Patent Description:
<NPL>, discloses a modelocked picosecond laser system based on the combination of an erbium-doped fiber as the active medium, and a saturated fiber Bragg grating (FBG) as a partial mirror of the cavity. The other side of the cavity consists of a semiconductor saturable absorber mirror acting also as the mode-locking device. The central wavelength of the FBG is selected by the application of axial strain giving rise to a tunable soliton over <NUM> on the C band with a variable spacing of <NUM>. The laser delivers <NUM> ps long pulses with a <NUM> bandwidth, an <NUM> repetition rate, a pulse energy of <NUM> pJ, and a peak power of <NUM> W.

Other examples of wavelength tuneable and/or wavelength stabilised modelocked picosecond laser systems including a strain or temperature controlled fiber Bragg grating are known from.

The present specification describes a wavelength-stabilised narrow-linewidth mode-locked picosecond laser system. The system comprises a laser cavity which includes an amplifier, a mode-locking element, and a fiber Bragg grating which acts as a narrowband reflector. The system includes a mount to which the fiber Bragg grating is mounted under tension, a tension control system to adjust the tension, and a measurement device to provide a measurement output for use in wavelength-stabilising the laser system. A microcontroller or other controller wavelength-stabilises the laser by controlling the tension control system responsive to the measurement output.

The techniques and systems described in the specification sacrifice the possibility of femtosecond time durations whilst employing mode-locking to produce relatively long ( ≥ <NUM> ps) pulses having a narrow linewidth, which can be wavelength-stabilised in a feedback loop by controlling the tension of the fiber Bragg grating reflector of the laser cavity. It has been found that this technique provides a very stable narrow-linewidth source in which the wavelength of the laser can be stabilised to within e.g. <NUM> or <NUM> picometers of a desired operating wavelength. Moreover, since the described techniques employ mode-locking and thus provide a pulsed output, they can be used to produce higher peak powers compared to other narrow-linewidth systems which do not employ the described techniques. In this way, a balance is struck between multiple inter-related factors to provide a laser which is particularly suited for applications in which high peak power is required in a stable, narrow, linewidth. Such applications include frequency conversion in nonlinear crystals and in particular frequency conversion to the deep ultraviolet in which variation of the laser wavelength by as little as <NUM> picometers can result in loss of efficiency.

The measurement output is related to (e.g. it may be a measure of) a current operating wavelength of the fiber Bragg grating (FBG). In some implementations, the measurement output may be a measure of a current operating wavelength of the laser system, or may be related to the current operating wavelength of the laser system.

The FBG may comprise a uniform grating period. That is, the FBG may be unchirped.

The FBG mount may be formed of a positive thermal expansion material such as aluminium, and the measurement device may comprise a temperature sensor configured to measure a temperature of at least a part of the mount. Since the FBG is mounted to the mount, the longitudinal tension of the FBG is dependent on the extent of thermal expansion of the mount. Since the current operating wavelength of the fiber Bragg grating is dependent on its longitudinal tension, measuring the temperature of at least part of the mount provides a measurement output which is related to the current operating wavelength of the FBG. Thus, the temperature of at least part of the mount may be used as a feedback signal for stabilising the current operating wavelength of the FBG, thereby to wavelength-stabilise the laser system.

Alternatively, the measurement device may comprise a spectrometer or other wavelength-measurement device arranged to measure the wavelength of radiation generated (e.g. output) by the laser cavity. Since the FBG is narrowband, the wavelength of radiation generated by the laser cavity is determined by, and will often be the same as, the current operating wavelength of the FBG. Thus, measuring the wavelength of radiation generated (e.g. output) by the laser cavity can provide a measurement output which is related to the current operating wavelength of the FBG and may thus be used to wavelength-stabilise the laser system. In some cases, nonlinear effects in the laser cavity may result in a change, for example a shift, between the FBG operating wavelength and the wavelength of the measured radiation. It will be understood that despite this shift / offset, the measured wavelength is nevertheless related to the current operating wavelength of the FBG and may thus be used as a feedback signal for wavelength-stabilising the laser system.

In some cases, a spectrometer or other wavelength-measurement device may measure the wavelength of radiation downstream of the output of the laser cavity, e.g. after nonlinear frequency conversion in one or more nonlinear elements. It will be understood that in this situation also, the measured wavelength will be related to the current operating wavelength of the FBG, because the wavelength of the frequency-converted radiation is dependent on the wavelength of radiation generated by the laser system. Thus, measuring the wavelength of radiation downstream of the output of the laser cavity (e.g. after nonlinear frequency conversion in one or more nonlinear elements) may also provide a feedback signal for stabilising the current operating wavelength of the FBG and thus for wavelength-stabilising the laser system.

