A SIMPLE LASER WITH AN IMPROVED PUMP SYSTEM FOR GENERATING LASER PULSES ON DEMAND

Abstract

The invention belongs to the field of lasers, 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 invention is an improved gain switched fiber laser, which enables changes of repetition frequency in a large range and which, at the same time, maintains a constant peak power and duration of generated laser pulses. The essence of the laser according to the invention is the pump system which is coupled to an oscillator and pumps the oscillator as well as the amplifier. The pump system comprises a primary and a secondary pump module, each at least with one pump laser diode, wherein the primary module generates short pump pulses and wherein the primary and secondary pump module operate at different peak powers. The power generated by the secondary module can be directly used to decrease the effect of active medium relaxation via spontaneous emission in-between laser pulses, which enables generation of short laser pulses on demand having constant parameters.

Claims

1. A simple laser with improved pump system, characterized in that the laser has the capacity to generate laser pulses with fixed parameters regardless of the repetition frequency, wherein the said laser comprises: an active element installed between two reflective elements, thereby 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, said primary and secondary pump module each comprises at least one pump laser diode, and wherein the primary pump module generates short pump pulses, and the primary and the secondary module operate at different peak powers, and on the other side of the oscillator a second active element, which serves as the amplifier for absorbing pump light transmitted through the oscillator, thereby ensuring optimal efficiency of the laser.

2. The simple laser with improved pump system according to claim 1, characterized in that the primary module based on control current generates short pump pulses, with a preferred duration up to 10 μs, with peak power from 10 W and up, wherein the energy of pump pulses is transferred with energy transitions in the active medium to the generated laser pulse; that the secondary module operates at low average power, preferably from 0,1 W to 10 W, wherein the output of the secondary module may be continued or modulated depending on the required control of the laser pulse parameters, while the generated power is used for decreasing the effect of active medium relaxation caused by spontaneous emission between laser pulses; and in that consequently the pump system with both modules can switch between a high pump power generating a laser pulse and a low pump power after said laser pulse is generated.

3. The simple laser with improved pump system according to claim 1 or 2, characterized in that the pump modules comprise one or more pump laser diodes with a wavelength corresponding to absorption peaks of the active medium, wherein the pump laser diodes of both modules may emit light at the same wavelength or at different wavelengths in order to cover different absorption peaks of the active medium.

4. The simple laser with improved pump system according to any of the preceding claims, characterized in that the number of pump laser diodes of the primary module depends on the required output power of the laser, preferably one pump laser diode in the secondary module and one or more pump laser diodes in the primary module.

5. The simple laser with improved pump system according to any of the preceding claims, characterized in that the active element is an optic fiber or a non-fiber element such as a crystal rod.

6. The simple laser with improved pump system according to any of the preceding claims, characterized in that the active medium present in the active element may be doped with rare earth dopants such as ytterbium, neodymium, erbium, thulium, holmium, samarium, wherein the preferred dopant is Ytterbium.

7. The simple laser with improved pump system according to claim 6, characterized in that the dopant is ytterbium and the primary pump module emits light with a wavelength around the absorption peak at 976 nm, while the secondary module emits light with a wavelength around the absorption peak at 920 nm.

8. The simple laser with improved pump system according to any of the preceding claims, characterized in that the control electronics of the pump system comprises inputs for control signals and optionally an input for a photodiode signal, which tracks the growth of the laser pulse, and two current outputs for the primary and secondary pump modules.

9. The simple laser with improved pump system according to any of the preceding claims, characterized in that the secondary pump system in-between pulses operates in a continuous mode at low power.

10. The simple laser with improved pump system according to any of the preceding claims, characterized in that the secondary pump module is modulated, the said laser having a photodiode and electronics with circuit for high sensitivity measurement in a lock-in approach, the photodiode being coupled from the side of the active element, thereby allowing monitoring of the intensity of the transmitted spontaneous emission in time and thus allowing adjustment of the pumping power from the secondary module to ensure repeatable conditions before the laser pulse is generated.

11. The simple laser with improved pump system according to any of the preceding claims, characterized in that the reflective elements are dichroic mirrors or preferably into the fiber written fiber Bragg gratings, wherein the grating 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.

12. The simple laser with improved pump system, characterized in that it generates laser pulses with fixed parameters regardless of the repetition frequency, wherein the said laser comprises: an active element installed between two reflective elements thereby forming the oscillator for generating laser pulses, An additional active element on the other side of the oscillator, the additional active element functioning as an amplifier for absorbing pump light transmitted through the oscillator, which ensures optimum efficiency of the laser, and One pump module, which is between pump pulses with high peak power modulated with a frequency needed for fluorescence measurement in a locked approach.

13. A method for generating laser pulses with constant parameters at arbitrary frequency using laser according to any claim from 1 to 12, wherein the method comprises the following steps: a) pulses may be generated based on the trigger signal or based on a pre-defined required sequence, wherein the trigger signal may be a digital or analogue electrical signal; b) the laser is prepared for arbitrary generation of pulses by turning on the secondary pump module, which pumps the active medium of the laser in a continuous mode or in a modulated mode with lock-in approach measurement, in order to prevent exceeding the threshold for relaxation oscillation; c) when the control electronics receives the trigger signal, the primary pump module is turned on, which causes a significant increase of population inversion in the laser and generates a laser pulse as a consequence of the relaxation oscillation; d) the primary module turns off the pumping to suppress secondary relaxation oscillations in the system, wherein the secondary module continues with pumping the system as in step b) and prevents additional loss to population inversion caused by spontaneous emission; e) by repeating the steps c) and d) laser pulses on demand can be generated at arbitrary times.

14. The method for generation of constant pulses with an arbitrary frequency with the laser according to claim 13, characterized in that the control electronics modulate pumping power of the secondary pump module in-between high pump pulses and adjust the peak value with regards to the signal provided by the photodiode.

Description

[0021] 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:

[0022] FIG. 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.

[0023] FIG. 2 Schematic view of the pump system, which is an integral part of the simple laser for generating pulses on demand.

[0024] FIG. 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).

[0025] FIG. 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.

[0026] FIG. 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.

[0027] 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.

[0028] FIG. 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.

[0029] FIG. 2 further shows key elements of the pump system of the simple laser with pulses on demand. The system thus comprises: control electronics 1a, which are used to set the electrical current for controlling the primary pump module 1b and the secondary pump module 1c. Both pump modules comprise one or more pump laser diodes 1d (LD) with their optical outputs coupled via a beam combiner 1e. 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 1a 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.

[0030] The primary 1b and the secondary 1c 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.

[0031] 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.

[0032] FIG. 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 FIG. 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.

[0033] FIG. 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 kHz 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 FIG. 4 shows the upper laser level population in case the pumping is performed with the above-described pump system according to the invention.

[0034] Further, FIG. 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.

[0035] 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.