Field of the invention

The present invention belongs to the field of laser devices for generating, multiplying, modulating or changing repetition frequency using stimulated light waves, more precisely to the field of constructional details of laser devices and laser devices for controlling intensity, frequency, duration, polarization or direction of emitted rays. The invention relates to a hybrid laser for generating laser pulses on demand, said pulses having constant energy, and to a method of generating laser pulses with constant energy using the said hybrid laser.

Background of the invention and the technical problem

Today's laser processing systems and methods demonstrate the need for adaptive laser sources capable of generating laser pulses on demand. The generation of laser pulses on demand is particularly important when using high-speed scanning systems (e.g. polygon or resonant scanning systems) where high flexibility of the laser system is desirable due to the inflexible scanning speed.

Hybrid lasers are lasers that combine different geometries of amplifier media, exploiting the different advantages of each type of amplifier media. For this purpose, a combination of fibre pre-amplifiers and additional solid-state power amplifiers is often used. The first provide high efficiency and high laser beam quality, whereas the second enable achieving high peak powers of the laser pulses and high pulse energies. One method for generating laser pulses on demand is to combine a primary laser source with at least one additional laser source that is used to generate idler pulses. The combined laser system also comprises laser amplifiers and an additional element to separate the idler pulses from the primary laser pulses. The idler pulses are used to control the amplification in all amplifiers of the laser system. Problems arise in cases where laser systems include two or more different types of amplifiers, such as one or more fibre amplifiers and one or more solid-state amplifiers. The different gain spectra of the fibre and solid-state amplifiers in combination with different spectral widths of the primary and idler pulses lead to a difference in the amplification between primary and idler pulses and thus to the occurrence of transient phenomena in laser pulse energies. As a result, the laser pulse amplitudes and pulse energies are no longer constant in pulse on demand operation, which is undesirable for laser operation and also for laser processing applications. For example, ytterbium-doped fibre amplifiers have a broad gain spectrum, resulting in the same amplification of the primary (with a broad spectrum) and idler pulses (with a narrow spectrum). On the other hand, solid-state amplifiers usually have a narrower gain spectrum, resulting in smaller amplification of the primary pulses compared to amplification of the idler pulses, which leads to transient optical phenomena, that are expressed as variations in a laser pulse energies during pulse on demand operation. This occurs because the laser does not operate in a stationary regime as a result of different gains in different type of amplifier.

For pulse on demand operation without transient optical phenomena, i.e. constant pulse energy, primary (desired) pulses with a broad spectrum and idler pulses must have the same gain in all amplification stages, especially in the case of hybrid lasers using a combination of fibre and solid-state amplifiers. The technical problem that this invention solves is therefore the design of a hybrid laser system that will allow pulses to be generated on demand with constant energy, which means that the aforementioned laser system must eliminate or prevent transient optical phenomena during the amplification of laser pulses. The invention must be suitable for integration into different laser devices and must be easily used in various laser processes where pulses on demand are desirable. State of the art

The pulse on demand operation of the laser can be achieved in several different ways, which are already well researched and described in the literature. In the case of lasers that do not require high pulse energies or high average power at the laser output, pulses on demand can be formed simply by using a light modulator at the output of the laser amplifier. In the case of increased power, however, more advanced techniques must be used to avoid optical damage in the modulator. More demanding techniques include so-called coherent combination of laser pulses from several separate laser amplifiers. In this case, by controlling the phase of individual pulses, constructive or destructive interference of pulses at the output of the laser system and thus operation in pulse on demand regime can be achieved. Pulse on demand generation is described in more detail in the patent EP3590160.

Since the coherent pulse combination is typically very challenging, other methods are typically used to generate laser pulses on demand, in which additional idler pulses are used to control the gain of the primary pulses in the laser amplifier. The idler pulses provide a constant population inversion of the amplifier and must be separated from the primary pulses at the output of the laser amplifier, for which different techniques can be used. Existing inventions based on the use of idler pulses are thus mainly distinguished in the methods of generation of idler and primary pulses and methods of separating idler and primary pulses at the output of the laser amplifier.