For example, UV radiation may be generated by providing one or more nonlinear elements after the laser system. This UV radiation may be measured to provide a measurement output which is related to the current operating wavelength of the fiber grating, thus providing a feedback signal for use in stabilising the current operating wavelength of the laser system.

More generally, any measurement output which is related to a current operating wavelength of the fiber Bragg grating may be used as a feedback signal for use in controlling the tension control system to wavelength stabilise the laser system.

The tension control system is electrically controlled. In particular, the controller is configured to receive a feedback signal derived from the measurement device and to generate a responsive control signal for controlling the tension control system.

The tension control system comprises one or more temperature control devices configured to control the temperature of the mount. As the temperature of the mount is changed, it thermally expands/contracts, thereby controlling the tension of the FBG which is mounted to it. In addition, the tension control system may optionally additionally comprise a piezoelectrically actuator configured to move one part of the mount relative to another, thereby to control the tension of the FBG.

So that the invention may be more easily understood, examples thereof will now be described with reference to the accompanying figures, in which:.

<FIG> illustrate an example laser system <NUM> which produces a modelocked train of picosecond pulses.

As shown in <FIG>, the laser system <NUM> includes a linear laser cavity comprising a ytterbium-doped fiber amplifier <NUM>, a modelocking element in the form of a semiconductor saturable absorber mirror (SESAM) <NUM>, and a fiber Bragg grating (FBG) <NUM>. The FBG <NUM> is a narrowband reflective device which limits the linewidth of the laser system <NUM> and thus ensures that picosecond pulses of an appropriate duration are output. For example, the linewidth of the laser system <NUM> may be less than <NUM> picometers. The pulses produced by the laser system <NUM> may have a duration of between <NUM> ps and <NUM> ps.

The FBG acts as a narrowband reflector around an operating wavelength determined by the longitudinal tension within the FBG <NUM>. As will be appreciated by those skilled in the art, the operating wavelength of the laser system <NUM> (e.g. the central wavelength at which the laser system operates) is dependent on the operating wavelength of the FBG <NUM>, and this wavelength varies dependent on the tension within the FBG <NUM>.

The laser system <NUM> includes a tension control system which is used to prevent drift of the laser operating wavelength which may otherwise occur due to environmental changes. Referring to <FIG>, the the FBG mount <NUM> may be formed of a positive thermal expansion material such as aluminium. The mount <NUM> includes an elongate groove <NUM> and the FBG <NUM> is fixed in the groove <NUM> under tension using an adhesive. The tension of the FBG <NUM> is adjustable using one or more thermoelectric coolers (TECs) <NUM> positioned on the underside of the aluminium mount <NUM>, as shown in <FIG>. It will be understood that the TECs <NUM> may be used to heat as well as cool the mount, responsive to an electrical control signal. When the temperature of the TEC is increased, the aluminium mount <NUM> heats up, causing it to expand. Since the FBG <NUM> is fixed to the mount <NUM>, and the expansion of the mount <NUM> is partially in the longitudinal direction of the FBG, the tension of the FBG increases as the mount thermally expands. Similarly, when the temperature of the TEC is reduced, the aluminium cools to a lower temperature and thus contracts, thereby reducing the tension in the FBG. During manufacture, the FBG may be fixed to the mount <NUM> under tension whilst the temperature of the aluminium mount is maintained at a chosen value; in operation this allows the tension of the FBG to be tuned by varying the temperature above and below this chosen value.

Returning to <FIG>, the TECs <NUM> comprise Peltier devices which can be heated or cooled responsive to an electric signal which is provided by controller <NUM>. Controller <NUM> includes a microcontroller and may additionally include further appropriate electronics for e.g. amplifying the temperature control signal <NUM> to an appropriate level for controlling the temperature of the TECs.

As shown, a measurement device in the form of a temperature sensor <NUM> is positioned on top of the aluminium mount to measure its temperature. Controller <NUM> receives a feedback signal <NUM> derived from the temperature sensor <NUM> and is configured to wavelength-stabilise the laser by adjusting the temperature control signal <NUM> so as to heat or cool the aluminium mount responsive to the feedback signal <NUM>. More specifically, the controller includes one or more processors configured to operate in accordance with computer-readable instructions to execute a feedback loop which adjusts the temperature control signal to ensure that the measured temperature of the aluminium mount, as determined by the feedback signal <NUM>, is maintained at a selected level, e.g. within a predefined tolerance.

