-studied and described in the literature. Articles by Petkovsek and Agrez (Opt. Express 22(2), 1366-1371 (2014)) and Larsen et al. (Opt. Express 22(2), 1490-1499 (2014)) disclose that a short oscillator has to be used in order to ensure short laser pulses with duration of a few tens of nanoseconds. The influence of other parameters such as laser wavelength, the ratio between the active and the passive part of the fiber resonator, the diameter of the active core, etc., is also well studied. The first gain switched lasers had problems with pulse stability, even if they operated at constant repetition.

A stable gain switched fiber laser comprising only an oscillator, which is based on active fiber doped with thulium, for generating pulses with duration of 10 ns has been described in patent US7787506. Different designs of exclusively fiber based embodiment of oscillator for gain switching have been described in document US2012069860. The latter does not mention generation of pulses on demand, but rather relates to speeding up generation of laser pulses, which is not connected with the technical problem of ensuring constant pulses at arbitrary times, which is solved by the present invention. The use of a laser system together with an oscillator for generating a super-continuum has been described in patent application EP2010081 1307. This document is the first to suggest that additional active fiber functioning as the amplifier and pumped with unabsorbed pump light from the oscillator may solve the low efficiency of short oscillators. This has been shown and discussed in the article by Petelin et al. (Opt. Express 22(17), 20588-20594 (2014)), while the device with reabsorption of the pump light has been later disclosed in document US20160099538A1. Providing an additional active fiber to the pulsed- pumped oscillator while retaining the optical connection for the pump light has been shown to enable higher peak powers and short pulses without the need to increase system complexity and costs.

All above-described solutions exhibit problems in ensuring constant parameters of laser pulses (duration and peak power) when the frequency of the laser operation is altered. A person skilled in the art of lasers knows that gain switched lasers generating individual pulses have the first pulse longer and with lower peak power as the following pulses in the pulse train at the same pumping conditions. This phenomenon could not be avoided until now, even if longer or stronger pump pulses were used. The additional amplifying fiber increases the efficiency of the laser, but the stability of laser pulse parameters decreases significantly and can change by a factor of 2 or more depending on the repetition frequency.

Description of the solution of the technical problem

The essence of the simple laser with improved pump system according to the invention is in that the laser can generate laser pulses with fixed parameters regardless of operation frequency at increased efficiency, wherein the said laser comprises:

- an active element installed between two reflective elements, all said elements forming an oscillator for generating laser pulses,

- a pump system having an output, which is coupled or spliced to the oscillator and pumps the oscillator and an amplifier, wherein the pump system comprises control electronics, a primary and a secondary pump module and a beam combiner, whereas the primary and secondary pump module each comprises at least one pump laser diode, and wherein the primary pump module generates short pump pulses, with a preferred duration up to 10 ps, more preferred from 10 ns to 2 ps, typically from 200 ns to 2ps, with peak power from 10 W and up, preferably from 100 W and up, typically few 100 W, while the secondary module operates at low average power, preferably from 0.1 W to 10 W, even more preferably 1 W, typically few W, and - on the other side of the oscillator a second active element, which serves as the amplifier pumped by the pump light transmitted through the oscillator, thereby ensuring optimal efficiency of the laser.

The pump modules comprise one or more laser diodes with their wavelength corresponding to the absorption peaks of the active material. The active element may be a fiber or a non-fiber element such as a crystal rod, preferably doped with ytterbium, whereas the dopant may also be any other suitable dopant of rare earths such as neodymium, erbium, thulium, holmium, samarium, in order to satisfy the need of operation at different wavelengths of the laser light. The amplifier is preferably doped with the same dopant as the oscillator. The number of pump laser diodes of the primary module depends on the required peak power of the laser, wherein one pump laser diode usually suffices for the secondary module. An important difference between the pump modules according to the invention is the range of pumping power. The primary module, with regards to the control current, generates short pump pulses, preferably with duration from 200 ns to 2 ps, with peak power from few 10 W and up, typically 100 W or few 100 W, wherein the laser is designed in such a way that the energy of the pump pulses is transferred to the generated laser pulse via energy transitions in the active medium. The secondary module operates at low pump power, preferably from 0.1 W do 10 W, typically 1 W or few W, wherein its output may be modulated or continuous, depending on the required type of individual laser pulse parameter control. The power, which is generated by the secondary module, is directly used for minimalizing the effect of active medium relaxation via spontaneous emission between laser pulses. Consequently, the two pump modules of the pump system according to the invention can switch between very high output pumping power (few 100 W) needed for laser pulse generation and low power (few W) between strong pump pulses, thus achieving a large contrast of pumping ratio, which enables optimal laser operation. Thus, improved gain switching technique enables generation of short laser pulses in the described oscillator-amplifier system and at the same time very reliably defines the lowest level of the population inversion that is reached after the laser pulse and maintains said value. In the least complex embodiment of the invention, wherein the pump system between high pulses operates in a continuous mode at low power, the secondary module ensures predefined amount of power needed to maintain a sufficient population of the upper laser level, wherein with changes of repetition frequencies smaller variations in population inversion occur. In the advanced mode, fluorescence intensity of the excited active medium is measured and used to adjust the pumping power of the secondary module, so as to ensure repeatable conditions before the generation of the pulse. The advanced embodiment of the invention is realized with a photodiode installed on the side of the active element, which tracks the intensity of the transmitted spontaneous emission in time. Control electronics are further arranged to comprise a circuit for high sensitivity measurement in a lock-in approach and can thus adjust the level of pumping between each pair of subsequent pulses. In this operation mode of the gain switched laser according to the invention tracking of the laser pulse evolution at the laser output for ensuring stable operation is not needed, which may in turn decrease the number of elements on the laser output.

