WO2008122289A2

WO2008122289A2 - A fiber laser comprising an intra cavity switch

Info

Publication number: WO2008122289A2
Application number: PCT/DK2008/050079
Authority: WO
Inventors: Jakob Dahlgren Skov; Carsten L. Thomsen; Claus Friis Pedersen; Thomas Vestergaard Andersen; Søren AGGER; Lasse Leick
Original assignee: Koheras A/S
Priority date: 2007-04-04
Filing date: 2008-04-01
Publication date: 2008-10-16
Also published as: WO2008122289A3

Abstract

The invention relates to a fiber laser comprising a first reflector and a second reflector, said reflectors defining a cavity, said cavity comprising at least one gain medium and said cavity further comprises at least one optical switch comprising at least one input port and at least one output port. The incorporation of a switch into the cavity allows for greater flexibility in the operation of the fiber laser.

Description

A FIBER LASER COMPRISING AN INTRA CAVITY SWITCH

The invention relates to a fiber laser comprising a first reflector and a second reflector, said reflectors defining a cavity, said cavity comprising at least one gain medium.

Generally, optical fibers provide stable, compact, rugged and relatively simple means for managing light relative to a light beam in free space. These advantages have been extensively exploited in the field of laser design providing so-called fiber lasers. However, one drawback of fiber lasers is that in order to avoid losses, fiber optic components are often spliced together and non-fiber components are often adhesively connected to the system.

Accordingly, a fiber laser may often be a static design not easily altered to obtain a variety of beam properties available from the same laser or to refurbish the laser with new components. Thus, there is a need to provide more flexibility/versatility within fiber lasers.

In the field of laser design it may be desirable to avoid lossy components in the cavity. A typical optical switch (latching type) has a single pass insertion loss of approximately 1 dB. It is surprisingly found that adequate laser operation is obtainable, even though a switch is inserted into the cavity, so that the switch is passed by light propagating both ways. In an embodiment the present invention relates to a fiber laser comprising a first reflector and a second reflector, said reflectors defining a cavity, said cavity comprising at least one gain medium wherein said cavity further comprises at least one optical switch. Said switch providing significant flexibility as explained from the discussion of the various embodiments of the invention.

The optical switch may in principle comprise as many ports as desired such as three ports with one input port and two output ports. In this way the switch may be incorporated to divide the fiber laser into two sections; one or more sections connect to the input port(s) and one or more sections connected to the output ports. In most preferable embodiments, one section will remain static, e.g. the input side, while the switch may be said to be capable of switching at least partially between elements on the other side of the, e.g. the output side. Through out this text the term input and output of the switch will refer to such inputs and outputs, respectively. In an embodiment the switch be a 1xN type switch where N>2, such as a N=2, such as a N>3, such as a N>4, such as a N>6, such as a N>8, such as a N>10. However, it is to be understood that the switch may be configured to switch between different optical elements on the input side, or that the two sides may be reversed so that the output side is kept static. In an embodiment the switch may also comprise an MxN type switch where M may take any of the values attributed to N above.

An optical element is to be understood as an optical component or an ensemble of connected optical components such as a fiber, a fiber-optic bragg grating, a WDM (Wavelenght Division Multiplexor), a switch, a mirror, and/or a pump light source etc.

Several types of optical switches exist in the art such as where switching is performed by mechanical means, such as physically shifting an optical fiber to one or more alternative fibers, or MEMS where a set of micromirrors deflect the light to appropriate path or fiber. Other methods are based on electro-optic effects, thermo-optic effects, magneto-optic effects, inkjet methods involving the intersection of two waveguides so that light is deflected from one to the other when an inkjet-like bubble is created, liquid crystals, thermal methods, or nonlinear effects, and acousto-optic effect any of which may be applied alone or in combination in the present invention. In an embodiment the switch is latching type obtained in fiber-optic systems for example by mechanically moving a fiber tip between alternate fibers. In one embodiment substantially all light, except any losses in the switch, is guided between one input port and one output port. In an embodimen a switch which guides light to more than one port is applied. Such a switch may be faster compared to a mechanical switch and may be applied when higher switching speed is desired. One example of such an application is the application of a switch, with a shorter response time than the round trip time of the laser, applied to select which pulses should at least partially leave the cavity and thus the switch functions as a pulse picker.

In an embodiment the switch guides light from an input port to a output port, so that said output port emits more of said light relative to any other output port(s), such as more than 5%, such as more than 25% more, such as more than 50% more, such as more than 100% more, such as more than 200% more, such as more than 500% more, such as more than 10000%. When more light is guided toward one element relative to another it is speculated that this path may experience larger net gain compared to other paths and therefore dominate the emission from the gain medium/media. It is therefore speculated that the laser will lase according to the properties of the cavity defined partially by this element. However, it is speculated that for some applications it may be advantageous to let the laser lase according to the cavities defined by two or more paths simultaneously.

As the invention relates to a fiber laser it is preferable that the switch is a fiber optic switch. However, any suitable switch may be applied as long as a suitable interface to an optical fiber can be made. One example may be a pigtailed acousto-optic switch made from semiconductor materials. In one embodiment, at least one of the ports of the switch comprises a connectorized fiber, so as to allow for easy connection to an optical element.

The laser comprises at least one gain medium which may be any gain medium suitable for the desired application of the laser. The gain medium of the fiber laser may comprise (or be comprised in) optical fiber waveguide (i.e. an active fiber), such as single or multimode fiber where a doped core acts as a gain medium. Furthermore, the active fiber may also be a sealed hollow core micro-structured fiber filled with a particular gas type. The application of an active fiber as gain medium has the advantage of being a simple, rugged construction relatively easily implemented into the fiber laser, such as by fusion splicing. However, the gain medium may also comprise (or be comprised in) a planar glass or semiconductor optical wave guide which has the advantage that higher doping concentration of the active material may be achieved. The fiber laser may also be designed to use other types of gain media or a combination of gain media types.

The gain medium may comprise one or more of the following active materials Yb, Eb, Tm, Pr, Ho, Sm, Nd all well known in the art for having suitable characteristics depending on the application - particularly distinguished by wavelength. However, the gain medium may in principle comprise any active material suitable for the desired application of the laser.