By maintaining the temperature of the aluminium mount at the selected level the tension of the FBG is also maintained, thereby stabilising the operating wavelength of the FBG and thus the operating wavelength of the laser system <NUM>.

The operating wavelength of the laser system <NUM> may also be more coarsely tuned by altering the temperature of the mount <NUM> using the TECs <NUM> until a desired operating wavelength is achieved. To identify that the laser has been tuned to the desired wavelength, the wavelength may either be measured using a spectrometer, or inferred from the temperature of the mount as measured by the temperature sensor <NUM>. In particular, appropriate calibration can be performed to determine the relationship between the measured temperature and the operating wavelength of the laser (an approximately linear relationship is expected).

Once the laser system <NUM> has been tuned to the desired wavelength, it is stabilised at this desired operating wavelength using the wavelength-stabilisation feedback loop described above. In this way, stable operation at any desired wavelength within the ytterbium gain band may be achieved, e.g. at any wavelength between <NUM> and <NUM>. It has been found that the techniques described herein permit stabilisation of the operating wavelength of the laser to within e.g. <NUM>%, <NUM>% or <NUM>% of the spectral linewidth. In some examples, the wavelength may be stabilised to within <NUM> picometers or <NUM> picometers of a desired operating wavelength.

<FIG> shows a more general schematic of an example wavelength-stabilised narrow-linewidth mode-locked picosecond laser system <NUM>. The laser system <NUM> includes a laser cavity which includes an amplifier <NUM>, a modelocking element <NUM>, and a fiber Bragg grating (FBG) <NUM> which acts as a narrowband reflector. The laser system <NUM> further comprises a mount <NUM> to which the FBG <NUM> is mounted under tension, a tension control system configured for adjusting the tension of the FBG <NUM>, a measurement device <NUM> to provide a measurement output related to a current operating wavelength of the FBG <NUM>, and a controller <NUM> to wavelength-stabilise the laser by controlling the tension control system responsive to the measurement output.

The amplifier may comprise a ytterbium-doped fiber amplifier as described above in relation to <FIG>, or it may comprises another type of fiber amplifier such an erbium-doped fiber amplifier (EDFA), a thulium-doped fiber amplifier, a holmium-doped fiber amplifier, or a praseodymium-doped fiber amplifier. Further alternatively, a bulk amplifier rather than a fiber amplifier may be used. The laser cavity may include a plurality of amplifiers of the same or different types, e.g. an amplifier chain.

The modelocking element <NUM> may comprises a suitable passive modelocking element, such as a saturable absorber e.g. a semiconductor saturable absorber mirror (SESAM) as described above in relation to <FIG>. In some examples, e.g. if a SESAM is used, the modelocking element may also act as a reflector <NUM> which, together with the FBG <NUM>, forms the laser cavity. However, alternatively, a separate reflector may be provided to form the cavity with the FBG <NUM>. In some examples, the modelocking element <NUM> may comprise an active rather than a passive element. For example, the modelocking element <NUM> may comprise a modulator such as an electro-optic modulator or acousto-optic modulator which is driven by a periodic electric signal so as to cause modelocking of the laser system <NUM>.

The FBG <NUM> may be substantially uniform, e.g. the grating period and/or the index modulation may vary by less than <NUM>%, such as less than <NUM>%, such as less than <NUM>%, such as less than <NUM>% along the length of the grating. As will be understood by those skilled in the art, the index modulation of the FBG may be set to obtain a desired bandwidth of the grating and the length of the grating may be used to set the peak reflectivity. The FBG acts as a reflector around an operating wavelength (e.g. center wavelength) determined by the grating period. The grating period may varied by varying the longitudinal tension of the FBG, thereby tuning its operating wavelength. The operating wavelength of the FBG may comprise the wavelength at which the spectral response of the FBG has its peak.

