Laser system for stabilizing laser emission

Abstract

The present disclosure relates to a laser system comprising: a cavity comprising a gain medium configured to generate laser emission in the form of laser pulses, each having a pulse bandwidth and pulse power; a pump source configured to optically pump the gain medium in the cavity; a photodetector located outside the cavity, the photodetector configured to monitor the laser emission from the cavity and provide a feedback signal to the pump source; and an optical filter located outside the cavity, wherein the optical filter is arranged in optical communication with the photodetector and configured to filter out a predefined range of wavelengths in order to stabilize one or more properties of the laser emission.

Description

[0001] LASER SYSTEM FOR STABILIZING LASER EMISSION

[0002] Technical field

[0003] The present disclosure relates to a laser system. In particular, the present disclosure relates to a laser system comprising an optical filter arranged outside the cavity of the laser system. The disclosure further relates to an ultra-fast fiber laser system for generating short laser pulses in the femtosecond or picosecond range.

[0004] Background

[0005] Some laser systems are required to operate for long periods of time or at varying temperatures. Existing laser systems sometimes suffer from negative effects caused by changes in the temperature of the laser system. In some cases, thermal effects cause the emission from the laser system to change.

[0006] As an example, for a pulsed laser system emitting a train of laser pulses, each laser pulse having a pulse amplitude and a pulse bandwidth, the amplitude and / or bandwidth may change with temperature. The change in pulse bandwidth may cause the pulse duration to change in some modes or operations of the laser system. This is undesired for many applications, such as in cutting applications, where it is important to have pulses with well- defined properties, e.g. in terms of power, bandwidth, energy, and / or pulse duration.

[0007] It is desired to obtain an improved laser system that is able to stabilize one or more properties of the laser emission.

[0008] Summary

[0009] Laser emission from a laser system may depend on the operating temperature of the laser system. Any changes in the properties of the laser emission due to thermal effects are typically undesired. The present inventor has realized that an optical filter, such as an optical bandpass filter, can be utilized to mitigate these thermal effects and thereby also stabilize the laser emission in terms of a variety of parameters, such as pulse power, pulse duration and / or pulse bandwidth.

[0010] In particular, it has been observed that in case the laser system is operated in a constant power mode, wherein the output pulse power output is constant, then temperature variations can cause the amplitude of the pulse(s) to change. A consequence hereof is that the pulse bandwidth changes to maintain a constant pulse power. As mentioned above and herein, these changes in pulse properties are undesired for some applications. The above-mentioned challenges are solved by providing a laser system comprising: a cavity comprising a gain medium configured to generate laser emission in the form of laser pulses, each having a pulse bandwidth and pulse power; a pump source configured to optically pump the gain medium in the cavity; a photodetector located outside the cavity, the photodetector configured to monitor the laser emission from the cavity and provide a feedback signal to the pump source; and an optical filter located outside the cavity, wherein the optical filter is arranged in optical communication with the photodetector and configured to filter out a predefined range of wavelengths in order to stabilize one or more properties of the laser emission. The one or more properties may be selected from the group of: pulse bandwidth, pulse duration, pulse power, and / or combinations thereof.

[0011] In accordance with some embodiments, the laser system comprises a cavity comprising a gain medium configured to generate laser emission in the form of one or more laser pulses, each laser pulse having a pulse duration and a pulse bandwidth; a pump source configured to optically pump the gain medium in the cavity; a photodetector located outside the cavity, the photodetector configured to monitor the laser emission from the cavity and provide a feedback signal to the pump source; and an optical bandpass filter located outside the cavity and arranged between the cavity and the photodetector, wherein the optical bandpass filter is configured to filter a predefined range of wavelengths in order to stabilize the pulse duration and / or the pulse bandwidth.

[0012] In accordance with some embodiments, the laser system comprises a cavity for generating laser light comprising one or more laser pulses, each pulse having a pulse duration and a pulse bandwidth; a pump source for generating pump light to be coupled into the cavity; a photo detector arranged outside of the cavity, said photo detector configured to receive at least a portion of laser light coupled out from the laser cavity; and an optical bandpass filter arranged between the photo detector and the laser cavity, said optical bandpass filter configured to filter a predefined range of wavelengths of light such that the photo detector receives a filtered portion of laser light. The laser system may further comprise a feedback control system for providing a feedback signal to the pump source for adjusting one or more parameters of the pump source, whereby the pulse duration and / or the pulse bandwidth are stabilized.

[0013] In accordance with some embodiments, the laser system comprises a laser cavity; an optical output coupler configured to couple out laser pulses from the cavity; a pump source operable to provide pump light to the laser cavity; an optical monitoring system positioned outside the laser cavity, comprising: optionally an optical splitter configured to direct a portion of the laser light generated in the laser cavity to an optical filter; an optical bandpass filter configured to filter a predefined range of wavelengths; a photodiode arranged to receive the filtered laser light and generate an electrical signal corresponding to a spectral characteristic of the laser light; and a feedback control system for generating a feedback signal based on the electrical signal; the feedback control system comprising a controller operatively connected to the pump source and configured to adjust the pump power in response to the feedback signal, such that the spectral bandwidth and / or the pulse duration of the laser pulses is stabilized against variations in operating temperature.