Furthermore, in one embodiment the fiber laser comprises at least one pump light source adapted for activating at least part of said gain medium. The suitability of a pump light source is primarily determined by its emitted wavelength, optical power and in some case by its pulse characteristics.

The method pumping of the gain medium/media of the fiber laser is not confined to applying one or more pump light sources. In principle any S

suitable method of pumping a gain medium may be applied such as electrical pumping.

The gain media emits light due to spontaneous and stimulated emission. Spontaneous emission is a stochastic process, which means that the output power and polarization fluctuates. By inserting a polarizing element in the cavity one polarization becomes dominant, which stabilizes the laser output power and increases the lasers tolerance towards environmental changes such as temperature and humidity. In an embodiment the fiber laser comprises a polarizing component preferably located within the cavity. Furthermore, it may be preferred that the laser comprises at least one polarization maintaining component, preferably the majority of the components comprised by the cavity are polarization maintaining, such as all of the components comprised by the cavity are polarization maintaining. Polarization maintaining is to be understood in a broad sense, wherein a polarization maintaining component may alter the polarization intentionally, such as the Faraday rotator discussed below. In this case, the polarization is preferably restored either by another component and/or may be re-passing the component. It is often preferred that a single polarization is maintained through the gain medium.

In one embodiment the fiber laser is considered a fiber laser due to the ! comprising a doped opttcai fiber as a gatn medium. In another embodimenf Hie fiber laser is an all-fiber laser where ail components within the laser cavity are fiber based, in another embodiment the fiber laser Is considered a fiber laser due Io the laser comprising optical fiber as part of the laser cavity, such as one or more fiber based componenls.

In the context of this text a component is considered fiber based or fiberoptic when at least one port of [he component is pigtaited.

In an embodiment the fiber laser comprises an output coupler for coupling light out of the cavity forming the output of the laser from which the output light of the laser emits during operation. In one embodiment, said output coupler comprises a coupler, such as a polarizing coupler. However, the output coupler may also comprise a semi transparent reflector, such as one of the reflectors forming the cavity. In an embodiment, two output couplers are applied so that one couples light of the cavity to be used for monitoring of the performance of the laser, and the other couples light of the cavity forming the output of the fiber laser.

The laser is preferably isolated so that substantially any light is prevented from propagating towards the cavity from the output port of the laser as such light may otherwise provide undesired feedback into the laser cavity which may cause instability. In an embodiment the fiber laser comprises at least one unit for substantially preventing any light propagating towards the cavity from the output of the laser, such as an isolator. However, in some applications of the laser there may be little or no risk of light propagating back into the laser and therefore said isolator may be omitted such as to save cost and/or complexity.

One method of designing the spectral properties of the fiber laser is to design the spectral response of one or more of the reflectors of the cavity. In an embodiment the first and/or second reflector comprises a grating where said grating may be a bulk grating and/or a grating comprised in an optical fiber. Gratings are preferable as they provide substantial flexibility with regard to the spectral response as well as the length of the grading may determine how much an impinging pulse may be stretched upon reflection. However, it may be preferable to apply a thin film filter as the first and/or second reflector. Similarly to gratings, thin film filters (or dichroic filters) are tunable reflecting light in one band and transmitting other light.

The reflectors discussed above are preferably designed to have a spectral response within the spectral response of the gain medium to ensure amplification of intra cavity light. In an embodiment first and/or second reflector has a spectral response of less than 100% of the 3dB spectral width of the gain medium, such as less than 50%, such as less than 20%, such as less than 10%, such as less than 5%, such as less than 1 %.

One method of introducing pump light into the cavity is through the first and/or second reflector. In an embodiment the fiber laser comprises at least one pump light source, wherein said first and/or second reflector are at least partially transparent for the light from said pump light source(s).

Another method of introducing pump light into the cavity is to include a WDM (Wavelength Division Multiplexer) in the fiber laser. The WDM is preferable designed to route pump light along one path and light from lasing of the fiber laser along a different path. In an embodiment the fiber laser comprises at least one pump light source, further comprising at least one WDM (Wavelength Division Multiplexer) for the introduction of light from said pump light source(s). The WDM may be placed inside the cavity and/or outside the cavity. In an embodiment a WDM placed outside the cavity guides the pump light from the pump light source toward a reflector at least partially transparent for pump light as discussed above. In an embodiment said reflector is semi transparent for light from lasing of the fiber laser. The part of said light transmitted through the reflector may then be guided away from the pump light source by the WDM and used as either output of the fiber laser and/or for monitoring of the fiber laser.

One method to achieve pulsed operation of the fiber laser is to employ a non-linear reflector as first and/or second reflector. In a embodiment said non-linear reflector comprises a saturable absorbable mirror (SAM) comprising a reflector surface, such as a semi-conductor saturable absorbable mirror. A SAM is characterized by increasing reflectivity with increased impinging intensity (at least over a particular range of intensity values) resulting in a preference for high intensity pulses in the cavity. In an embodiment the initial pulse arises from thermal fluctuation in the laser. R

In an embodiment said SAM is connected to an optical fiber forming part of the cavity by a connector unit, such as an FC/PC, FC/UPC or similar connector. In an embodiment said connector unit comprises a hybrid adaptor unit where the fiber connector is inserted in one side and a SAM mount holding said SAM on the other side, so that said fiber is butt coupled to the SAM. In an embodiment the SAM is glued to the plane surface of a SAM mount. In an embodiment the tip of said fiber is pressed onto the SAM by the feather force in the connector so that the force on the SAM is 100 N, such as 10 N or less, such as 1 N or less, such as 0.1 N or less, such as 0.01 N or less, such as 0.001 N or less to ensure good contact to the SAM and reduce the optical loss when reflected light is recoupled to the fiber. Connecting the SAM via a connector further has the advantage that a SAM may relatively easily be exchanged without the need for splicing or gluing.

In one embodiment said connector unit comprises a fiber patch-cord where the SAM is connected to one end of the patch-cord. The SAM may be connected to the patch-cord via a second connector unit such as the connector unit described above or the SAM may be pigtailed to one end of the patch cord i.e. fixed (in an embodiment glued) to a bare fiber end of the patch cord. In an embodiment the SAM is pigtailed to a connectorized fiber and connection to a fiber forming part of the cavity is performed via an adapter.