The peak reflection, also referred to as the reflectivity unless otherwise clear, may be less than or equal to <NUM>%, such as less than or equal to <NUM>%, such as less than or equal to <NUM>%, such as less than or equal to <NUM>%, such as less than or equal to <NUM>%, such as less than or equal to <NUM>%, such as less than or equal to <NUM>%, such as less than or equal to <NUM>%, such as less than or equal to <NUM>%, The reflectivity may be more than or equal to <NUM>%, such more than or equal to <NUM>%, such more than or equal to <NUM>%, such more than or equal to <NUM>%, such more than or equal to <NUM>%, such more than or equal to <NUM>%, such more than or equal to <NUM>%, such more than or equal to <NUM>%, such more than or equal to <NUM>%, such more than or equal to <NUM>%, such more than or equal to <NUM>%. In one embodiment the reflectivity is between <NUM>% and <NUM>% such as <NUM>%. In one embodiment the reflectivity is between <NUM>% and <NUM>%, such as <NUM>. In one embodiment the reflectivity is between <NUM>% and <NUM>%, such as <NUM>%.

The width of the FBG spectral response may be less than <NUM> picometers, e.g. less than <NUM> picometers, e.g. less than <NUM> picometers, e.g. less than <NUM> picometers.

The tension control system is configured to increase or reduce the tension within the FBG <NUM> responsive to an electric signal derived from the controller <NUM>. The tension control system is a temperature-based system, e.g. as described above in relation to <FIG>, which controls the temperature of at least a portion of the mount such that it thermally expands or contracts and thereby adjusts the tension of the FBG <NUM>.

The mount <NUM> may comprise any suitable material. For temperature-based tension control, the mount may comprise any suitable positive thermal expansion material such as aluminium or a ceramic material. The mount <NUM> may include a substrate and a holder within which the FBG is glued in place. One or more temperature controllers may be provided to control the temperature of the mount to cause thermal expansion or contraction of the mount (or at least part thereof), thereby to adjust the tension of the fiber Bragg grating. Alternatively, in examples not in accordance with the invention where only piezoelectric tension control is used, the mount may include an actuator including a suitable piezoelectric element which causes movement of one FBG <NUM> mount point relative to another, responsive to an electric signal derived from the controller <NUM>. In some cases both temperature-based and piezoelectric-tension control may be employed to achieve improved control of the tension of the FBG <NUM> and thus of the operating wavelength of the laser system <NUM>.

The measurement device <NUM> provides a measurement output related to a current operating wavelength of the FBG <NUM>. The measurement device <NUM> may comprise a temperature sensor which measures the temperature of the mount <NUM>, e.g. as described above in relation to <FIG>. In this case, the temperature of the mount is related to the operating wavelength of the FBG <NUM> because the tension within the FBG <NUM> is dependent on the extent to which the mount has thermally expanded or contracted, and the operating wavelength of the FBG is dependent on its tension. In particular, an approximately linear relationship is expected between the temperature of the mount and the FBG operating wavelength.

Alternatively, however, the measurement device <NUM> may be configured to measure the operating wavelength of the laser system <NUM> directly. For example, the measurement device <NUM> may comprise a spectrometer which receives a portion of the radiation generated by the laser cavity (e.g. via a tap coupler) and measures its wavelength; this can then be equated with the current operating wavelength of the FBG <NUM>. More generally, any measurement which is related to a current operating wavelength of the FBG <NUM> or of the laser system <NUM> may be used for the purpose of wavelength stabilisation.

The controller <NUM> may comprise a microcontroller or other data processing apparatus. The controller <NUM> includes one or more processors and a memory storing computer-readable instructions which, when executed by the processor cause the processor to perform one or more operations. <FIG> illustrates a wavelength stabilisation procedure <NUM> which may be executed by the controller <NUM>. As shown, the controller receives <NUM> a measurement signal (e.g. feedback signal) derived from the measurement device <NUM>. The controller <NUM> determines <NUM> whether the measurement signal indicates that the current operating wavelength of the FBG <NUM> has deviated from a desired wavelength by more than a threshold amount. If it has, the controller <NUM> sends <NUM> a control signal to the tension control system to cause the tension control system to adjust the tension of the FBG <NUM> to bring its current operating wavelength towards the desired wavelength, i.e. by either increasing or decreasing the tension as appropriate. The process <NUM> may be carried out continuously during operation of the laser system <NUM> so as maintain the operating wavelength of the laser system <NUM> at the desired wavelength with a high degree of stability, e.g. within <NUM> picometers or within <NUM> picometers of the desired wavelength.