[0014] In accordance with some embodiments, the laser system comprises a cavity, such as a linear cavity, configured to generate laser emission in the form of one or more laser pulses, each laser pulse having a pulse bandwidth, a pulse duration, and a pulse power; a pump source, such as a laser diode, configured to optically pump a gain medium in the cavity; an optical bandpass filter located outside the cavity, wherein the optical bandpass filter is configured to filter a predefined range of wavelengths; and a photodetector, such as a photodiode, located outside the cavity, wherein the optical bandpass filter is arranged between the cavity and the photodetector, and wherein the photodetector is configured to monitor the laser emission from the cavity, such as the spectrum of the laser emission; wherein the laser system is further configured to provide a feedback signal to the pump source in order to stabilize the pulse duration and / or the pulse bandwidth.

[0015] In accordance with some embodiments, the laser system comprises a cavity, such as a linear cavity, configured to generate intracavity laser output in the form of one or more laser pulses, each laser pulse having a pulse bandwidth, a pulse duration, and a pulse power; a pump source configured to optically pump a gain medium in the cavity; an optical bandpass filter located outside the cavity, wherein the optical bandpass filter is configured to filter a predefined range of wavelengths; a photodetector, such as a photodiode, arranged outside the cavity, wherein the optical bandpass filter is arranged between the cavity and the photodetector, wherein the photodetector is configured to monitor the intracavity laser output based on a filtered optical signal received from the optical bandpass filter; and a feedback control system configured to provide a feedback signal, wherein the feedback control system is further configured to adjust a pump power or pump current of the pump source in response to the feedback signal, whereby the pulse duration and / or the pulse bandwidth is stabilized with varying temperatures.

[0016] In some embodiments, the laser system is an ultra-fast fiber laser system for generating short laser pulses in the femtosecond or picosecond range, wherein the ultra-fast fiber laser system comprises the optical filter disclosed herein. The present disclosure further relates to a laser system comprising: a laser cavity including a gain medium, and an optical output coupler configured to emit laser pulses; an optical monitoring system positioned outside the laser cavity, said monitoring system comprising: an optical splitter configured to direct a portion of the laser light generated in the laser cavity to an optical filter; and a photodiode arranged to receive the filtered laser light and generate an electrical signal corresponding to a spectral characteristic of the laser light; a dispersion control system within the laser cavity, said dispersion control system comprising a tunable dispersion element configured to modify the dispersion of the laser cavity; and a feedback control system for providing a feedback signal based on the electrical signal from the photodiode; and a controller operatively connected to the tunable dispersion element and configured to adjust the dispersion of the laser cavity in response to the feedback signal, such that the pulse duration and / or the spectral bandwidth of the laser pulses is stabilized against variations in operating conditions.

[0017] In some embodiments, the feedback control system further comprises a comparison circuit configured to generate an error signal based on a difference between the electrical signal from the photodiode and a reference signal corresponding to a desired spectral characteristic of the laser pulses.

[0018] The present disclosure further relates to a method of stabilizing a pulsed output from a laser system, such as the laser system disclosed herein, the method comprising the steps of: providing pump light into the cavity for generating laser light comprising one or more laser pulses, each pulse having a pulse duration and a pulse bandwidth; coupling out at least a portion of the laser light generated in the cavity; filtering at least a portion of the laser light coupled out from the cavity, whereby filtered laser light is obtained; monitoring the filtered laser light using a photo detector; generating a feedback signal based on the monitored filtered laser light; and adjusting one or more parameters of the pump light based on the feedback signal, whereby the pulse duration and / or the pulse bandwidth are stabilized. The disclosed method may be carried out, fully or partly, by any of the embodied laser systems disclosed herein.

[0019] The present disclosure further relates to a method for stabilizing the spectral bandwidth of laser pulses generated by a laser system, such as the laser system disclosed herein, the method comprising: generating laser pulses within a laser cavity; directing a portion of the laser light generated in the laser cavity to an optical filter configured to transmit a selected spectral portion of the laser light; directing the filtered selected spectral portion of the laser light from the optical filter to a photodiode; generating, by the photodiode, an electrical signal corresponding to a spectral characteristic of the laser light; generating a feedback signal based on the electrical signal; adjusting a tunable dispersion element within the laser cavity based on the feedback signal to modify the dispersion of the laser cavity; and stabilizing the spectral bandwidth of the laser pulses against variations in operating conditions through the adjustment of the tunable dispersion element. The disclosed method may be carried out, fully or partly, by any of the embodied laser systems disclosed herein.

[0020] Brief description of the drawings

[0021] Fig. 1 shows an embodiment of a laser system according to the present disclosure.

[0022] Fig. 2 shows another embodiment of a laser system according to the present disclosure, said embodiment comprising some additional components to the system shown in figure 1.

[0023] Fig. 3 shows experimental data related to a laser system with an architecture shown in figures

[0024] 1 or 2 excluding the optical filter.

[0025] Fig. 4 shows a similar experiment and setup as described in relation to figure 3, with the difference that the laser system includes the optical filter.

[0026] Fig 5. shows a similar experiment and setup as described in relation to figure 3, with the difference that the pulse repetition frequency was set to 2 MHz.

[0027] Fig. 6 shows a similar experiment and setup as described in relation to figure 5, with the difference that the laser system includes the optical filter.

[0028] Fig. 7 shows a plot of the spectral density of a laser pulse versus the wavelength of the laser pulse. The plot shows a plurality of graphs or traces corresponding to different temperatures.

[0029] Detailed description

[0030] The laser system may comprise a cavity, said cavity comprising a gain medium configured to generate laser emission. A cavity may in the present context be understood as an optical cavity or an optical resonator. The cavity may also be referred to as a laser cavity. The laser emission may be in the form of laser pulses. The laser pulses may be described in terms of amplitude, energy, power, bandwidth, and / or combinations thereof. In particular, each laser pulse in the laser emission may have a certain pulse bandwidth and pulse power. In this disclosure, the terms pulse bandwidth and spectral width of a pulse, are used interchangeably.