The light impinging on the SAM is, in an embodiment, substantially perpendicular to the reflector surface, such that the angle of incident light to the normal of the reflector surface is |20°| or less, such as |10°| or less, such as |5°| or less, such as |2°| or less, such as |1°| or less, such as |0.5°| or less, such as |0.05°| or less.

In an embodiment the fiber laser comprises one or more components for monitoring the performance of the fiber laser providing at least one monitor signal. Alternatively, the fiber laser is adapted to interface with one or more such components. Such components may include detectors, power meters, P,

spectrometers, pulse analyzers, etalons, and any other device applied in laser analysis in the art.

In one embodiment of the invention the switch comprises two or more ports, wherein each of at least two ports of said optical switch are connected to substantially identical optical elements to enable extension of the life time of the fiber laser. This embodiment may be particularly useful when one or more components have an expected life time which is significantly shorter than for the remaining components of the fiber laser. This component is referred to as a limiting component or limiting optical element which may comprise one or more limiting components. It is then preferable that two or more limiting elements are connected to the output ports of the fiber laser, so that laser operates substantially with a single of these elements at a time. A variety of approaches may then be applied to extend the life time of the laser, such as:

• switching between limiting elements at regular intervals to diverse the load and extend the overall life time of the fiber laser,

• operating the laser connected to a first limiting element and switching to a second limiting element when the expected life time of the first limiting element approaches,

• operating the laser connected to a first limiting element and switching to a second limiting element when measurement(s) indicates that the end of the life time of the first limiting element is approaching,

• operating the laser connected to a first limiting element and switching to a second limiting element combined with exchange/replacement of one or more of the at least partially spent limiting elements. Said limiting elements may also be exchanged/replaced based on other eventualities such as if they are found to be faulty or an upgrade is available. in

• or, a combination of the above stated approaches.

To indicate the remaining life time of a limiting element the fiber laser, in one embodiment, comprises at least one indicator of the remaining life time of one or more components referred to as a life time indicator. Such an indicator may, as an example, be provided by correlating the pumping power to the laser output power, where for example an increase in necessary pumping power to obtain constant output power may indicate that e.g. a SAM is approaching the end of its life time.

To allow for automatic operation of the switch, one embodiment of the fiber laser may comprise a trigger for triggering said optical switch based on one or more indicators for indicating the remaining life time of one or more of the components of the laser and/or based on at least one monitor signal.

To allow simple exchange, the limiting element is preferable connected to the optical switch by at least one optical fiber connector selected from the group: an FCC connector (such as FC/PC, FC/APC and FC/UPC), ST connector, SC, LC, Biconic, D4, ESCON, FDDI, Opti-Jack, MT-RJ, MU, SMA, E2000, F3000, LSA and TOSLINK. Said optical fiber connector is preferable a fiber-to-fiber connector.

In a embodiment, the limiting optical element comprises a SAM, such as a semi-conductor saturable absorbable mirror, a pump light source, and/or a gain medium.

Correspondingly, in one embodiment the invention relates to a method of extending the life time of a fiber laser comprising a first reflector and a second reflector forming a cavity, said cavity comprising at least one gain medium and said cavity further comprises at least one optical switch comprising two or more ports, wherein at least two ports of said optical switch each are connected to substantially identical optical elements to enable extension of the life time of the fiber laser. In an embodiment, the method further comprises

a. operating the laser in connection to at least one to one of said optical elements,

b. monitoring the performance of the laser producing at least one monitor signal to establish the remaining life time and/or approximate remaining life time of one or more components of said optical element,

c. comparing said established life time with a preset value, and

activating said optical switch to a second optical element when the established life time is equal to or less than said preset value.

Preferable the above method provides the capability of substantially continues operation as an optical element may be exchanged while the fiber laser utilizes a second optical element. In an embodiment the method comprises exchanging, refurbishing, repairing, and/or replacing an optical element while the laser is in optical communication with a second optical element through said switch.

To facilitate easy exchange the optical element in one embodiment be connected to the switch via en optical connector, such as a fiber-to-fiber connector. Accordingly, it may be preferred that at least one of said optical elements are connected to the optical switch by at least one optical fiber connector comprising at least one of: an FCC connector (such as FC/PC, FC/APC and FC/UPC), ST connector, SC, LC, Biconic, D4, ESCON, FDDI, Opti-Jack, MT-RJ, MU, SMA, E2000, F3000, LSA, and TOSLINK.

To provide the option of altering the repetition rate of the fiber laser, the fiber laser, in one embodiment, comprises a switch comprising two or more ports, 1 ?

wherein each of at least two ports of said optical switch are connected to optical elements of different optical path length.

Here it is preferable that at least one of said optical elements comprises optical fiber with low non-linearity to avoid non-linear effects, particularly self-phase modulation, from the added optical path length from affecting the performance of the laser. A fiber with low non-linearity is understood as a fiber with a non-linearity parameter, γ, less than 3-10³, such as less than 2-10³, such as less than 1 -10³, such as less than 0.5-10³, such as less than 0.1 -10³, such as less than 0.01 -10³ at a wavelength of 1064nm. As γ may be assumed to decrease linearly with wavelength these values may be scaled accordingly at other wavelengths. Such that for a wavelength of 1550 nm, a low non-linearity parameter is understood to be less than 2.1 -10³, such as less than 1.4-10³, such as less than 0.69-10³, such as less than 0.34-10³, such as less than 0.069-10³, such as less than 0.0069-10³. In one embodiment a low non-linearity fiber is a fiber with a relatively large mode field area relative to conventional single mode fibers designed for the same wave length.

As the path length increases, dispersion effects may affect the pulse width and other properties of the laser output. In an embodiment it is preferable that at least one of said optical elements comprises an optical path, said path comprising a section with anomalous dispersion. In one embodiment said section comprises microstructured optical fiber and/or holey fiber and/or photonic bandgab fiber, which may designed to provide suitable dispersion compensating characteristics. Alternatively, or in combination, said section comprises one or more free-space gratings suitable for dispersion compensation. As an alternative or supplement to the above described approach to provide the option of altering the repetition rate of a laser the invention relates to a method of altering the repetition rate of the output light of a pulsed laser, said method comprising splitting the output light into at least two arms by a first coupler and combining the light of one or more arms by a second coupler, wherein said arm comprises unequal optical path length and thereby altering the repetition rate of the laser. In an embodiment, the method comprise the option of controlling said alteration by activating at least one unit for disrupting the path from said first coupler to said second coupler in at least one of said arms. The two couplers and the two arm may be said to form an unbalanced Mach-Zehnder interferometer.