The US8149886 patent describes how to operate in a pulse on demand regime using a single source to generate both idler and primary pulses and how to separate them based on different polarizations of primary and idler pulses. This solution differs from the present invention because the latter does not use different polarizations to separate primary and idler pulses, and further uses two or more sources to generate primary and idler pulses. The US8717670 patent describes separation of idler and primary pulses based on light frequency conversion or based on the use of a light modulator at the amplifier output. In this patent a gain switched diode is used to generate both idler and primary laser pulses. The idler pulses are generated with a much higher repetition frequency than the primary pulses, making the peak power of the idler pulses much lower than the peak power of the primary pulses. This allows for effective separation of the idler and primary pulses by frequency conversion of light. This solution differs from the present invention because the latter does not use optical modulators at the output of the laser amplifier and because the present invention allows for the formation of primary pulses with picosecond or femtosecond duration.

Instead of a gain switched diode, a laser oscillator may also be used to form primary and idler pulses, as described in the document DE102014017568. In addition to the laser oscillator another crucial component presented in this document is an optical modulator, which allows for the modulation of optical pulses from the oscillator, and thus for the formation of primary and idler pulses. The primary pulses have the largest amplitude, whereas idler pulses have a much smaller amplitude and a much higher repetition frequency. Due to the lower amplitude and the higher repetition frequency, the idler pulses have much lower peak power than the primary pulses. Due to the lower peak power, it is again possible to separate primary and idler pulses based on frequency conversion of light. In this document an in-depth knowledge of the amplifier dynamics is required which allows for achieving different output energies of the primary pulses. Similar approaches are also described in the patent US9570877. In-depth knowledge of the amplifier dynamics is also crucial for the device described in document DE102017210272, where they use either one or two pulsed light sources to generate primary and idler pulses.

Two light sources for the generation of primary and idler pulses are also used in the device described in the patent US7885298. Like before the primary and idler pulses are also separated by frequency conversion of light. The mentioned document describes the control of pulse amplification in one type of amplifier but does not mention the combination of different types of amplifiers with different gains, which is conceptually different from the present invention. In the case of a combination of different types of amplifiers, the mere use of an additional source for the formation of idler pulses is not sufficient for successful control of pulse amplification due to the differences in the gain spectra already mentioned above.

Description of the solution of the technical problem

With regards to the above described known solutions the aim of the invention is thus to ensure additional control of the secondary light source, which generates idler laser pulses, which will enable constant gain in two different types of optical amplifiers and consequently constant energy of the desired pulses after separation from idler pulses with a suitable separator. The technical problem is solved by a hybrid laser, as described in the independent patent claim, while the dependent claims present the specific embodiment of the laser according to the invention. None of the solutions previously known to date foresees a change or adjustment of the spectrum of idler pulses in order to control laser pulse amplification.

The essence of the hybrid laser for generating pulses on demand with constant energy according to the invention is in that it controls the spectrum of idler pulses which are generated by one, two or more laser sources, in order to stabilize amplification of laser pulses in all laser amplifiers. The change in spectrum of the idler pulses can be achieved using different techniques. In a case where a fibre or a solid-state laser is used to generate idler pulses, the appropriate idler spectrum can be achieved using an appropriate adjustable optical band-pass filters in the laser oscillator or with appropriate adjustable reflective elements in the laser oscillator such as diffraction gratings or Bragg gratings. According to an embodiment of the present invention a laser diode is used as a source of idler pulses, where the change in the idler light spectrum can be achieved either by temperature adjustment of the laser diode or with the use of an appropriate fibre Bragg grating, which enables stabilization of the laser diode wavelength. The simplest method is to adjust the temperature of the laser diode, which the user can selectively change by appropriate control of the installed thermoelectric elements in the laser diode itself. In the preferred embodiment of the invention, the temperature of the idler source shall be set in the range between 10 and 50 °C by adjusting the current on the thermoelectric element, depending on the desired wavelength shift, since the spectrum of the idler pulses depends on the temperature of the laser source. More than a change in temperature, an appropriate change in the idler wavelength is essential. The wavelength of the idler pulses must be shifted appropriately (optimally) from the centre of the gain of the solid-state amplifier. The desired adjustment of the wavelength of the idler pulses is possible with advance knowledge or measurement of the gain spectrum of the solid-state amplifier and the spectrum of primary laser pulses, in which case the desired wavelength of the idler pulses can be calculated in advance. The calculation of the desired wavelength of idler pulses is based on Frantz-Nodvik equations [1] describing the amplification of light in laser amplifiers and which can be written as