[0031] The gain medium may be a medium which can amplify the power of light to compensate for losses in the cavity. Thus, the gain medium may be suitable for adding energy to the amplified light in the laser cavity. The gain medium may comprise one or more active fibers. An active fiber may be understood as an optical fiber which has one or more laser-active dopants in the fiber core. As an example, the active fiber may be a rare-earth-doped fiber, wherein the fiber is doped with a rare-earth element. In some embodiments, the gain medium comprises a rare-earth doped fiber with a dopant selected from the group of ytterbium, erbium, or thulium.

[0032] In some embodiments, the active fiber is a single-mode fiber or a few-mode fiber. In other embodiments, the active fiber is a large mode area fiber or a polarization-maintaining fiber. The cavity may comprise a plurality of optical fibers, of which only one or more of said fibers are active fibers. In preferred embodiments, the cavity comprises a plurality of polarization-maintaining (PM) optical fibers.

[0033] The cavity may be defined by two ends, wherein a first reflector is located at a first end of the cavity, and a second reflector is located at a second end of the cavity. As an example, a fiber Bragg grating (FBG) may be located at a first end of the cavity, and a semiconductor saturable-absorber mirror (SESAM) may be located at a second end of the cavity, opposite to the first end. The SESAM may constitute a mode-locking element, such as a passive modelocking element. The SESAM may be configured to act as a reflector which, together with the FBG, forms the laser cavity. The cavity may alternatively be defined by other optical components, such as dielectric mirrors, dichroic mirrors, reflectors, distributed Bragg reflectors (DBRs), FBGs, saturable absorber mirrors, SESAMs, and / or combinations thereof. One end of the cavity may be provided with a component configured as an output coupler. The output coupler may have a significant transmission for the laser emission, whereas the component at the opposite end may be highly reflecting. The cavity may be a linear cavity and / or a fiber laser cavity. A linear cavity may also be referred to as a standing-wave cavity or a standingwave resonator. Alternatively, the cavity may be a ring cavity, where the light can do round trips in two different directions in the cavity. The cavity may further comprise one or more extension optical fibers for extending the cavity length.

[0034] The laser system may comprise a pump source. The pump source may be selected from the group of: laser diodes, fiber-coupled diode lasers, or fiber lasers. The pump source may be configured to emit and deliver an optical signal to the cavity. In particular, the pump source may be configured to optically pump the gain medium in the cavity. This may be achieved by providing a wavelength that matches one or more absorption bands of the dopants in the gain medium.

[0035] In some embodiments, the pump source is configured to emit a central wavelength in the range of 900 nm to 1000 nm, such as in the range 910 nm to 980 nm. As an example, the pump source may be configured to emit a wavelength suitable for optically pumping an Ytterbium-based laser. The two primary pump wavelengths for ytterbium lasers are around 915 nm and 976 nm, since these wavelengths correspond to the energy levels of ytterbium ions in the laser gain medium. In some embodiments, the pump source is configured to emit a central wavelength in the range of 750 nm to 800 nm, or in the range of 1700 nm to 1800 nm. As an example, the pump source may be configured to emit a wavelength suitable for optically pumping a Thulium- based laser. The two primary pump wavelengths for thulium lasers are around 785 nm and 1750 nm since these wavelengths correspond to the energy levels of thulium ions in the laser gain medium.

[0036] The laser system may further comprise one or more multiplexers, such as wavelength division multiplexers (WDM), couplers, splitters, and / or combinations thereof. As an example, the system may comprise a WDM arranged between the pump source and the cavity. The coupler, or WDM, may be configured to split one or more first optical channels to two or more second optical channels. Alternatively, it may be configured to combine two or more first optical channels to one or more second optical channels. In some embodiments, the WDM is connected to the pump source via an optical fiber, and the WDM is further configured to split the optical signal from the pump source in two different optical channels, such as in two different optical fibers. The WDM may be further configured to deliver the optical signal from the pump source to the laser cavity via one of said optical channels. The other optical channel may comprise an optical fiber, an optical isolator, and a fiber cable termination, such as a fiber optic pigtail.

[0037] The laser system may comprise a photodetector, such as a photodiode, located external to the cavity. In some embodiments, the photodetector is selected from the group of: semiconductor diodes, photodiodes, high-speed photodiodes, photodiode arrays, and photomultiplier tubes. The photodetector may be configured to monitor the laser emission from the cavity and provide a feedback signal, such as a real-time feedback signal, to the pump source. Advantageously, the photodetector has a linear response in the spectral range of interest. When the laser emission is monitored, typically only a portion, such as a small fraction, of the laser emission is coupled out of the cavity. Thus, the laser system may comprise one or more splitters or couplers for coupling out a portion of the generated laser emission, such that the photodetector receives said portion of light coupled out from the cavity. The portion of laser emission received by the photodetector may be filtered by an optical filter, as explained further herein. The portion may constitute a small fraction of the intracavity laser output. The laser system may comprise a current-to-voltage converter, such as a transimpedance amplifier (TIA), for converting a photodiode current to a voltage, which may form the basis for a feedback signal. The system may further be configured to generate a feedback signal based on the monitored portion of the intracavity laser output. The laser system may be configured to adjust the current and / or power of the pump source based on the feedback signal, whereby the pulse bandwidth, pulse duration, and / or pulse power of the laser emission is adjusted and / or stabilized. Thus, by adjusting e.g. the current to the pump source in response to the feedback signal, the pulse bandwidth, and / or the pulse duration, of the laser pulses can be stabilized, such as stabilized with varying temperatures, at least to a greater degree than without the optical filter. The laser system may further comprise a proportional-integral-derivative (PID) controller arranged in a feedback loop with the photodetector and the pump source. As an example, the pump power of the pump source may be adjusted via a current driver controlled by the output from the PID controller.