Correspondingly, in one embodiment the invention relates to a pulsed laser wherein the output of the laser is split into at least two arms by a first coupler and a second coupler combining the light of one or more arms, wherein said at least two arms comprises unequal optical path length. In an embodiment, at least one of said arms comprises a unit for disrupting the path from said first coupler to said second coupler. In an embodiment said unit comprises an optical switch and/or a variable optical attenuator.

In one embodiment at least one switch of the fiber laser comprises two or more ports, wherein each of at least two ports of said optical switch are connected to optical elements comprising dissimilar spectral response providing an ability to alter spectral properties of the output light such as wavelength and/or pulse length.

Flexibility in wavelength selection may be useful in some applications such as fluorescence imaging where the ability to target different contrast agent by wavelength selection without the need for multiple light sources may be beneficial. Another example is a fiber laser using Ytterbium as gain medium.

To achieve maximum output laser operation around the gain peak at 1030 nm is preferable. However, for other applications it is advantageous to work at the YAG laser wavelength of 1064 nm, since this has become a de-facto industry standard. Different wavelengths may for example be obtained by letting the optical elements comprise Bragg gratings of different wavelength. The set of available wavelengths may be limited by the spectral response of the other components in the cavity.

Flexibility in pulse length selection may be beneficial e.g. such as having a choice between picosecond output pulses for UV-generation by frequency doubling or materials processing and femtosecond output pulses for terahertz generation.

Accordingly, in an embodiment the dissimilar spectral response may comprise different center wavelength to select different center wavelengths of the output light and/or different spectral width in order to select the pulse length. It is often preferred that the spectral response of the optical element overlaps the spectral characteristics of the gain medium, so that the spectral width of the spectral response of at least one of said optical elements is more than 1 % of the 3dB spectral width of the gain medium, such as more than 3%, such as more than 5%, such as more than 10%, such as more than 20%, such as more than 50%, such as more than 75%, such as more than 100%, such as more than 200%.

In an embodiment, the optical elements comprises one or more of the following: a Bragg grating, a saturable absorbable mirror, a semi-conductor saturable absorbable mirror, a mirror, a broad band mirror and a thin film filter all of which may be designed with specific spectral properties.

In regard to pulse length, it may be preferable to a have spectrally broad pulse, such as a pulse which is particularly suitable for compression into a short temporal pulse such as a femtosecond pulse. In one embodiment this is obtainable by letting the optical element comprise a broad band mirror. Due to the broader spectrum dispersion may significantly affect the pulse. Accordingly, the laser preferable further comprises at least one component with anomalous dispersion preferable comprised in said optical element. In one embodiment at least one of said optical elements comprises a Faraday rotator and a Faraday mirror and preferable further comprise at least one component with anomalous dispersion. As it is sometimes difficult to design an optical component to have anomalous dispersion and be polarization maintaining simultaneously for wavelengths less than 1.3 μm, the Faraday components are included in this example in order to compensate for any polarization rotation in this component.

As discussed above, one suitable component having anomalous dispersion is a microstructured optical fiber. In an embodiment said component with anomalous dispersion comprises a microstructured fiber and/or holey fiber and/or photonic bandgab fiber.

To allow further flexibility, said optical element may comprise a gain medium, such as the gain medium of the laser. This may provide further flexibility with regard to selection of wavelength.

Similarly the laser may comprise two or more pump light sources, such as a pump light source for each optical element. In one embodiment at least one optical element comprises a pump light source. Multiple pump light sources provide the flexibility of changing parameters of the pump light to match different gain media and/or changing other pump parameters such as wavelength, pulsed or cw operation and/or pulse characteristics

One pump light source may also be applied to feed one or more optical elements for example by incorporating a second switch to switch pump light between optical elements. This is particularly relevant when pump light is introduced through one of the reflectors of the cavity such as discussed above. Alternatively, one pump light source may feed the laser regardless of which optical elements is in use by introducing the pump light inside the cavity, such as via a WDM. 1 fi

Corresponding to the above embodiments regarding a fiber laser with selectable repetition rate, wave length and/or pulse length, an embodiment of the invention relates to a method of changing at least one property of the output light of a fiber laser, said fiber laser comprising a laser cavity and a switch distributing light entering on one or more input ports to one or more output ports of said switch, where each of at least two of said input and/or output ports are connected to optical elements with different optical property/properties, wherein at least one of said optical elements forms part of the laser cavity and the method comprises activating said switch to alter the distribution of light between said input ports and output ports of the switch. In an embodiment all light entering said input ports is guided to a single output port and/or all light entering said output ports is guided to a single output port.

In a first embodiment of the method, said optical property comprises optical path length in order to alter the repetition rate of the fiber laser. In a second embodiment of the method, said optical property comprises spectral response in order to alter the wavelength and/or pulse length of the fiber laser. In a third embodiment said first and second embodiments are combined.

Any of the embodiments described above may be combined to achieve the option of selecting different parameters for the output light. This may be performed by optical elements having two or more dissimilar parameters, such as spectral response and optical path length, and/or by including multiple switches to allow a combination of optical elements. In one embodiment on optical elements comprises a switch connected to two or more optical elements, such as an optical element comprising a Bragg grating and further comprising a switch connected to two or more optical elements with dissimilar optical path length. In another embodiment a combination of multiple embodiments may be realized by incorporating an M X N optical switch providing the option of having multiple optical elements on both sides of the switch.

By the invention it is realized that a large variety of combination of optical elements and switches may be obtained and that the present invention therefore provide the designer of a fiber laser with a tool to incorporate virtually any flexibility desired.

Similarly to the perception of optical switches, connectors are regarded in the art as lossy components but more importantly connectors are also regarded as difficult to operate reproducibly providing an added unknown to a system. Accordingly, fusion splicing and/or adhesive connections are commonly applied to connect components in a fiber laser. However, by the inventors it was surprisingly found that by suitable choice of connector type reliable laser operation is achievable. In an embodiment the invention relates to a fiber laser comprising a first reflector and a second reflector forming a cavity comprising at least one gain medium wherein said cavity further comprises at least one optical fiber connector. Said connector provides significant flexibility with regards to the exchange of one or more components of the laser.

