WO2011160234A2 - Active optical device component with large area bragg grating - Google Patents

Abstract

The large area bragg grating can be inscribed on a cladding or a large core, for instance, using ultrashort pulses of light and cover a cross-sectional area of over 500 μm².

Description

ACTIVE OPTICAL DEVICE COMPONENT WITH LARGE AREA BRAGG

GRATING

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of United States provisional application 61/357,575 filed 23 June 2010.

BACKGROUND

[0002] It was known to inscribe fiber bragg gratings in small cross-sectional area cores of optical fibers using the ultraviolet inscription technique. Attempts of using this inscription technique on larger areas met limited success.

[0003] For instance, in order to reduce the fiber length of double-clad fiber lasers, there has been attempts made to place a pump light reflector at the end of the fiber length in order to reflect residual pump light back in the fiber in reversed propagation. In Seungin, B. et al. "A Cladding-Pumped Fiber Laser With Pump-Reflecting Inner-Cladding Bragg Grating", IEEE Photonics Technology Letters, Vol. 16, No. 2, Feb. 2004, an optical fiber especially designed for this application was used. The ring cladding contains a relatively high level of germanium oxide in order to provide photosensitivity in that region, thereby allowing ultraviolet inscription with techniques known in the art. A pump reflectivity of only about 46.5% was achieved. It is noted however that it is difficult to write a FBG in a large photosensitive structure such as the ring cladding of Seungin because the index-changing radiation is absorbed over a very short distance in the photosensitive material. It is then difficult to achieve a good reflectivity for all modes of the pump light since the FBG may not be written over the entire section of the germanium cladding. Furthermore, doping a large cross- sectional region of optical fiber with a photosensitive element such as germanium can represent control challenges and adds cost and complexity to the manufacturing process of the optical fiber.

[0004] There thus remained room for improvement, in particular, there remained a need for a solution to inscribing Bragg gratings achieving better reflectivity on large areas.

SUMMARY

[0005] A solution is to inscribe Bragg gratings using ultra short pulses of light. [0006] This solution gives rise to several applications : a fiber Bragg grating can be inscribed in the cladding to reflect pump light and thus reduce the length of an active fiber, or to prevent incoming signal leaks from interfering with the pump light source; or fiber Bragg gratings can be inscribed in a large core of an active fiber to define a laser cavity, or in a large core of a passive fiber to lock the wavelength of the pump light source, for instance.

[0007] Henceforth, in accordance with one aspect, there is provided a method of making an active optical device component comprising inscribing a Bragg grating on a cross- sectional area of at least 500 μιη² of a longitudinally-extending waveguide section of the active optical device component by exposing the waveguide section to pulses of light in the picosecond or smaller range. The waveguide section can be a large core or a cladding of an active or passive optical fiber for instance.

[0008] In accordance with another aspect, there is provided an active optical device component comprising a longitudinally-extending waveguide, having a Bragg grating having a reflectivity of at least 50% at a central wavelength, over at least 500 μιη2 cross-sectional area of the waveguide section.

[0009] In accordance with another aspect, there is provided an active optical device component comprising a pump light source optically coupled to a longitudinally-extending waveguide section having a Bragg grating over a cross-sectional area of at least 500 μιη² of the waveguide section wherein the Bragg grating has a central wavelength and a reflectivity configured and adapted to lock and narrow a spectral emission of the pump light source.

[0010] In this specification, the expression active optical device refers to optical devices having a gain medium, such as amplifiers, lasers and other sources such as fiber lasers, fiber amplifiers and amplified spontaneous emission (ASE) sources. The component of such devices where the Bragg grating is inscribed can be a large core passive optical fiber connecting the light source to the active optical fiber of the device, or the active optical fiber itself, for instance. The core of the active optical fiber is typically rare-earth doped to provide the gain medium. Large waveguide cross-sectional areas are typically multi mode at wavelengths of interest. The expression waveguide section can refer to a cladding, a core, or both of an optical fiber for instance, or to another form of waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

[001 1] Fig. 1 is a schematic view of an example of a fiber laser which uses a Fiber Bragg Grating (FBG) cladding reflector inscribed using ultrashort pulse light; [0012] Fig. 2 is schematic cross-sectional view of a double-clad optical fiber used in the fiber laser of Fig.1 ;

[0013] Fig. 3 is a graph showing the refractive index profile of the optical fiber of Fig.2; and

[0014] Fig. 4 is a schematic view of a system for inscribing a FBG cladding reflector in a double-clad optical fiber in accordance with one embodiment.