[0038] As an alternative to adjusting the pump power of the pump source, the laser system may be configured to adjust the dispersion within the laser cavity to achieve the purpose of stabilizing the properties of the intracavity laser light, e.g., stabilizing the spectral bandwidth of the laser pulses against variations in operating conditions. Specifically, the laser system may include a tunable dispersion element in the laser cavity, such as a grating pair, prism pair, or a chirped fiber Bragg grating. The system may further comprise an actuator (e.g., a piezoelectric transducer or thermal control element) configured to fine-tune the dispersion dynamically. A similar feedback control may be employed as described herein. In this case, the feedback signal may be used to adjust the tunable dispersion element to counteract bandwidth variations caused by temperature changes. This embodiment has the advantage that it avoids potential thermal instabilities or nonlinear effects associated with pump power modulation. Furthermore, it provides precise control over the spectral properties of the laser pulses while maintaining a constant gain medium excitation.

[0039] The laser system may comprise an optical filter, such as an optical bandpass filter, located externally to the cavity. The optical filter may be arranged in combination with the photodetector, such as upstream of the photodetector, and configured to filter out a predefined range of wavelengths. Specifically, the optical filter may be placed in optical communication with the photodetector. As an example, the output from the optical filter may be connected to the input of the photodetector. The two components may be connected via an optical fiber. Similarly, the optical filter may be connected to a coupler in the cavity via an optical fiber. Thus, the optical filter may be a fiber-coupled component. The present inventor has realized that an optical filter in such an arrangement is particularly useful for stabilizing one or more properties related to the laser emission, such as the pulse bandwidth, the pulse duration, the pulse power, and / or combinations thereof. The optical filter may be selected from the group of: bandpass filters, long-pass filters, or short-pass filters. In preferred embodiments, the optical filter is an optical bandpass filter (BPF). The optical filter may be arranged between the cavity and the photodetector to filter a predefined range of wavelengths of the optical signal provided to the photodetector. In this way, the monitoring of the laser emission is more sensitive to changes in the bandwidth of the laser pulses. The optical filter thus ensures a more stable monitoring of the laser emission. Without the optical filter, the monitored signal becomes more noisy and the feedback signal is not able to efficiently correct the changes due to thermal effects. Thus, the optical filter in combination with the photodetector ensures a more efficient monitoring of the laser emission and feedback, and it further ensures stabilized laser pulses, e.g., in terms of pulse duration and / or pulse bandwidth. This is evident from the experimental data shown in figures 3 to 6.

[0040] In some embodiments, the optical filter has a transmission bandwidth in the range of 1 nm to 10 nm, such as 2 nm to 6 nm, such as a transmission bandwidth of about 4 nm. Advantageously, the optical filter has a minimum transmission bandwidth, or pass bandwidth, of 2 nm. Generally, the optical filter may be configured to allow light of wavelengths within said bandwidth to pass through the filter. Furthermore, the optical filter may have a center wavelength in the range of 1030 nm to 1050 nm, such as about 1038 nm ± 1 nm. Thus, the optical filter may be configured to allow light inside the transmission bandwidth to pass through the filter, such as light having a similar wavelength as the center wavelength of the filter. The transmission bandwidth of the filter may be centered around said center wavelength. Advantageously, the optical filter is arranged to filter wavelengths at a red edge of the spectrum from the intracavity laser output. A red edge may be understood as wavelengths in the upper range of the spectrum from the laser. The spectrum of the laser emission may include wavelengths from about 1025 nm to about 1040 nm.

[0041] In some embodiments, the laser system comprises a feedback control system. The feedback control system may comprise one or more components for providing the feedback signal to the pump source. In some embodiments, the feedback control system comprises the photodetector and the proportional-integral-derivative (PID) controller described herein. The feedback control system may further comprise a current-to-voltage converter, such as a transimpedance amplifier (TIA), for converting a photodiode current to a voltage, which may form the basis for the feedback signal. In some cases, the feedback signal constitutes an error signal formed by e.g. subtracting the signal measured by the photodetector from a reference signal. The feedback signal, or error signal, may reflect deviations in e.g. the spectral bandwidth of the pulse(s) due to thermal variations. The reference signal may correspond to a desired spectral bandwidth under stable operating conditions (e.g. measured during system calibration). The feedback control system may further comprise a current driver for controlling the pump power of the pump source, e.g. based on an output from the PID controller.

[0042] The laser system may be an ultra-fast fiber laser system for generating short laser pulses in the femtosecond or picosecond range. As an example, the laser system may be a medicalgrade femtosecond fiber laser. The laser system may be suitable for applications such as optogenetics, ophthalmology, medical device manufacturing, and material processing, such as cutting applications.

[0043] The laser system may be configured to generate and output a train of laser pulses. The laser pulses may be generated by mode-locking or Q-switching. As an example, the laser emission may be in the form of ultrashort pulses, such as mode-locked ultrashort pulses. The laser cavity may comprise a mode-locking element, such as a saturable-absorber e.g. a semiconductor saturable-absorber mirror (SESAM). The laser system may be a mode-locked fiber laser system. In some embodiments, the laser pulses have a pulse duration in the range of femtoseconds or picoseconds. As an example, the laser pulses may be ultrashort pulses having a pulse duration from about 10 fs to about 1 ps, such as from about 50 fs to about 500 fs. The average power output from the laser system may be more than 1 W, such as more than 5 W, such as more than 10 W. The pulse energy of each laser pulse may be more than 1 pJ, such as more than 10 pJ, such as more than 20 pJ.