In an embodiment the optical connector is selected from the group of: FCC connector (such as FC/PC, FC/APC and FC/UPC), ST connector, SC, LC, Biconic, D4, ESCON, FDDI, Opti-Jack, MT-RJ, MU, SMA, E2000, F3000, LSA, and TOSLINK.

It is preferable that the mating of two connectors may be performed as reproducible as possible while minimizing variation due the skill-level of the operator performing the mating. The inventors has found that connectors of the type E2000 are particular suitable as these connector type are clicked together, so there is little room for variations due to the operator such as in e.g. a FC connector where the assembly procedure may affect the position 1 R

and angle of the fiber tip which may cause loss variations. Another advantage of the connector is that it is equipped with a dustcover which automatically covers the fiber tip when the connector is disengaged. This significantly reduces the risk of damage to any component, such as the connector itself, due to dirt on the connector which may incinerate during laser operation.

Correspondingly, an embodiment of the invention relates to a method of connecting an optical element comprising an optical connector to the cavity of a fiber laser comprising an optical connector, said method comprising mating said optical connector of the cavity with said optical connector of the optical element. In an embodiment the method further comprises refurbishing, altering and/or upgrading an optical element of said fiber laser wherein said optical element is connected to the cavity of the fiber laser via optical connectors. Hereby the invention provides a method which is simple and effective while reducing the requirement to prior training servicing and/or assembly of a fiber laser. Such servicing may for example be used to extend the life time of the laser by enabling the exchanges of a limiting optical component.

In the following a series of embodiment will be described with reference to the accompanying figures. These embodiments should not be inferred to limit the invention which is defined by the accompanying set of claims.

The invention will be explained more fully below in connection with a embodiment and with reference to the drawings in which:

FIG. 1 a fiber laser comprising a switch for switching between two SAMs.

FIG. 2 a fiber laser comprising a switch for switching between two SAMs, each connected to a port of the switch via an optical connector.

FIG. 3 a fiber laser comprising a switch for switching between two optical elements with different optical path length. 1 Q

FIG. 4 a fiber laser comprising a switch for switching between two optical elements with different optical path length and further comprising a component with anomalous dispersion.

FIG. 5 a fiber laser comprising an unbalanced Mach-Zehnder interferometer coupled to the output port..

FIG. 6 a fiber laser comprising a switch for switching between Bragg gratings.

FIG. 7 a fiber laser comprising a switch for switching between two Bragg gratings and further comprising a WDM for introducing pump light into the cavity.

FIG. 8 a fiber laser comprising a switch for switching between a Bragg grating and a broadband mirror.

FIG. 9 a fiber laser comprising a switch for switching between a Bragg grating and a Faraday mirror combined with a Faraday rotator.

FIG. 10 a fiber laser comprising a switch for picking pulses.

FIG. 11 a fiber laser.

The figures are schematic and may be simplified for clarity. Throughout, the same reference numerals are used for identical or corresponding parts.

Figure 11 shows a fiber laser 1 comprising a WDM 2 for introducing pump light via an input 4. The laser cavity is defined by the Bragg grating 5 and a

SAM 13 acting as first and second reflectors. The gain medium, here shown as an active fiber 6, amplifies spontaneous emission emitted from the same gain medium, which has been filtered by reflection in the Bragg grating 5. In an embodiment the Bragg grating forces the laser to lase at the Bragg wavelength. In an embodiment the Bragg grating is uniform and/or chirped.

Spikes in the spontaneous emission are favored in the cavity due to the non- linear reflectivity (saturable absorption) of the SAM 13 causing the output of the fiber laser to be pulsed. In an embodiment the SAM has a saturable absorption of more than or equal to 5%, such as more than or equal to 10%, such as more than or equal to 15%, such as more than or equal to 25%. In an embodiment the roundtrip time of the cavity is longer than the relaxation time of the SAM which is preferably shorter than or equal to 5 ps, such as shorter than or equal to 2 ps, such as shorter than or equal to 1 ps, such as shorter than or equal to 0.5 ps, such as shorter than or equal to 0.1 ps. The spectral width of the Bragg grating may determine the pulse length (wider spectral width gives shorter pulses). The output coupler 7 couples part of the light traveling in the cavity to the output port 15 and light is prevented from entering the cavity through the output coupler 7 by the isolator 16. In an embodiment the cavity comprises polarization maintaining components. In one such embodiment the output coupler is polarizing, so that output coupler 7 may have low loss in the slow axis but large loss in the fast axis. The WDM 2 further comprises a port 3 through which light passing the Bragg grating 5 may be monitored. Depending on the reflectivity of the Bragg grating 5, this port may also be utilized as an output port.

Figure 1 shows a fiber laser according to one embodiment of the invention suitable for extending the life time of said fiber laser comprising a switch 9 with an input port 10 and two output ports 11 and 12. The output ports of the switch 11 ,12 are each connected to a SAM 13,14. For a latching type switch the switch 9 is shown to be in optical communication with the SAM 13 whereas the SAM 14 is disconnected. However, as discussed above, several suitable types of switches are available in the art, some of which works to guide substantially more of the light entering the input to one output port. For a switch of this type, this illustration show the switch guiding more light towards the SAM 13 than the SAM 14. The switch is connected to the output coupler 7, which in turn is connected to the remaining cavity. The cavity is defined by the Bragg grating 5 and the SAM 13 acting as primary and secondary reflectors. The SAM 13,14 are connected to the switch either 91

by pigtailing and/or by butt-coupling to an optical connector such as described above. As discussed above, extended life-time may be obtained by switching between the two identical optical elements, in this embodiment defined by the SAM, the coupling to the SAM and any fiber not comprised by the switch.

Figure 2 shows a fiber laser according to one embodiment of the invention similar the fiber laser of figure 1. In this embodiment the SAMs 13,14 are connected to the output ports 11 ,12 of the switch 7 via optical connectors 202.