The pump system further comprises control electronics and a beam combiner (BC). The latter may be built in any suitable way, using for example fiber technology or free-space optics. The control electronics comprises inputs for control signals, an input for a signal coming from a photodiode, which tracks the growth of the optical pulse and on the basis of which time modulation of current to the primary and secondary pump module can be determined, as well as two current outputs for the primary and secondary pump module. Reflective elements may be fiber Bragg gratings (FBG), wherein the FBG with high reflectivity above 90% is installed between the pump system and the oscillator, while the grating with lower reflectivity is installed between the active fiber of the oscillator and the active fiber of the amplifier. The invention could also be realized with free- space optical elements, where the FBG are replaced with dichroic mirrors. The requirement for the reflective elements forming the oscillator is that they retain the optical connection between the oscillator and the amplifier for the pump light. The laser according to the invention may have other components, for example it may be equipped with polarization maintaining fibers and/or single polarization optical elements.

Laser system constructed in the said manner is able to generate any sequence of laser pulses with minimum deviations in their duration and peak power from the average value. The sequence may follow the pre-loaded sequence on the control electronics processor or the pulses may be generated based on a trigger signal, which is in some embodiments generated by the processing device itself. The trigger signal may be a digital or an analogue electric signal generated in the response to the detected event in the laser application/use. The laser is prepared for pulse on demand operation by turning on the secondary pump module, which pumps the active part of the laser continuously (in the simple embodiment of the invention) or in a modulated manner for advanced operation with a lock-in approach so that the threshold for relaxation oscillation is not exceeded. When the control electronics receive the trigger signal, the primary pump module is turned on, which causes a quick rise of the population inversion and emitted laser pulse as a consequence of relaxation oscillation. The primary module turns off the pumping so that it suppresses secondary relaxation oscillations in the system, wherein the secondary module continues to pump the system at lower power and prevents additional losses to population inversion caused by spontaneous emission. In case of upgraded lock-in approach for measuring the pumped state of the of the upper laser level, the secondary pump module may increase or decrease the pump power in order to achieve the optimal value of the population inversion before request for a new laser pulse and the primary pump module is turned on again. The described laser operation mode enables generation of laser pulses on demand.

The described simple pulsed laser has been described in a preferred embodiment of a gain switched fiber laser comprising the oscillator and the additional amplifier, as the advantages of the invention are most obvious in this execution. However, it is certainly possible to use the described pump system with an increased dynamic range of pumping in other embodiments of laser oscillators generating pulses on demand further amplified in additional active medium.

The simple laser for generating laser pulses on demand according to the invention will be described in further detail on the basis of exemplary embodiments and figures, which show:

Figure 1 Schematic view of the simple laser for generating pulses on demand, which shows all key components and is represented by an exemplary embodiment of a monolithic all fiber laser, which does not exclude embodiments with free-space elements.

Figure 2 Schematic view of the pump system, which is an integral part of the simple laser for generating pulses on demand.

Figure 3 Comparison of operation of the gain switched pulsed laser system in known embodiments of generating pulses on demand (top) and in case of use of the pump system and the pumping method according to the invention (bottom).

Figure 4 Comparison of the upper laser level population at two different repetition frequencies in dependency on time for known laser sources (top) and in the laser source according to the invention (bottom). The secondary pump module in the invention ensures constant population level between laser pulses.

Figure 5 Comparison of duration of primary pump pulses (top) and peak power

(bottom) of the output laser pulses in dependence on repetition frequency. Squares represent the response without and dots with the use of solutions according to the invention.

In the scope of the invention as described here and defined in the claims other embodiments of the lasers clear to the skilled person in the art of laser technology are possible, which does not limit the essence of the invention as described here and defined in the claims. In a possible embodiment of low peak power laser system according to the invention, which needs a smaller range of pumping power, it would be possible to use only one pump module functioning as a primary as well as a secondary pump module as described above. This means that only one pump laser diode is used, operated to emit high pump power pulses and in between them modulated with a frequency necessary for measuring fluorescence in a lock-in approach. Furthermore, the pump system having a large dynamic range may be used in optical pumping of other laser types, such as semiconductor lasers.