[0044] The laser system may be configured to output the laser pulses at a pulse repetition rate from about 0.5 MHz to about 100 MHz, such as from about 0.5 MHz to about 20 MHz, such as from about 1 MHz to about 10 MHz. Advantageously, the bandwidth of the photodetector exceeds the pulse repetition rate, since this provides an even more accurate monitoring of the laser emission. The laser pulses may have a spectral width in the range from about 8 nm to about 24 nm, such as from about 10 nm to about 16 nm. The laser pulses may have a central wavelength from about 1025 nm to about 1050 nm, such as about 1030 nm. In some embodiments, the laser emission has a central wavelength in the range of 1030 nm to 1120 nm. In other embodiments, the laser emission has a central wavelength in the range of 1530 nm to 1550 nm. In other embodiments, the laser emission has a central wavelength in the range of 2010 nm to 2080 nm.

[0045] Detailed description of the drawings

[0046] The presently disclosed laser system is further described in the following exemplary and non-limiting embodiments.

[0047] Fig. 1 shows an embodiment of a laser system according to the present disclosure. In this embodiment, the laser system comprises a cavity 1 comprising a gain medium 2 configured to generate laser emission in the form of laser pulses, a pump source 3 configured to optically pump the gain medium 2 in the cavity 1 , a photodetector 4 located external to the cavity 1 , the photodetector 4 configured to monitor the laser emission from the cavity 1 and provide a feedback signal to the pump source 3; and an optical filter 5 located external to the cavity 1 , wherein the optical filter 5 is arranged in combination with the photodetector 4 and configured to filter out a predefined range of wavelengths in order to stabilize the pulse bandwidth, the pulse duration and / or the pulse power of the laser emission. In this embodiment, the optical filter is placed in optical communication with the photodetector via an optical fiber. Furthermore, in this embodiment, the cavity is a linear cavity, wherein the cavity comprises a fiber Bragg grating (FBG) 6 located at a first end of the cavity, and a semiconductor saturable-absorber mirror (SESAM) 7 located at a second end of the cavity, opposite to the first end. Thus, the laser cavity may constitute a linear laser cavity comprising a mode-locking element in the form of a semiconductor saturable-absorber mirror (SESAM) and a fiber Bragg grating (FBG). As an alternative to using an FBG and a SESAM, other types of reflectors, mirrors, or saturable absorber mirrors may be utilized. One alternative example could be the use of two FBGs to define the laser cavity. The embodiment may further comprise a coupler 11 , such as a fiber optic coupler, for coupling out at least some light from the cavity 1. The coupler 11 may be optically connected to the photodetector 4 via the optical filter 5, such that light from the cavity 1 can be monitored by the photodetector 4. The coupler 11 may also be referred to as an optical splitter.

[0048] Fig. 2 shows another embodiment of a laser system according to the present disclosure. In this embodiment, the laser system comprises some additional components in addition to the components mentioned in relation to figure 1. In this embodiment, the laser system further comprises an extension optical fiber 12 in the cavity 1 for defining the cavity length, an optical isolator 9 for allowing light to propagate through it in one direction but not in the opposite direction, and a fiber cable termination 13, such as a fiber optic pigtail. In this embodiment, the laser system further comprises a wavelength division multiplexer (WDM) 8 arranged between the pump source 3 and the cavity 1 , wherein the WDM 8 is configured to split the optical signal from the pump source 3 in two separate optical fibers, wherein a first optical fiber is connected to the fiber cable termination 13 via the optical isolator 9 and a second optical fiber is connected to the cavity 1. In this embodiment, the laser system further comprises a second photodetector 14, such as a photodiode, and a coupler 10, such as a fiber optic coupler, for coupling or splitting a first optical signal from an optical fiber into two separate optical fibers. Thus, in this embodiment, the laser system comprises a first photodetector 4 for monitoring at least a part of the laser emission from the cavity 1 via the optical filter 5, and a second photodetector 14 for monitoring the laser emission without filtration of the light. The coupler 10 may be a fiber optic coupler configured to split the light coupled out from the cavity in two equal signals, such that the coupler provides a 50:50 ratio.

[0049] Fig. 3 shows experimental data related to a laser system with an architecture shown in figure 1 or 2 but without the optical filter 5. The laser system may be a femtosecond fiber laser configured to deliver ultra-short femtosecond pulses. For this experiment, the laser cavity was placed in a thermal chamber for controlling the air temperature surrounding the laser cavity. The figure shows two graphs: a first graph (blue) having a sawtooth profile, which is the air temperature inside the thermal chamber (values on right y-axis), which was ramped up and down over time during the experiment, and a second graph (red) showing the pulse duration measured in terms of full width at half maximum (FWHM) of a given laser pulse output by the laser system as monitored by a photodetector 14 outside the cavity, with no optical filter placed in front of the photodetector. For this experiment, the pulse repetition frequency was 5 MHz. The x-axis shows the time in terms of time stamps, which are similar to both graphs. It is observed that the FWHM changes with temperature. In particular, there are some larger peaks in pulse duration when the temperature is near a minimum. These peaks are undesirable, and thus it is an object of the presently disclosed laser system to minimize or ideally entirely remove said peaks. Ideally, the pulse duration in terms of FWHM should be substantially constant with varying temperature, or at least vary minimally.