Figure 3 shows a fiber laser according to one embodiment of the invention suitable for providing selectable repetition rate. The fiber laser is similar to the fiber laser of figure 1 , however; the output port 11 of the switch 9 is connected to an optical element comprising a SAM 13 as well as a fiber 301 providing additional path length. Accordingly, the total length of the cavity may be altered by switching between output ports 11 and 12 resulting in a different repetition rate of the laser output for the two positions of the switch.

Figure 4 shows a fiber laser according to one embodiment of the invention suitable for providing selectable repetition rate. The fiber laser is similar to the fiber laser of figure 3, however; the output port 11 of the switch 9 is connected to an optical element further comprising a component 401 with anomalous dispersion which may be designed to compensate for dispersion, such as the dispersion induced by the fiber 301 providing additional optical path length.

Figure 5 shows a fiber laser according to one embodiment of the invention suitable for providing selectable repetition rate. The fiber laser is similar to the fiber laser of figure 11 , however; the output of the isolator 16 is connected to an unbalanced Mach-Zehnder interferometer comprising a first coupler 501 splitting the light into two arms 502,503 comprising unequal path length and a second coupler 504 combining the light from the two arms 502,503 into the output 505. In one embodiment a Mach-Zehnder interferometer is combined with selectable repetition rate such as implemented in figure 4.

Figure 6 shows a fiber laser according to one embodiment of the invention suitable for providing selectable wavelength and pulse length. The fiber laser is similar to the fiber laser of figure 11 , however; a switch 609 is connected to two optical elements each comprising a fiber based Bragg grating 601 ,602, a WDM 603,604 having an input port 607,608 for introducing pump light into the cavity and a port 605,606 which may be used similarly to the port 3 of figure 11. The difference in spectral response of the two Bragg gratings 601 ,602 determines what property/properties of the output light that is selectable.

Figure 7 shows a fiber laser according to one embodiment of the invention suitable for providing selectable wavelength and/or pulse length. The fiber laser is similar to the fiber laser of figure 6, however; a single WDM 2 is implemented inside the cavity. The advantage relative to the fiber laser of figure 6 is a reduction in the number of components. Relative to the fiber laser of figure 6, this embodiment has the drawbacks that the insertion loss of the WDM may affect laser performance and that the additional fiber length of the WDM limits the minimum length of the cavity and thereby the maximum repetition rate obtainable. In one alternative embodiment the switch 609 is placed on the other side of the gain medium 6 so that each of the optical elements connected to the output ports of the switch 609 comprises a gain medium preferably optimized to the spectral properties of the Bragg grating, such as the wavelength. In one embodiment with selectable wavelength, these two gain media are adjusted to provide the same gain at two different wavelength e.g. by different length active fibers.

Figure 8 shows a fiber laser according to one embodiment of the invention suitable for providing selectable pulse length. The fiber laser is similar to the fiber laser of figure 7, however; one output 11 of the switch 609 is connected 93

to an optical element comprising a broadband mirror 802. This optical element preferable comprises a component 801 with anomalous dispersion which is preferable polarization maintaining. The component 801 is inserted to compensate for dispersion of the remaining cavity which may be particularly disruptive for a broadband pulse. In one embodiment such a configuration provides for a linearly chirped output which can easily be compressed to fs pulses.

Figure 9 shows a fiber laser according to one embodiment of the invention suitable for providing selectable pulse length. The fiber laser is similar to the fiber laser of figure 8, however; alternatively to a the component 801 with anomalous dispersion, which may be difficult to obtain as polarization maintaining, the optical element connected to the output 11 of the switch comprises a faraday mirror 902 and a faraday rotator 901 which may be used to obtain the effect of polarization stability of the reflected light.

Figure 10 show a fiber laser according to one embodiment of the invention, wherein a switch 1101 is incorporated to function as the output coupler 7 of any of the figures 1 to 9. In this manner extensive flexibility with regard to repetition rate is obtained. In an embodiment the switch 1101 has a response time shorter than the roundtrip time of the cavity so that single pulse may be selectively picked to couple to the output 12 of the switch 1101 forming the output of the laser 15. In one embodiment substantially all of the picked pulse may be coupled out and in another embodiment only part of the pulse is picked, such as discussed with regard to switches in general above.

It should be noted that any of the distinguishing parts/components/ implementations of the embodiments figures discussed above may be combined. In particular it should be noted that optical connectors such as 202 may be applied to connect any component of laser, such as any type of optical element. 94

It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other stated features, integers, steps, components or groups thereof.

The term embodiment is, unless otherwise clear, understood to mean a group of realizations of the invention sharing the feature(s) stated to signify the embodiment. Unless otherwise specified, the specification of embodiments presented above has been in regard to a single or a few features out of the total set of features contained in a complete embodiment. As will be recognized by a person skilled in the art some or all of said features disclosed with regard to individual embodiments may be combinable. Such combinations should be understood as to be within the scope of the present invention.

Claims

95CLAIMS

1. A fiber laser comprising a first reflector and a second reflector, said reflectors defining a cavity, said cavity comprising at least one gain medium and said cavity further comprises at least one optical switch comprising at least one input port and at least one output port.

2. The fiber laser of claim 1 , wherein said optical switch is adapted to guide light from an input port to a output port so that said output port emits more of said light relative to any other output port(s), such as more than 5%, such as more than 25% more, such as more than 50% more, such as more than 100% more, such as more than 200% more, such as more than 500% more, such as more than 10000%.

3. The fiber laser of claim 2, wherein all of the light is guided to said output port.

4. The fiber laser of any of the preceding claims, wherein said optical switch comprises a latching type switch.

5. The fiber laser of any of the preceding claims, wherein said optical switch comprises a 1xN type switch where N>2, such as N=2, such as a N>3, such as a N>4, such as a N>6, such as a N>8, such as a N>10.

6. The fiber laser of any of the preceding claims, wherein said optical switch comprises a fiber optic switch.

7. The fiber laser of any of claims 1 to 6, wherein at least one of said ports comprises a connectorized fiber.

8. The fiber laser of any of the preceding claims, wherein said gain medium comprises an optical fiber waveguide.

9. The fiber laser of claim 8, wherein said gain medium comprises a semiconductor optical wave guide.

10. The fiber laser of any of claims 8 or 9, wherein said gain medium comprises at least one of the following active materials Yb, Eb, Tm, Pr, Ho, Sm, Nd.

