Patent Application
20180261970
The invention generally relates to pulsed fiber lasers and in particular to an arrangement for reducing the number of amplifier pumps required to energize gain sections in fiber lasers which incorporate nonlinear amplified loop mirrors.
A nonlinear amplified fiber optic loop mirror (NALM) can be used as a means of mode locking fiber optic lasers either in the normal or the anomalous dispersion regime. However, laser incorporating a NALM are restricted in power because a NALM may operate only at particular power levels. In order to increase the power of pulses output from a laser with a NALM, it is conventional to amplify the laser by using one or more additional gain sections and pump source at what may be considerable additional cost and complexity.
It is an object of the present invention to go at least some way toward overcoming the limitations of the prior art, or to provide the public with a useful choice. Other objects of the invention may become apparent from the following description which is given by way of example only.
According to one embodiment, the invention consists in a laser apparatus comprising: a first component loop comprising at least a first gain component; a second loop comprising a fiber nonlinear amplified loop mirror (NALM), the NALM comprising a second gain component and coupled to the first component loop by a bidirectional optical coupling component; a first pump source operatively coupled to the first gain component in the first loop; a third gain component; a second pump component coupled to the third gain component then the second gain component and operative to sequentially pump both the third and second gain components; and wherein the second pump source is operable to pump the third gain component in excess of saturation such that surplus pump light is provided to the second gain component.
In some embodiments, the third gain component is located in the first component loop.
In some embodiments, the second pump source is configured to couple to the third gain component, be transmitted through the bidirectional optical coupling component to the second gain component.
In some embodiments, the first component loop further comprises an output coupler configured to couple at least some light to an output from the first component loop, and the third gain component is operatively couple to the output.
In some embodiments, the second pump source is configured to couple to the third gain component, be transmitted through the output coupler and bidirectional optical coupling component to the second gain component.
In some embodiments, the laser is constructed from all fiber components;
In some embodiments, the first loop further comprises: an optical isolator component; a length of single mode fiber; an output coupler component; and an optical filter component.
According to another aspect the invention consists in a method of operating a laser apparatus comprising a first component loop comprising at least a first gain component; a second loop comprising a fiber nonlinear amplified loop mirror (NALM), the NALM comprising a second gain component and coupled to the first component loop by a bidirectional optical coupling component; a first pump source component operatively coupled to the first gain component in the first loop; a third gain component; and a second pump source component coupled to the third gain component then the second gain component and operative to sequentially pump both the third and second gain components; wherein the method comprises: energizing the first pump source to supply pump light to the first gain component; and energizing the second pump component to supply pump light to the second and third gain components, the third gain component pumped in excess of saturation such that surplus pump light is provided to the second gain component.
According to another embodiment, the invention consists in a laser apparatus operable to generate giant chirp pulses, comprising optical fiber based components arranged to form a loop, the components comprising: a first amplifying section comprising a first gain section, a first pump source and a coupler configured to couple light from the first pump source to the first gain section, an optical isolator, a length of single mode fiber, a nonlinear amplified loop mirror passive mode locking device consisting of a gain section and a coupler configured to couple the mirror to the loop, a second amplifying section comprising a gain section, a second pump source and a coupler configured to couple light from the second pump source to the second gain section, an output coupler, and an optical filter, wherein the optical filter is configured between an output of the passive mode locking device and an input of the first gain medium to close the loop; and wherein the nonlinear amplified loop mirror is arranged to receive pump light from the second pump source, located outside of the NALM loop, in a bidirectional configuration.