[0050] Fig. 4 shows experimental data related to a laser system with an architecture shown in figure 1 or 2 including the optical filter 5. The laser system may be a femtosecond fiber laser configured to deliver ultra-short femtosecond pulses. The experiment performed is identical to the experiment described in relation to figure 3, apart from the inclusion of the optical filter in front of the photodetector 4. Thus, the laser system shares the same components as the one described in relation to figure 3, with the addition of the optical filter 5. Also, the temperature was varied in the same range and the pulse repetition frequency of the laser emission was the same in both experiments. It is observed that the optical filter has the advantage of minimizing the aforementioned peaks in the pulse duration (FWHM). Accordingly, the optical filter has the technical effect of stabilizing the pulse duration and / or the pulse bandwidth with varying temperatures. It is seen that the pulse duration varies in a narrower range without the larger peaks observed without the filter, cf. figure 3.

[0051] Fig 5. shows a similar experiment and setup as described in relation to figure 3, with the difference that the pulse repetition frequency was set to 2 MHz. For this experiment, the laser system did not include the optical filter in front of the photodetector. Once again, a number of undesired peaks are observed in the laser pulse duration (FWHM). In particular, the peaks appear to correlate roughly with the extrema of the temperature. It is an object of the presently disclosed laser system to remove or minimize said peaks, corresponding to stabilizing the pulse duration (FWHM) of the emitted laser pulses.

[0052] Fig. 6 shows a similar experiment and setup as described in relation to figure 5, with the difference that the laser system includes the optical filter 5 placed between the cavity and the photodetector 4. For this experiment, the pulse repetition frequency was set to 2 MHz similar to the experiment shown in figure 5. The inclusion of the optical filter is seen to minimize the peaks present in figure 5. Accordingly, the optical filter has the technical effect of stabilizing the pulse duration and / or the pulse bandwidth with varying temperatures.

[0053] Fig. 7 shows a plot of the spectral density of a laser pulse versus the wavelength of the laser pulse. The plot shows a plurality of graphs or traces, said graphs corresponding to the observed spectral density and wavelength at different temperatures as controlled by the thermal chamber. It is observed that both the amplitude of the pulse and the spectral width (bandwidth) of the pulse changes with temperature. It is also observed that one side of the pulse changes more with temperature than the other side of the pulse. In particular, the upper end of the pulse, i.e. the higher wavelength end, changes to a greater extent than the lower end of the pulse at shorter wavelengths. Thus, it is an advantage to select an optical filter having a center wavelength around the upper range of the spectrum of the pulse. Such an optical filter has the technical effect of further stabilizing the pulse bandwidth, the pulse duration, and / or the pulse power of the laser emission. In particular, a stabilization with varying temperatures is achieved. For this given example shown here, this would correspond to an optical filter having a center wavelength in the range of 1035 nm to 1040 nm, such as about 1038 nm ± 1 nm, or 1038 nm ± 0.5 nm.

[0054] Further details of the disclosure

[0055] 1 . A laser system comprising:

[0056] - a cavity comprising a gain medium configured to generate laser emission in the form of laser pulses, each having a pulse bandwidth and pulse power;

[0057] - a pump source configured to optically pump the gain medium in the cavity;

[0058] - a photodetector located outside the cavity, the photodetector configured to monitor the laser emission from the cavity and provide a feedback signal to the pump source; and

[0059] - an optical filter located outside the cavity, wherein the optical filter is arranged in combination with the photodetector and configured to filter out a predefined range of wavelengths in order to stabilize one or more properties of the laser emission.

[0060] 2. The laser system according to item 1 , wherein the one or more properties are selected from the group of: pulse bandwidth, pulse duration, pulse power, and / or combinations thereof.

[0061] 3. The laser system according to any of the preceding items, wherein the optical filter is selected from the group of: bandpass filters, long-pass filters, or short-pass filters. 4. The laser system according to any of the preceding items, wherein the optical filter is arranged between the cavity and the photodetector.

[0062] 5. The laser system according to any of the preceding items, wherein the laser system is configured to change the current and / or the power of the pump source based on the feedback signal in order to stabilize the power of the laser emission.

[0063] 6. The laser system according to any of the preceding items, wherein the laser system is configured to change the current and / or the power of the pump source based on the feedback signal in order to stabilize the pulse bandwidth and / or the pulse duration with varying temperatures.

[0064] 7. The laser system according to any of the preceding items, wherein the laser system comprises a proportional-integral-derivative (PID) controller arranged in a feedback loop with the photodetector and the pump source.

[0065] 8. The laser system according to any of the preceding items, wherein the laser system further comprises a current driver configured to adjust the pump power and / or the pump current to the pump source.

[0066] 9. The laser system according to any of the items 7-8, wherein the current driver is configured to adjust the pump power in response to an output from the PID controller.

[0067] 10. The laser system according to any of the preceding items, wherein the photodetector is selected from the group of: semiconductor diodes, photodiodes, high-speed photodiodes, photodiode arrays, and photomultiplier tubes.

[0068] 11 . The laser system according to any of the preceding items, wherein the photodetector has a linear response in a predefined spectral range, such as a range substantially corresponding to the spectrum of the laser emission.

[0069] 12. The laser system according to any of the preceding items, wherein the pump source is selected from the group of: laser diodes, fiber-coupled diode lasers, or fiber lasers. 13. The laser system according to any of the preceding items, wherein the pump source is configured to emit a central wavelength in the range of 900 nm to 1000 nm, such as in the range 950 nm to 1000 nm, such as about 970 nm to 980 nm.

[0070] 14. The laser system according to any of the preceding items, wherein the optical filter has a transmission bandwidth in the range of 1 nm to 10 nm, such as 2 nm to 6 nm, such as about 4 nm.