(1 )

_D ¹ Ini ^ex P [ ^2E ' ^E l °‘ ^sE ^ ^c r[2E _¾ /£,] + !/ ¾ -! where E _out is the energy of the laser pulses at the output of the amplifier, E _s is the saturation energy of the amplifier, E _in is the input energy of the pulses, D _ouί is the population inversion in the amplifier, directly after the individual pulse leaves the amplifier, a is the stimulated emission cross section of the amplifier and L is the length of the amplifier. Small signal gain g _m , in the moment when a laser pulse enters the amplifier can be described as g _in = exp (aLA _in ), where A, _n is the population inversion of the amplifier at the time the pulse enters the amplifier.

Since the durations between laser pulses in pulse on demand operation can be larger than typical time scales of the processes related to pumping of the amplifier, the pumping must be taken into account when modelling the laser operation in pulse on demand regime. Optical pumping influences the inversion population of the amplifier and can be described whit equation where D is a time dependant inversion population of the amplifier after the laser pulse leaves the amplifier. Parameters A and B are introduced as A = 2 a _p cn _p - l/t and B = o _p cn _p N, where a _p is the absorption cross section for the pump laser light and n _p is the number density of the pump photons T is the upper state lifetime of the amplifier media and A/is the number density of the active ions in the amplifier media. Pumping of the amplifier can be neglected during the time in which the laser pulse is in the amplifier.

By transforming equations (1) we can get the condition for the appropriate wavelength of the idler pulses through the numerical solution of the equation where a _/ is the effective stimulated emission cross section of the idler pulses in the solid-state amplifier, L is the length of the amplifier, D is the population inversion of the amplifier, E _out is the energy of the laser pulses at the output of the amplifier, Ej _n is the energy of the laser pulses at the input of the amplifier, Ti is the reduced Planck constant and w ₀ is the central frequency of the laser pulses. The effective cross section for stimulated emission of idler pulses is defined as where S /\J is the gain spectrum of the solid-state amplifier and Oo is the stimulated emission cross section at the wavelength that corresponds to the maximum of the gain spectrum of the solid-state amplifier. With the calculated value for oi, that we get from equation (3), we can further calculate the desired wavelength of the idler pulses through equation (4). The output laser pulse energy E _out can be determined experimentally or we can calculate it by solving Frantz-Nodvik equations (1) for the primary laser pulses.

The preferred embodiment of the hybrid laser comprises at least:

- a source of primary laser pulses with broad spectrum (typically broader than 2 nm), - at least one element such as pulse picker, which allows the repetition frequency of the primary pulses to be changed, except where the source of the primary pulses itself allows pulses to be generated with different repetition frequencies or pulses on demand,

- at least one additional source for generating idler pulses, the wavelength of which can be adjusted in a range of a few nanometres, typically from 0,5 nm to 5 nm,

- at least one element for combining primary and idler pulses,

- at least two amplifiers having different gain spectra, preferably at least one fibre amplifier and at least one solid-state amplifier,

- optional pulse compressor,

- pulse separator to separate primary (desired) pulses from idler pulses, and

- control electronics for switching between primary and idler pulses.