Figure 1 shows a scheme of a preferred embodiment of the simple laser for pulse on demand generation, wherein the laser is designed as a monolithic all fiber laser, however this does not exclude embodiments with free-space elements. The laser according to the first embodiment comprises a pump system 1 , which is optically coupled to an active fiber of an oscillator 2, a first fiber Bragg grating 3a with high reflectivity, which is installed between the pump system and the active fiber of the oscillator, a second fiber Bragg grating 3b of an additional amplifier 4, a measuring photodiode 5 for measuring in a lock-in approach, an output 6 from the laser and an electric input 7 for control and triggering signals. Operation of the first embodiment is the same as described above. The presented design of the laser enables efficient use of the pump light, as the length of the amplifier 4 is chosen in a manner which allows absorption of all transmitted light from the oscillator 2. The length of the later may be adjusted to generation of laser pulses with duration tailored to the selected final application.

Figure 2 further shows key elements of the pump system of the simple laser with pulses on demand. The system thus comprises: control electronics 1 a, which are used to set the electrical current for controlling the primary pump module 1 b and the secondary pump module 1 c. Both pump modules comprise one or more pump laser diodes 1 d (LD) with their optical outputs coupled via a beam combiner 1 e. Operation of the laser depends on the shape of the pump light emitted at the output of the pump system 1. Laser pulses are generated as a consequence of pump pulses with high power and subsequent fast increase in population of the upper laser level according to the gain switching principle. For ensuring pulses on demand the pump system 1 enables additional functionality. In the simplified embodiment of the invention, the pump system 1 ensures low power continued pumping power between high pump pulses. This power is calibrated to the range of repetition frequencies to be used and physical properties of the laser system, such as the length of the active medium, its cross-section and its dopant. In the advanced mode, which ensures high stability of laser pulses on demand the control electronics 1 a modulates in time the pump power between the high pump pulses and adjusts its peak power according to the feedback signal from the photodiode 5.

The primary 1 b and the secondary 1 c pump modules may have one or more pump laser diodes 1d, wherein the pump laser diodes of both modules emit light at the same wavelength or the wavelengths differ in such manner to cover different absorption peaks of the active medium.

According to a possible embodiment of the invention, wherein Ytterbium active fibers are used, it is beneficial to pump with a combination of different wavelengths. The primary pump module ensures light with a wavelength around the absorption peak at 976 nm and the secondary pump module ensures light with a wavelength around the absorption peak at 920 nm. The advantage of this embodiment is a more favourable distribution of the absorbed power along the laser system, wherein the primary module deposits more pumping power in the short active fiber of the oscillator, while the pumping power of the secondary pump module is more equally distributed between the oscillator and the amplifier due to lower absorption. It is obvious to the person skilled in the art of pumping Ytterbium doped active fibers that absorption of the pump light depends on its wavelength, which consequently means that other combinations of pump light wavelengths may be chosen and the same effect may be achieved by tuning the ratio of absorption coefficients.

Figure 3 shows a comparison of pulsed gain switched laser operation. Such lasers exhibit stable operation at constant repetition frequency. In case of different time delays between laser pulses and in the case of switching on the laser, variations in the pulse duration and peak power occur. The figure obviously shows that the first pulse has longer duration and lower power than the rest of pulses. The bottom part of figure 1 shows operation of the gain switched laser, wherein the above described pumping capable of a large dynamic range and switching between pump pulses with high power and low power between laser pulses is used. This eliminates variations of the output laser pulse parameters, as all pulses have the same duration and peak power, which is the aim of this invention.

Figure 4 shows a comparison of the upper laser level population depending on time during generation of a laser pulse with a gain switched laser. Two repetition frequencies are shown, wherein the dashed line represents occupancy of the upper laser level at two times higher repetition frequency. This figure also shows that between individual oscillations and consequently emitted laser pulses the inversion population is lower. This is due to the spontaneous emission, as between laser pulses the system is below the threshold for stimulated operation. For gain switched system operating at repetition frequencies lower than 10 kFIz or operating in the single shot regime, the effect of spontaneous emission on the output power and duration of laser pulses is significant. The bottom part of figure 4 shows the upper laser level population in case the pumping is performed with the above-described pump system according to the invention.

Further, figure 5 shows measurements of primary pump pulse duration and output laser pulse peak power dependence on repetition frequency. Squares represent the response without and dots with the use of solutions according to the invention. This figure shows that the peak power of laser pulses decreases with lowering repetition frequency, while the optimal duration of pump pulse increases. Despite this the longer pump pulse cannot compensate unbalanced pumped condition caused by the spontaneous emission along the active fiber of the oscillator and the amplifier. By using the present invention almost constant output peak power with minimal increase of the pump pulse duration is achieved.

The simple laser with improved pump system thus reliably ensures constant pulses generated at arbitrary times, wherein the number of laser components is kept low, thus ensuring the price of the described laser lower in comparison to known, more complex solutions of the technical problem. The laser according to the invention may be integrated into various systems or may be used in applications where pulses on demand with constant duration and peak power are desired. Additionally, the laser source enables tuning the peak power of laser pulses for adjustment to requirements of each application.