[0071] 15. The laser system according to item 14, wherein the optical filter is configured to allow light of wavelengths within the transmission bandwidth to be transmitted through the optical filter.

[0072] 16. The laser system according to any of the preceding items, wherein the optical filter has a center wavelength in the range of 1030 nm to 1050 nm, such as about 1038 nm.

[0073] 17. The laser system according to item 16, wherein the optical filter is configured to allow light having a similar wavelength as the center wavelength of the filter to pass through the filter.

[0074] 18. The laser system according to any of the preceding items, wherein the cavity comprises one or more polarization-maintaining (PM) fibers.

[0075] 19. The laser system according to any of the preceding items, wherein the cavity is a linear cavity and / or a fiber laser cavity.

[0076] 20. The laser system according to any of the preceding items, wherein the cavity comprises a fiber Bragg grating (FBG) located at a first end of the cavity.

[0077] 21. The laser system according to any of the preceding items, wherein the cavity further comprises a semiconductor saturable-absorber mirror (SESAM) located at a second end of the cavity, opposite to the first end.

[0078] 22. The laser system according to any of the preceding items, wherein the laser system further comprises an optical splitter, such as a fiber tap coupler, for directing a fraction of the intracavity laser emission to the optical filter. 23. The laser system according to item 22, wherein the optical splitter has a splitter ratio of at least 90:1 , such as at least 99:1 , such that a minimal disruption of intracavity dynamics is ensured.

[0079] 24. The laser system according to any of the preceding items, wherein the laser system further comprises a wavelength division multiplexer (WDM) arranged between the pump source and the cavity.

[0080] 25. The laser system according to any of the preceding items, wherein the laser system is an ultra-fast fiber laser system for generating short laser pulses in the femtosecond or picosecond range.

[0081] 26. The laser system according to item 25, wherein the laser pulses are generated by mode-locking or Q-switching.

[0082] 27. The laser system according to any of the preceding items, wherein the laser emission is in the form of ultrashort pulses, such as mode-locked ultrashort pulses.

[0083] 28. The laser system according to any of the preceding items, wherein the laser system is a mode-locked fiber laser system.

[0084] 29. The laser system according to any of the preceding items, wherein the ultrashort pulses have a pulse duration in the range of femtoseconds.

[0085] 30. The laser system according to any of the preceding items, wherein the ultrashort pulses have a pulse duration from about 10 fs to about 1 ps, such as from about 50 fs to about 500 fs.

[0086] 31 . The laser system according to any of the preceding items, wherein the laser system is configured to output the ultrashort pulses at a pulse repetition rate from about 1 MHz to about 20 MHz.

[0087] 32. The laser system according to any of the preceding items, wherein the ultrashort pulses have a spectral width in the range from about 12 nm to about 16 nm.

[0088] 33. The laser system according to any of the preceding items, wherein the ultrashort pulses have a central wavelength from about 1025 nm to about 1040 nm. 34. The laser system according to any of the preceding items, wherein the gain medium comprises a rare-earth doped fiber.

[0089] 35. The laser system according to any of the preceding items, wherein the gain medium comprises a rare-earth element selected from the group of: Erbium, Thulium, or Ytterbium.

[0090] 36. The laser system according to any of the preceding items, wherein the laser emission has a central wavelength in the range of 1030 nm to 1120 nm.

[0091] 37. The laser system according to any of the preceding items, wherein the laser emission has a central wavelength in the range of 1530 nm to 1550 nm.

[0092] 38. The laser system according to any of the preceding items, wherein the laser emission has a central wavelength in the range of 2010 nm to 2080 nm.

[0093] 39. The laser system according to any of the preceding items, wherein the pump source is configured to emit a central wavelength in the range of 750 nm to 800 nm, or in the range of 1700 nm to 1800 nm.

[0094] 40. The laser system according to any of the preceding items, wherein the laser system is configured for material processing applications, such as cutting.

[0095] 41. The laser system according to any of the preceding items, wherein the laser emission is monitored in real-time and / or wherein the feedback signal is provided in real-time.

[0096] 42. The laser system according to any of the preceding items, wherein the laser system comprises a feedback control system for providing a feedback signal.

[0097] 43. The laser system according to any of the preceding items, wherein the photodetector is configured to generate an electrical signal corresponding to a spectral characteristic of the laser light.

[0098] 44. The laser system according to any of the preceding items, wherein the feedback control system comprises a comparison circuit configured to generate an error signal based on a difference between the electrical signal and a reference signal corresponding to a desired spectral characteristic of the laser pulses.

[0099] 45. The laser system according to any of the preceding items, wherein the feedback control system comprises a controller, such as a proportional-integral-derivative (PID) controller.

[0100] 46. The laser system according to item 45, wherein the controller is operatively connected to the pump source and configured to adjust the pump power of the pump source to stabilize the spectral bandwidth and / or pulse duration of the laser pulses against variations in operating conditions.

[0101] 47. The laser system according to any of the preceding items, the laser system further comprising a dispersion control system arranged within the cavity

[0102] 48. The laser system according to item 47, wherein the dispersion control system comprises a tunable dispersion element configured to modify the dispersion of the cavity.

[0103] 49. The laser system according to any of the items 45-48, wherein the controller is operatively connected to the tunable dispersion element and configured to adjust the dispersion of the laser cavity in response to the error signal.

[0104] 50. The laser system according to any of the preceding items, wherein the feedback control system is configured to stabilize the spectral bandwidth of the laser pulses against variations in operating conditions.