11. The fiber laser of any of claims 8 to 10, further comprising at least one pump light source adapted for activating at least part of said gain medium.

12. The fiber laser of any of the preceding claims, wherein at least one component of the fiber laser is polarization maintaining.

13. The fiber laser of any of the preceding claims, wherein all components of the fiber laser are polarization maintaining.

14. The fiber laser of any of the preceding claims, further comprising a polarizing component.

15. The fiber laser of any of the preceding claims, further comprising an output coupler-coupling optical element for coupling light out of the cavity forming the output of the laser.

16. The fiber laser of claim 15, wherein said output coupler-coupling optical element comprises a coupler, such as a polarizing coupler.

17. The fiber laser of any of claims 15 or 16, wherein said output couplerout-coupling optical element comprises a semi transparent reflector.

18. The fiber laser of any of the claims 15 to 17, further comprising at least one unit for preventing substantially any light propagating towards the cavity from the output of the laser, such as an isolator. 97

19. The fiber laser of any of the preceding claims, wherein said first and/or second reflector comprises a grating.

20. The fiber laser of any of the preceding claims, wherein said first and/or second reflector has a spectral response of less than 100% of the 3dB spectral width of the gain medium, such as less than 50%, such as less than 20%, such as less than 10%, such as less than 5%, such as less than 1 %.

21. The fiber laser of any of the preceding claims, wherein said first and/or second reflector comprises a thin film filter.

22. The fiber laser of any of the preceding claims comprising at least one pump light source at least one of said first and/or second reflector are at least partially transparent for the light from said pump light source(s).

23. The fiber laser of any of the preceding claims comprising at least one pump light source, further comprising at least one WDM

(Wavelength Division Multiplexer) for the introduction of light from said pump light source(s).

24. The fiber laser of claim 23, wherein at least one of said WDM is placed in said cavity.

25. The fiber laser of claims 22 or 23, wherein at least one of said WDM is placed outside said cavity.

26. The fiber laser of any of the preceding claims, wherein said first and/or second reflector comprises a non-linear reflector.

27. The fiber laser of claim 26, wherein said non-linear reflector is saturable absorbable mirror (SAM) comprising a reflectorreflective surface, such as a semi-conductor saturable absorbable mirror.

28. The fiber laser of claim 27, wherein said SAM is connected to the cavity by a connector unit.

29. The fiber laser of claim 28, wherein said connector unit comprises a hybrid adaptor where the fiber connector is inserted in one side and the SAM mount on the other side, so that the fiber-tip is butt coupled to the

SAM

30. The fiber laser of any of claims 27 to 29 where the angle of incident light to the normal of the reflective surface is |20°| or less, such as |10°| or less, such as |5°| or less, such as |2°| or less, such as |1°| or less, such as |0.5°| or less, such as |0.05°| or less.

31. The fiber laser of any of the claims 29 to 30 where the SAM is glued to the plane surface of the SAM mount.

32. The fiber laser of any of claims 28 to 31 , where said connector unit is an adaptor and a fiber patch-cord where the SAM is fixed to one end of the patch-cord.

33. The fiber laser of any of claims 28 to 32, where the SAM is glued on one end of the patch-cord.

34. The fiber laser of any of the preceding claims further comprising one or more components for monitoring the performance of the fiber laser providing at least one monitor signal.

35. The fiber laser of any of the preceding claims where in said switch comprises two or more output ports, wherein at least two of said output ports of said optical switch are connected to substantially identical optical elements to enable extension of the life time of the fiber laser.

36. The fiber laser of claim 34, wherein said optical element comprise a

SAM, such as a semi-conductor saturable absorbable mirror.

37. The fiber laser of any of claims 34 and 36, wherein said optical element comprises a pump light source, such as said pump light source.

38. The fiber laser of any of the claims 34 to 37, wherein said optical element comprises a gain medium.

39. The fiber laser of any of the preceding claims, further comprising at least one life time indicator.

40. The fiber laser of any of claims 34 to 39, comprising a trigger for triggering said optical switch based on one or more indicators for indicating the remaining life time of one or more of the components of the laser and/or based on the at least one monitor signal.

41. The fiber laser of any of the claims 34 to 40, wherein at least one of said optical elements are connected to the optical switch by at least one optical fiber connector selected from the group: an FCC connector (such as FC/PC, FC/APC and FC/UPC), ST connector, SC, LC, Biconic, D4, ESCON, FDDI, Opti-Jack, MT-RJ, MU, SMA, E2000, F3000, LSA and

TOSLINK.

42. The fiber laser of claim 41 , wherein said optical fiber connector is a fiber-to-fiber connector.

43. The fiber laser of any of the preceding claims said switch comprising two or more ports, wherein each of at least two ports of said optical switch are connected to optical elements of different optical path length.

44. The fiber laser of claim 43, wherein at least one of said optical elements comprise an optical fiber with low non-linearity, such as a fiber with a larger mode-field diameter of the light. 3D

45. The fiber laser of any of claims 43 and 44, wherein at least one of said optical elements comprises an optical path, said path comprising a section with anomalous dispersion.

46. The fiber laser of claim 45, wherein said section comprises microstructured optical fiber and/or holey fiber and/or photonic bandgab fiber.

47. The fiber laser of any of claims 45 and 46, wherein said section comprises one or more free-space gratings.

48. The fiber laser of any of the preceding claims wherein at least two output ports of the optical switch are connected to optical elements comprising dissimilar spectral response.

49. The fiber laser of claim 48, wherein said dissimilar spectral response comprises different center wavelength.

50. The fiber laser of any of claims 48 and 49, wherein said dissimilar spectral response comprises different spectral width.

51. The fiber laser of any of claims 48 to 50, wherein the spectral width of the spectral response of at least one of said optical elements is more than 1 % of the 3dB spectral width of the gain medium, such as more than 3%, such as more than 5%, such as more than 10%, such as more than 20%, such as more than 50%, such as more than 75%, such as more than 100%, such as more than 200%.

52. The fiber laser of any of claims 48 to 51 , wherein at least one of said optical elements comprises one or more of the following: a Bragg grating, saturable absorbable mirror, a semi-conductor saturable absorbable mirror, a mirror, a broad band mirror and a thin film filter.