The source of the primary pulses is a laser oscillator, which can exploit either gain switching technique, q-switching technique or mode-locking for generation of laser pulses. The preferred embodiment of the invention uses a mode-locked laser oscillator for primary pulse generation, generating pulses of less than 10 ps in length at a repetition frequency above 1 MHz and an average power of about 1 mW at 1030 nm. In this case, an additional pulse picker shall be used for desired primary pulses selection and which can be used to select individual primary laser pulses, that are then amplified in the following laser amplifiers.

The pulse picker is a device that can be used to switch between individual laser pulses very fast, leaving only the selected pulses in the remaining pulse train. The pulse picker may be either an acousto-optic modulator, an electro-optic modulator, or a semiconductor optical amplifier. The preferred embodiment of the invention uses an acousto-optic modulator as a pulse picker with a response time of less than 100 ns.

The doped optical fibre and associated pumping system are considered a fibre amplifier, thus achieving the amplification of the laser pulses. As a preferred embodiment of the invention, fibre amplifiers are made up of optical fibres doped with ytterbium, but other dopants known in the field may also be used. A laser system may consist of one or more fibre amplifiers, where the exact location of the pulse picker and pulse combiner can be arbitrary as long as at least one fibre amplifier and at least one solid-state amplifier remain after the pulse combiner.

A solid-state amplifier is a doped crystal or glass in which laser pulses are not guided by an amplifier (as in the case of fibre amplifiers), preferably YAG crystal doped with ytterbium ions, pumped with diode lasers at a wavelength of about 940 nm or about 969 nm.

In the case of an ultra-short laser pulse source (e.g. mode-locked laser oscillator) as the source of the primary pulses, a laser pulse stretcher may be additionally used in front of the amplifiers. In this case, a laser pulse compressor may also be used after the last amplifier.

There may be an arbitrary number of sources for generating idler pulses, preferably one and alternatively two or more. In the case of one source of idler pulses, it is important that the amplitude and spectrum of the pulses from this source can be adjusted. However, in the case of two or more sources of idler pulses, it is important that at least one source of idler pulses is spectrally shifted relative to the other and that the amplitude of pulses from this source can be adjusted. The source of the idler pulses can be a DFB laser diode, where the preferred duration of the idler pulses is about 30 ns. A preferred method of wavelength adjustment in the case of one source of the idler pulses is temperature regulation, however an appropriate fibre Bragg grating could also be used. If two or more sources of idler pulses are used, the wavelength adjustment is achieved in the same way as in the case of only one source, for at least one of the sources of idler pulses. In this case one source has the wavelength that corresponds approximately to the peak of the solid-state gain spectrum, whereas the other has a wavelength that falls outside of the solid-state gain spectrum but is still within the gain spectrum of the fibre amplifier, therefore not influencing the amplification of the laser pulses in the fibre amplifier. The operation principle in this case is as follows: - The energy of both idler pulses must be constant and their sum must be the same as the energy of primary pulse, and

- amplitude of the idler pulses must be adjustable in order to achieve appropriate amplification in the solid-state amplifier.

It is important that both sources of idler pulses generate pulses that lie on different locations along the solid-state gain spectrum, therefore exhibiting different amplification in the solid-state amplifier, where one must have a higher gain than primary pulses and the other must have a lower gain than primary pulses in the solid- state amplifier.

For separation of primary and idler pulses a separator based on polarization separation (in the case of different polarizations of idler and primary pulses), on frequency conversion of light (in the case of different time durations of primary and idler pulses) or on combination of both methods. The preferred embodiment of the invention is based on a second harmonic generation (SHG) that doubles the frequency of the laser pulses coming from the compressor.

The hybrid laser according to the invention is further equipped with appropriate elements (e.g. acousto-optic modulator or electro-optic modulator) and associated control electronics, that allow changing the repetition frequency of the primary pulses, and control electronics, that allow control over the source of the idler pulses in such a way, that it can generate idler pulses with arbitrary repetition frequency.