[0105] Although some embodiments have been described and shown in detail, the disclosure is not restricted to such details, but may also be embodied in other ways within the scope of the subject matter defined in the following claims. In particular, it is to be understood that other embodiments may be utilized, and structural and functional modifications may be made without departing from the scope of the present disclosure. Furthermore, the skilled person would find it apparent that unless an embodiment is specifically presented only as an alternative, different disclosed embodiments may be combined to achieve a specific implementation and such specific implementation is within the scope of the disclosure. Reference numerals

[0106] 1. Laser cavity

[0107] 2. Gain medium

[0108] 3. Pump source 4. Photodetector

[0109] 5. Optical filter

[0110] 6. Fiber Bragg grating

[0111] 7. Semiconductor saturable-absorber mirror

[0112] 8. Wavelength division multiplexer 9. Optical isolator

[0113] 10. Coupler / Splitter

[0114] 11. Coupler / Splitter

[0115] 12. Extension optical fiber

[0116] 13. Fiber cable termination 14. Photodetector

Claims

Claims1. A laser system comprising:- a cavity comprising a gain medium configured to generate laser emission in the form of one or more laser pulses, each laser pulse having a pulse bandwidth, a pulse duration, and a pulse power;- a pump source configured to optically pump the gain medium in the cavity;- a photodetector located outside the cavity, the photodetector configured to monitor the laser emission from the cavity and provide a feedback signal to the pump source; and- an optical bandpass filter located outside the cavity and arranged between the cavity and the photodetector, wherein the optical bandpass filter is configured to filter a predefined range of wavelengths in order to stabilize the pulse duration and / or the pulse bandwidth.

2. The laser system according to claim 1 , wherein the laser system is configured to change the current and / or the power of the pump source based on the feedback signal.

3. The laser system according to any of the preceding claims, wherein the laser system is capable of stabilizing the pulse power based on the feedback signal.

4. The laser system according to any of the preceding claims, wherein the laser system comprises a proportional-integral-derivative (PID) controller arranged in a feedback loop with the photodetector and the pump source.

5. The laser system according to any of the preceding claims, wherein the laser system further comprises a current driver configured to adjust the pump power and / or the pump current to the pump source.

6. The laser system according to any of the claims 4-5, wherein the current driver is configured to adjust the pump power in response to an output from the PID controller.

7. The laser system according to any of the preceding claims, wherein the optical bandpass filter has a transmission bandwidth in the range of 1 nm to 10 nm.

8. The laser system according to any of the preceding claims, wherein the optical bandpass filter has a transmission bandwidth in the range of 2 nm to 5 nm.

9. The laser system according to any of the preceding claims, wherein the optical bandpass filter has a center wavelength in the range of 1030 nm to 1050 nm.

10. The laser system according to any of the preceding claims, wherein the cavity comprises one or more polarization-maintaining (PM) fibers.

11. The laser system according to any of the preceding claims, wherein the cavity is a linear cavity.

12. The laser system according to any of the preceding claims, wherein the cavity comprises a fiber Bragg grating (FBG) located at a first end of the cavity13. The laser system according to any of the preceding claims, wherein the cavity further comprises a semiconductor saturable-absorber mirror (SESAM) located at a second end of the cavity, opposite to the first end.

14. The laser system according to any of the preceding claims, wherein the cavity is delimited by two fiber Bragg gratings (FBGs).

15. The laser system according to any of the preceding claims, wherein the laser system is configured for generating laser pulses in the femtosecond range.

16. The laser system according to any of the preceding claims, wherein the laser system is configured for generating laser pulses in the picosecond range.

17. The laser system according to any of the preceding claims, wherein the laser pulses are generated by mode-locking or Q-switching.

18. The laser system according to any of the preceding claims, wherein the laser pulses have a pulse duration from about 10 fs to about 500 fs.

19. The laser system according to any of the preceding claims, wherein the laser system is configured to output the laser pulses at a pulse repetition rate in the range from about 1 MHz to about 20 MHz.

20. The laser system according to any of the preceding claims, wherein the laser pulses have a spectral width in the range from about 8 nm to about 24 nm.

21. The laser system according to any of the preceding claims, wherein the laser pulses have a spectral width in the range from about 12 nm to about 16 nm.

22. The laser system according to any of the preceding claims, wherein the optical bandpass filter is a fiber-coupled component.

23. A method of stabilizing a pulsed output from the laser system according to any of the claims 1-22, the method comprising the steps of:- providing pump light into the cavity for generating laser light comprising one or more laser pulses, each pulse having a pulse duration and a pulse bandwidth;- coupling out at least a portion of the laser light generated in the cavity;- filtering at least a portion of the laser light coupled out from the cavity, whereby filtered laser light is obtained;- monitoring the filtered laser light using a photo detector;- generating a feedback signal based on the monitored filtered laser light; and- adjusting one or more parameters of the pump light based on the feedback signal, whereby the pulse duration and / or the pulse bandwidth are stabilized.

24. The method according to claim 23, wherein the pump light is provided by a pump source, such as a pump laser, arranged outside the cavity.

25. The method according to any of the claims 23-24, wherein the step of filtering at least a portion of the laser light is done using an optical bandpass filter configured to filter a predefined range of wavelengths of light.

26. The method according to any of the claims 23-25, wherein the photo detector is a photo diode.

27. The method according to any of the claims 23-26, wherein the feedback signal is provided by a feedback loop between the photo detector and the pump source.

28. The method according to any of the claims 23-27, wherein the feedback loop includes a proportional-integral-derivative (PID) controller.

29. The method according to any of the claims 23-28, wherein the one or more parameters of the pump light are selected from the group of: pump power, pump current, pump wavelength, and / or combinations thereof.

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