The controller <NUM> may also be used to tune the current operating wavelength of the laser system <NUM> to another designed wavelength by adjusting the tension control system until the measurement signal indicates that the current operating wavelength of the FBG <NUM> corresponds to the desired wavelength. Once the laser system <NUM> has been tuned to the desired wavelength, it may be stabilised at this desired operating wavelength using the wavelength-stabilisation procedure <NUM> shown in <FIG>. In this way, tunable, wavelength-stabilised operation can be achieved using the laser system <NUM>.

In various implementations, the pulses produced by laser systems described in this specification may have a duration of greater than <NUM> ps, e.g. greater than <NUM> ps or greater than 20ps.

In various implementations, the pulses produced by laser systems described in this specification may have a duration of less than <NUM> ps, e.g. less than <NUM> ps or less than <NUM> ps. In some examples the laser system may produce pulses having a duration between <NUM> ps and <NUM> ps. Pulse durations may be measured based on the full width at half maximum (FWHM) measure.

In various implementations, the peak powers generated by laser systems described in this specification may be greater than 1kW, such as greater than 10kW or greater than <NUM> kW.

The laser systems described in this specification may be used to achieve efficient frequency conversion in nonlinear elements such as nonlinear crystals. As will be appreciated by those skilled in the art, suitable frequency conversion arrangements may be used to reach various wavelengths of interest for particular applications, for example conversion from around <NUM> micron to the ultraviolet or deep ultraviolet (e.g. below <NUM>) is possible using such techniques.

It is important to note that the systems <NUM>, <NUM> combine multiple characteristics, in particular narrow linewidth, wavelength stability and pulsed operation, which work together to achieve high efficiency in frequency conversion. Narrow linewidth leads to efficient frequency conversion because of phase matching constraints, since only a narrow linewidth can typically be converted, with any energy outside the phase matching bandwidth remaining unconverted. While narrow linewidth may be achieved using CW lasers, pulsed operation leads to higher peak powers and thus higher efficiency conversion. Wavelength stability also leads to efficiency because any variation of the laser wavelength may result in efficiency losses. For example, in conversion to the deep ultraviolet, a variation in the laser wavelength by as little as <NUM> picometers can result in loss of efficiency.

<FIG> illustrates a frequency conversion arrangement <NUM> using one or more of the wavelength-stabilised narrow-linewidth mode-locked picosecond laser systems <NUM>, <NUM> described above. As shown, picosecond pulses from the laser system <NUM>, <NUM> are launched (e.g. using suitable optics) into one or more nonlinear elements <NUM> (e.g. one or more nonlinear crystals), where they undergo frequency conversion by way of one or more nonlinear effects, such as harmonic generation (e.g. second-harmonic generation). In some examples frequency converted light generated in one nonlinear element may be combined in a second nonlinear element with unconverted light (or light generated a third nonlinear element) to generate further wavelengths using e.g. sum-frequency generation, difference-frequency generation or parametric frequency conversion. In some cases the outputs of two or more wavelength-stabilised narrow-linewidth mode-locked picosecond laser systems may be combined together in a nonlinear element of the frequency conversion arrangement, thereby further expanding the possibilities for the generated wavelengths.

A wavelength-stabilised laser system as described in this specification may also be used to provide a stable seed source in a larger laser system which employs amplification in a solid state crystal. This is particularly advantageous where the solid state crystal comprises a Nd:YAG crystal, or another other solid state crystal with a limited gain bandwidth, since in such cases any wavelength variation of the seed may result in loss of efficiency. For example, a wavelength-stabilised laser system operating at <NUM> may be used as a seed in a larger (e.g. bulk) laser system which employs amplification in a Nd:YAG crystal.

Claim 1:
A wavelength-stabilised narrow-linewidth mode-locked picosecond laser system (<NUM>) comprising:
a laser cavity which includes:
an amplifier (<NUM>);
a modelocking element (<NUM>); and
a fiber Bragg grating (<NUM>) which acts as a narrowband reflector,
a mount (<NUM>) to which the fiber Bragg grating is mounted under tension;
a tension control system to adjust the tension of the fiber Bragg grating;
a measurement device (<NUM>) to provide a measurement output related to a current operating wavelength of the fiber Bragg grating, and
a controller (<NUM>) to wavelength-stabilise the laser by controlling the tension control system responsive to the measurement output, wherein the controller is configured to receive a feedback signal derived from the measurement device and to generate a responsive control signal for electrically controlling the tension control system,
wherein the tension control system comprises one or more temperature control devices (<NUM>) to control the temperature of at least part of the mount to cause thermal expansion or contraction of the at least part of the mount, thereby to adjust the tension of the fiber Bragg grating.