According to another embodiment, the invention consists in a laser apparatus operable to generate giant chirp pulses, comprising optical fiber based components arranged to form a loop, the components comprising: a first amplifying section comprising a first gain section, a first pump source and a coupler configured to couple light from the first pump source to the first gain section, an optical isolator, a length of single mode fiber, a nonlinear amplified loop mirror passive mode locking device consisting of a gain section and a coupler configured to couple the mirror to the loop, an output coupler, and an optical filter connected to the input of the first amplifying section to close the loop, wherein the laser further comprises, on the outside of the loop, a second amplifying section comprising a gain section, a second pump source and a coupler configured to couple light from the second pump source to the second gain section, and wherein the nonlinear amplified loop mirror is arranged to receive pump light from the second pump source in a bidirectional configuration.
According to another embodiment, the invention consists in a laser apparatus comprising: a nonlinear amplified loop mirror; and a pump source located outside for the loop mirror configured to transmit pump light into the loop mirror.
A laser apparatus wherein the second pump source is configured to pump a gain section located outside of the nonlinear amplified loop mirror.
According to another embodiment, the invention consists in a laser apparatus operable to generate giant chirp pulses, comprising: optical fiber based components arranged to form a loop, the components arranged in a unidirectional sequence of: a first gain medium, a length of single mode fiber, a nonlinear loop mirror passive mode locking device consisting of a gain section and a coupler configured to couple the mirror to the loop, a second amplifying section comprising a gain section, a second pump source and a coupler configured to couple light from the second pump source to the second gain section, an output coupler, and an optical filter optically coupled to the first gain medium to close the unidirectional sequence, wherein the loop further comprises an optical isolator located between the first gain medium and the passive mode locking device in the unidirectional sequence, and wherein the optical isolator is oriented to configure the giant chirp pulses to follow the unidirectional sequence, and wherein the nonlinear amplified loop mirror is arranged to receive pump light from at least the second pump source in a bidirectional configuration.
According to another embodiment, the invention consists in a laser apparatus operable to generate giant chirp pulses, comprising: optical fiber based components arranged to form a loop, the components arranged in a unidirectional sequence of: a first amplifying section comprising a first gain section, a first pump source and a coupler configured to couple light from the first pump source to the first gain section, a length of single mode fiber, a nonlinear loop mirror passive mode locking device consisting of a gain section and a coupler configured to couple the mirror to the loop, a second amplifying section comprising a gain section, a second pump source and a coupler configured to couple light from the second pump source to the second gain section, an output coupler, and an optical filter optically coupled to the first gain medium to close the unidirectional sequence, wherein the laser further comprises, on the outside of the loop, a second amplifying section comprising a gain section, a second pump source and a coupler configured to couple light from the second pump source to the second gain section, and wherein the nonlinear amplified loop mirror is arranged to receive pump light from at least the second pump source in a bidirectional configuration.
As used herein the term “and/or” means “and” or “or”, or both. As used herein “(s)” following a noun means the plural and/or singular forms of the noun. The term “comprising” as used in this specification means “consisting at least in part of”. When interpreting statements in this specification which include that term, the features, prefaced by that term in each statement, all need to be present but other features can also be present. Related terms such as “comprise” and “comprised” are to be interpreted in the same manner.
The entire disclosures of all applications, patents and publications, cited above and below, if any, are hereby incorporated by reference.
To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting.
In this specification, where reference has been made to external sources of information, including patent specifications and other documents, this is generally for the purpose of providing a context for discussing the features of the present invention. Unless stated otherwise, reference to such sources of information is not to be construed, in any jurisdiction, as an admission that such sources of information are prior art or form part of the common general knowledge in the art.
The invention will now be described by way of example only and with reference to the drawings in which:
A NALM is a device comprising a gain section and a central optical coupler to which it is optically connected to other optical components. Conventional practice is to provide pump light to the gain section of the NALM from a wavelength division multiplexing (WDM) coupler which is configured inside the NALM loop.
The laser shown in
A gain component can be provided by a number of different devices such as a single mode fiber, Ytterbium (Yb), an Erbium (Er), Neodymium, Holmium other rare-earth doped fibers. Those skilled in the art will appreciate the particular gain component used will be related to the desired output wavelength of the pulses to be generated and sustained by the laser cavity. Further, those skilled in the art will appreciate a pump source characteristics required to enable a particular fiber to act as a gain component will depend on the particular characteristics of the gain component selected.