The method of generating laser pulses with constant energy with a hybrid laser according to the invention consists of the following steps: a) generation of primary pulses with primary laser source, selection of desired primary pulses, optional pulse stretching with Bragg grating and optional amplification of the pulses with at least one fibre amplifier, b) generation of idler pulses with at least one additional laser source and adjustment of the wavelength of the idler pulses in the range of wavelengths within the gain spectrum of the solid-state amplifier, c) amplification of laser pulses formed in steps a) and b) with at least one first amplifier, preferably with a fibre amplifier, d) amplification of laser pulses from step c) with at least one second amplifier, which has a different gain spectrum than the first amplifier, preferably with a solid-state amplifier, e) optional pulse compression of laser pulses from step d) with a compressor, and f) separation of primary and idler laser pulses from step e) with an appropriate separator.

The hybrid laser and the method of generating ultra-short laser pulses with the same energy according to the invention allow the formation of primary pulses, which all have the same energy, which further means that a specific laser pulse is generated at the desired moment with the desired characteristics, which are relevant for different laser applications with either resonant or polygonal scanning systems. The temperature control of the source of idler pulses is easy to perform and to control, which makes the resulting system compact, reliable and without unnecessary structural complications.

A different embodiment of a laser is also possible with a larger number of amplifiers of different types, which have different gain spectra, whereby the constant energy of the desired pulses is achieved in the same way as above by adjusting the wavelength of the idler laser pulses. Such a laser would include at least two sources of idler pulses, but the rationality of such a laser is questionable, and is therefore not a preferred embodiment.

The hybrid laser for generating pulses on demand with constant energy according to the invention, its operation and method of generation of said pulses will be described in further detail based on exemplary embodiments and figures, which show:

Figure 1 Amplification of laser pulses in a fibre amplifier with a broad gain spectrum

Figure 2 Amplification of laser pulses in a solid-state amplifier with a narrow gain spectrum

Figure 3 The dependence of idler pulse wavelength on the temperature of the source of idler pulses

Figure 4 Amplification of laser pulses in a solid-state amplifier with wavelength adjustment of idler pulses from a single idler pulse source Figure 5 Amplification of laser pulses in a solid-state amplifier with wavelength adjustment of idler pulses from two idler pulse sources Figure 6 A schematic of the hybrid laser for generating ultra-short laser pulses on demand according to a specific embodiment of the invention

The spectrum of laser pulses and amplification of these pulses in a fibre amplifier with a broad gain spectrum are shown in Figure 1. On the left side there are primary pulses (1) and idler pulses (2) at the input of the fibre amplifier, both with the same energy. On the right side there are amplified primary pulses (6) and idler pulses (7) at the output of the fibre amplifier. Since the fibre amplifier has a broad gain spectrum (3), which completely overlaps with both the spectrum (4) of primary pulses (1) and also the spectrum (5) of the idler pulses (2), the amplification of both primary and idler pulses is the same in this amplifier.

The spectrum of laser pulses and amplification of these pulses in a solid-state amplifier with a narrow gain spectrum are shown in Figure 2. On the left side there are primary pulses (1) and idler pulses (2) at the input of the solid-state amplifier, both with the same energy. On the right side there are amplified primary pulses (6) and idler pulses (7) at the output of the solid-state amplifier, where a variation in the amplitudes and therefore energies of the laser pulses is evident (6). Since the solid- state amplifier has a narrow gain spectrum (3a), it completely overlaps with the whole spectrum (5) of idler pulses (2), however only partially overlaps with the spectrum (4) of the primary pulses (1) (a part of spectrum 4a falls outside of the gain spectrum of the solid-state amplifier), which consequently means, that the amplification of primary and idler pulses is not the same in the solid-state amplifier. Figure 3 shows the dependence of the idler pulse spectrum on the temperature of the idler pulse source, where the maximum of the spectrum at 20 °C is around 1028,7 nm, and moves towards 1032 nm with increasing temperature up to 47 °C.