53. The fiber laser of any of claims 48 to 52, wherein at least one of said optical elements comprises a Faraday rotator and a Faraday mirror.

54. The fiber laser of any of claims 48 to 53, wherein at least one of said optical elements comprise a component with anomalous dispersion

55. The fiber laser of any of claims 48 to 54, wherein said component comprises a microstructured optical fiber and/or holey fiber and/or photonic bandgab fiber.

56. The fiber laser of any of claims 48 to 55, wherein said optical element comprises a gain medium, such as the gain medium of the laser.

57. The fiber laser of any of claims 48 to 56, comprising two or more pump light source, such as a pump light source for each optical element.

58. The fiber laser any of claims 48 to 57, further comprising a unit for switching light from one or more pump light sources between one or more Bragg gratings.

59. The fiber laser any of claims 48 to 58, wherein said fiber laser comprises at least one pump light source and further comprises at least one WDM (Wavelength Division Multiplexer) for the introduction of light from said pump light source(s).

60. The fiber laser of any of the preceding claims said switch comprising two or more ports comprising at least one input port and one output port, wherein at least one of said ports form at least part of the output of the laser and at least one input port and at least one output port are part of the cavity of the fiber laser. 3?

61. The fiber laser of any of the preceding claims, comprising two or more optical switches each arranged as the switch of any of the claims 35 to 60.

62. The fiber laser of any of the preceding claims, comprising an M X N optical switch where each of the M inputs is arranged as the switch of any of the claims 35 to 61.

63. A fiber laser comprising a first reflector and a second reflector forming a cavity comprising at least one gain medium wherein said cavity further comprises at least one optical fiber connector.

64. The fiber laser of 63, wherein said optical fiber connector comprises a connector selected from the group: FCC connector (such as FC/PC, FC/APC and FC/UPC), ST connector, SC, LC, Biconic, D4, ESCON, FDDI, Opti-Jack, MT-RJ, MU, SMA, E2000, F3000, LSA, and TOSLINK.

65. The fiber laser of any of the claims 63 and 64, further comprising any feature of claims 1 to 62.

66. A method of extending the life time of a fiber laser comprising a first reflector and a second reflector forming a cavity, said cavity comprising at least one gain medium and said cavity further comprises at least one optical switch comprising two or more ports, wherein at least two ports of said optical switch each are connected to substantially identical optical elements to enable extension of the life time of the fiber laser.

67. The method of claim 66, further comprising

a. operating the laser in connection with at least one of said optical elements,

c. comparing said established life time with a preset value, and

d. activating said optical switch to a second optical element when the established life time is equal to or less than said preset value.

68. The method of any of the claim 66 or 67 comprising exchanging, refurbishing, repairing, and/or replacing an optical element while the laser is in optical communication with a second optical element through said switch.

69. The method of any of the claims 66 to 68 wherein at least one of said optical elements are connected to the optical switch by at least one optical fiber connector comprising at least one of: an FCC connector (such as FC/PC, FC/APC and FC/UPC), ST connector, SC, LC, Biconic, D4, ESCON, FDDI, Opti-Jack, MT-RJ, MU, SMA, E2000, F3000, LSA, and TOSLINK.

70. The fiber laser of claim 69, wherein said optical fiber connector is a fiber-to-fiber connector.

71. The method of any of the claims 68 to 70 further comprising any of the features of claims 1 to 67.

72. A method of changing at least one property of the output light of a fiber laser, said fiber laser comprising a laser cavity and a switch distributing light entering on one or more input ports to one or more output ports of said switch, where at least two of said input and/or output ports are connected to optical elements of different optical property/properties, wherein at least one of said optical elements forms part of the laser cavity and said method comprising activating said switch to alter the distribution of light between said input ports and output ports of the switch.

73. The method of claim 72, wherein all light entering said input ports is guided to a single output port and/or wherein all light entering said output ports is guided to a single output port.

74. The method of any of claims 72 to 73, wherein said optical property comprises optical path length in order to alter the repetition rate of the output light.

75. The method of any of claims 72 to 74, further comprising any of the features of claims 43 to 47.

76. The method of any of claims 72 to 75, wherein said optical property comprises spectral response in order to alter the wavelength and/or pulse length of the output light.

77. The method of any of claims 72 to 76, further comprising any of the features of claims 48 to 59.

78. A pulsed laser wherein the output of the laser is split into at least two arms by a first coupler and a second coupler combining the light of one or more arms, wherein said at least two arms comprises unequal optical path length.

79. The pulsed laser of claim 78, wherein at least one arm comprises a unit for disrupting the path from said first coupler to said second coupler.

80. The pulsed laser of claim 79, wherein said unit for disrupting comprises and optical switch.

81. The pulsed laser of any of claims 78 to 80, further comprising any of the features of claims any of the preceding claims.

82. A method of altering the repetition rate of output light of a pulsed laser, said method comprising splitting the output light into at least two arms by a first coupler and combining the light of one or more arms by a second coupler, wherein said arm comprises unequal optical path length and thereby altering the repetition rate of the laser.

83. The method of claim 82, further comprising the step n of controlling said alteration by activating at least one unit for disrupting the path from said first coupler to said second coupler in at least one of said arms.

84. The method of any of the claims 82 to 83 wherein the laser comprises any of the features of claim 1 to 81.

85. A method of connecting an optical element comprising an optical connector to the cavity of a fiber laser comprising an optical connector, said method comprising mating said optical connector of the cavity with said optical connector of the optical element.

86. The method of claim 85 further comprising refurbishing, altering, assembling and/or upgrading an optical element of said fiber laser wherein said optical element is connected to the cavity of the via optical connectors.

87. The method of any of claims 85 to 86, further comprising any of the features of claims 63 to 65.

88. A method of providing flexibility in the repetition rate of a fiber laser comprising a switch, said switch comprising two or more ports comprising at least one input port and one output port, wherein at least one of said ports form at least part of the output of the laser and at least one input port and at least one output port are part of the cavity of the fiber laser, and said method comprising activating said switch to couple at least part of a pulse to the output.

89. The method of claim 88 further comprising any of the features of the preceding claims.

90. An optical system product comprising any of the features of claims 1 to 89.