For example, if the desired output wavelength is around 1 micrometer, the potential rare-earth doped fibers gain component that are operable to provide amplification at this wavelength are Ytterbium, or Neodymium doped fibers as they provide amplification of light around 1 micrometer. For example, if the desired output wavelength is around 1.5 micrometers, an Erbium-doped fiber would provide amplification around this wavelength provided that suitable normal dispersion fibers at that wavelength were used. Similarly, if the desired output wavelength is around 2 micrometers, a Thulium or Holmium-doped fiber would provide amplification. A rare earth-doped fiber may also be considered to be a preferable gain component as they usually provide greater amplification compared to a single-mode fibers (using Raman or parametric gain).
One limitation of the laser of
Further, gain fibers should be pumped by light with a wavelength not more than 0.5 nm from the optimum gain fiber absorption wavelength for efficient operation. Pump sources generate significant amounts of heat and therefore require strict temperature stabilization in order to prevent the output frequency from drifting. In addition to avoiding cost and additional components to support additional pumps, it is advantageous to avoid further pump sources in what is often a compact laser product, which may suffer detrimentally from too many heat generating sources within a given enclosure.
In embodiments of the invention discussed herein, a laser is provided that is operable to generate much more power than the laser depicted by
In embodiments of the invention discussed herein, there is a laser that includes a NALM and pump light is provided to a NALM through the central optical coupler used to connect the NALM to other optical components in the laser. Further, that pump light is shared with one or more other gain sections located outside of the NALM. The NALM therefore does not have a WDM coupler within the NALM loop itself or pump light provided directly to the gain section in the NALM itself.
As mentioned, one advantage embodiments of the invention provide is avoiding the requirement for an additional pump source and WDM coupler dedicated to the gain section of the NALM. A pump associated with an external amplifier, which is always required at the output of the laser to amplify the output pulses, is also used to supply the gain component of the NALM whilst also pumping the amplifier at the laser output. However, other significant advantages are also realized and are discussed below. The advantages can be realized either by adding a WDM and associated gain section in the main laser loop instead of in the NALM loop as discussed below with reference to the embodiment depicted by
In a first exemplary embodiment, there is provided a fiber loop laser having three gain sections in the loop pumped by two pump sources, where one of the three gain sections forms part of a NALM. In a second exemplary embodiment, there is provided a fiber loop laser having two gain sections inside a loop, where one of these two gain sections forms part of a NALM, and a third gain section outside the loop. The three gain sections are again energized by two pump sources.
In the first exemplary embodiment, a loop laser is provided comprising a first gain section, a NALM comprising a second gain section, and a third gain section. The first and third gain sections are located outside the NALM. The first and third gain section are configured to receive pump light from respective pump sources via a WDM coupler in the conventional way. However, the direction of the pump source supplying the first and third gain sections is orientated such that pump light exiting the first and third gain sections propagates through the loop in opposing directions, and toward the NALM.
However, optimal performance is realized when the first gain section is pumped below saturation and no or minimal pump light exists the first gain section. This is to avoid phenomena such as multiple pulse propagation and Q-switching. Further, the third gain section is pumped with surplus light to ensure saturation of the third gain section and to ensure sufficient pump light exits the third gain section to be split by the central coupler of the NALM and provides bidirectional pumping of the second gain section inside the NALM. This arrangement not only ensures bidirectional pumping of the NALM gain section, but also provides for increased power output from the laser itself, as the pulse exiting the NALM is amplified before the output coupler.
Bidirectional pumping of the gain section in the NALM leads to the abovementioned additional advantages—it has been discovered that operational performance of the NALM is improved due to uniform population inversion inside the gain section of the NALM. A further advantage is that the NALM loop can be shortened due to the improved operational performance.