Figure 4 shows amplification in a solid-state amplifier, which is a part of a hybrid laser and where only one source of the idler pulses is used. The hybrid laser according to the embodiment I therefore consists of:

- a source of primary laser pulses with corresponding pulse picker and a pulse stretcher,

- one additional source for the generation of idler pulses the wavelength of which can be modified by changing the temperature of the source, the source temperature being between 10 and 50 °C, preferably between 40 and 45 °C, and the said source being preferably a 1030 nm DFB laser diode,

- ytterbium doped fibre amplifier,

- Yb:YAG solid-state amplifier,

- pulse compressor,

- pulse separator for separation of primary (desired) and idler pulses, and

- control electronics for switching between primary and idler pulses, for control of the pulse picker, and for the control of the source of the idler pulses.

By adjusting (5a) of the wavelength of the idler pulses (5) as shown in Figure 4, the uniformity of the outgoing primary pulses is achieved.

Figure 5 shows amplification in a solid-state amplifier, which is a part of a hybrid laser and where two sources of the idler pulses are used. The hybrid laser according to the embodiment II therefore consists of:

- a source of primary laser pulses with corresponding pulse picker and a pulse stretcher,

- two additional sources for generation of idler pulses, where one has a wavelength close to the maximum of the gain spectrum of the solid-state amplifier, and the wavelength of the second idler source is sufficiently shifted so that the gain in the solid-state amplifier for the second idler source is significantly lower than for the first idler source and both of these sources are preferably 1030 nm DFB laser diodes,

- ytterbium doped fibre amplifier,

- Yb:YAG solid-state amplifier,

- pulse compressor,

- pulse separator for separation of primary (desired) and idler pulses, and

- control electronics for switching between primary and idler pulses, for control of the pulse picker, and for control of the amplitudes of the laser pulses from both idler sources.

In this variant, the emphasis is not on the exact adjustment of a specific (optimal) wavelength, as in case I. The important thing in this case is that the wavelength of one idler source is sufficiently shifted away from the wavelength of the other idler source, so that it falls somewhere on the edge or even outside of the solid-state amplifier gain spectrum. By adjusting the amplitude of the two idler sources, the same effect is then achieved as in embodiment I.

On the left side of the Figure 5 primary pulses (1) and first idler pulses (2a) and second idler pulses (2b) can be seen, where the sum of the energies of first and second idler pulses equals to the energy of the primary pulses (1). On the right side amplified primary (6) and idler (7a, 7b) pulses are shown. The wavelength of the first idler pulses (5a) is in the proximity of the maximum of the solid-state gain (3a), whereas the wavelength of the second idler pulses (5b) is sufficiently shifted, so that the amplification of the second idler pulses in the solid-state amplifier is significantly lower than the amplification of the first idler pulses. With appropriate difference in the amplitudes (2c) between first (2a) and second (2b) idler pulses, a uniformity of the output primary pulses is achieved (6), where the energy of the primary pulses (6) equals the sum of the energies of the first (7a) and second (7b) idler pulses.

Figure 6 shows embodiment III of the hybrid laser system according to the invention, where the laser system comprises: - front-end module (a), which comprises a fibre oscillator, that generates picosecond laser pulses with 30 MHz pulse repetition frequency, a pulse stretcher, and a fibre amplifier and a pulse picker;

- two additional fibre amplifiers (c) and (d) and an Yb:YAG solid-state amplifier (e);

- pulse compressor (f);

- a source of idler pulses (b) with 32 ns duration, that is situated after the front-end module (a), where the idler pulses can replace the desired primary pulses from module (a);

- control electronics for controlling the pulse picker, the source of the idler pulses and for generating the desired sequences of primary pulses; and

- a SHG separator, that is a 2 mm long LBO (lithium triborate) crystal at 55 °C, to filter out the long idler laser pulses from the short primary laser pulses that are consequently the output pulses (g) from the hybrid laser.

In the scope of the invention as described herein and defined in the claims, other embodiments of the hybrid laser obvious to a person skilled in the art are possible, which does not limit the essence of the invention as described herein and defined in the claims.

References:

1. L. M. Frantz and J. S. Nodvik, "Theory of Pulse Propagation in a Laser Amplifier," J. Appl. Phys. 34(8), 2346-2349 (1963).