It should be understood that the location of the isolator 43 could be anywhere in the loop, however it is most preferable to locate the isolator between the first gain section 41 and the third gain section 60 relative to the propagation direction of the loop. Further, the filter 45 could also be placed anywhere in the loop, however it is preferable to locate the filter after the output coupler, and between the third gain component 60 and the first gain component 41 relative to the propagation direction of the loop.
The component arrangement of the laser depicted in
The length of the third gain component 60 can vary from zero to several meters depending on the doping level of the fiber and the output power desired. It is essential to adjust the pump power though the pump coupling component so that the third gain component 60 is oversaturated such that there is sufficient pump energy passed through the third gain medium and transmitted to energize the NALM 50. That is, so that the second pump 61 sequentially pumps the third then second gain components.
It should be understood that the location of the isolator 43 could be anywhere in the main loop 40.
For each of the above described embodiments, the central coupler 30 provided as part of the NALM 50 can be from 50/50 to 60/40 in split ratio.
The third gain component 60 is optionally added to the laser to increase the output power. The third gain component 60 can be located in either the main laser loop 40 as shown in
Putting the third gain component 60 inside the main laser loop 40 enables all the pump power from the pump configured to pump the third gain component 60 to be used for the third and second gain components, but can lead to excessive power circulating in the main laser loop 40.
Putting the third gain component outside the laser loop and separating the preamplifier from the laser can enable higher output power, but some of the second pump laser power is then lost though the output coupler (but typically only 20% if the output coupler 44 is 80/20). Both configurations retain the advantage of using only one pump laser to power the NALM loop and the third gain component 60.
The third gain component can be between zero and several meters in length depending upon the dopant level in the gain fiber and the power of the pump laser. Once again, the pump source for the third gain component is configured to pump the third gain component beyond saturation to ensure ample pump light is emitted from the third gain section and to the NALM gain section 51.
It should be noted that pump light provided to the NALM 50 from the second pump source 61 may be redirected toward and re-enter the second pump 42 after propagating through the NALM 50. In some embodiments, an isolator may be present on the output of the second pump component to prevent re-entry of pump light.
It should further be noted that start-up and operational stability are optimized when the power levels of the two pump sources 61, 42 are controllable. Whilst further variations of the above described embodiments are envisaged where the output of a single pump source is split a directed to two or more fiber gain sections, it is noted that control over startup and operational stability is compromised. However, locating a variable attenuation device on each split from a single pump source may go at least some way toward recovering control over optimum startup and operational stability.
Where an adjustable attenuator 71 is used, as depicted, it may be in either path, and the magnitude of attenuation balanced with the output power of the pump 70 to thereby provide the desired output power to each of the first and second pump paths. Whilst dependent on the particular gain components used, the second pump path will typically require the most pump energy since that energy is provided to two gain mediums, whereas the first pump source 42 supplies only a single gain component. Therefore, it may be most advantageous to locate the attenuator in the pump path that requires the least energy, that is, the first pump path. Similarly, an attenuator may be located at each output of the splitter for control of the pump power provided to each pump source. However, it is most important that stable pumping of the laser operation is stable and that is achieved by accurate control of the pump energy provided to each pump path. Therefore an attenuator 71 may be located in either pump path.
It is further envisaged that a single pump may be divided into three or more paths and provided independently to each gain component, typically with an adjustable attenuator in at least two paths, or at least splitter ratio matched to the pump energy requirements of each gain medium. However, the advantages of the invention in this configuration are diminished by the requirement for additional splitter and attenuator components, and the increased complexity of balancing three pump source paths.
Where in the foregoing description reference has been made to elements or integers having known equivalents, then such equivalents are included as if they were individually set forth. Although the invention has been described by way of example and with reference to particular embodiments, it is to be understood that modifications and/or improvements may be made without departing from the scope or spirit of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 729928 | Mar 2017 | NZ | national |