[0015] Fig.5 is a schematic view of an amplification portion of a fiber amplifier which uses a FBG cladding reflector in accordance with another embodiment;

[0016] Fig. 6 is a schematic view of an amplification portion of a fiber amplifier wherein a FBG reflector is used in the pump delivery fiber to lock the emission wavelength of the pump source, in accordance with another embodiment;

[0017] Fig.7 is a schematic view of an amplification portion of a fiber laser wherein FBG signal reflectors are inscribed in the large and highly multimode core of the amplification fiber, in accordance with another embodiment;

[0018] Fig. 8 is a schematic view of an amplification portion of a fiber laser combining a FBG reflector is used in the pump delivery fiber to lock the emission wavelength of the pump source, FBG signal reflectors are inscribed in the large and highly multimode core of the amplification fiber and a FBG cladding reflector is used, in accordance with another embodiment;

[0019] Fig. 9 is a schematic view of an amplification portion of a fiber amplifier wherein a FBG cladding reflector is used to reflect signal light leaking in the cladding in order protect the pump source, in accordance with another embodiment

[0020] Fig. 10 is a variant of the amplifier shown in Fig. 5, with a FBG in a passive extension of an active optical fiber.

[0021] It will be noted that throughout the appended drawings, like features are identified by like reference numerals.

DETAILED DESCRIPTION

[0022] Now referring to the drawings, Fig. 1 shows a fiber laser 10 which uses a Fiber Bragg Grating (FBG) cladding reflector 22. The fiber laser 10 comprises a rare-earth doped double-clad optical fiber 12 which is used as a gain medium. The optical fiber 12 is pumped with pump light provided by a pump source 14 and coupled to optical fiber 12 using a pump coupler 16. In this case, pump light is carried in an optical fiber at the output of the pump source 14. The pump coupler 16 may consist of a fused fiber coupler or a fusion splice for example. A first FBG signal reflector 18 is placed at the input of optical fiber 12 and a second FBG signal reflector 20 is placed near the output of the optical fiber 12 to form a laser cavity 24. FBG signal reflector 18 is a high reflectivity grating and FBG signal reflector 20 has a lower reflectivity and is used as a collection mirror at the output of laser cavity 24 in order for the signal light to exit the laser cavity 24. Both FBG signal reflectors 18, 20 have central wavelengths corresponding to the wavelength of the laser signal and are inscribed in the optical core of optical fiber 12 as known in the art. The fiber laser 10 also comprises a FBG cladding reflector 22 which is used to reflect pump light at end of the optical fiber length.

[0023] Pump light is said to be in forward injection because it propagates in the same direction as signal light exiting fiber laser 10.

[0024] Pump light overlaps the rare-earth-doped core of the optical fiber 12 and is therefore absorbed by as it propagates along the optical fiber 12 for amplification of signal light propagating in the optical core. Pump light has its maximum power at the input of the optical fiber 12. Pump light power decreases along the optical fiber 12 as it is absorbed. Pump power needs to be absorbed in order to provide amplification gain for signal light. However, in double-clad optical fibers, the pump light overlap with the rare-earth core is small and the absorption rate of the pump light is low. Laser cavities with large optical fiber lengths are thus required for optimum results.

[0025] The FBG cladding reflector 22 is used to decrease the length of amplification fiber 12 in laser cavity 24. The FBG cladding reflector 22 reflects pump light that remains unabsorbed at the end of laser cavity 24 back in the amplification fiber 12 in backward propagation. Pump power is then allowed to be absorbed more by propagating back again in the amplification fiber 12. This can allow to reduce the length of optical fiber in the laser cavity 24 significantly. This can also allow a more uniform pumping throughout the length of the amplification fiber 12.

[0026] It is noted that even though the fiber laser is illustrated herein with forward injection of pump power, an FBG cladding reflector 22 may also be used with backward injection of pump power, if placed at the end of the optical fiber that is opposite to output end where signal light exits, for instance. [0027] One will understand that various fiber lasers based on the scheme illustrated in Fig. 1 may be designed for generation of various signal wavelengths and power. The scheme illustrated in Fig. 1 is meant to illustrate a general configuration that can be customized to be adapted to various specific applications. The specifications of the amplification fiber 12, the pump source 14, the coupler 16, the FBG signal reflectors 18, 20 and the FBG cladding reflectors may be selected as a function of various possible fiber laser designs.

[0028] Figs. 2 and 3 illustrate a refractive index profile of a double-clad optical fiber 30 for use as the amplification fiber in a fiber laser such as the one described above with reference to Fig. 1. Fig. 2 is a schematic cross-sectional view of optical fiber 30 while Fig. 3 is a graph illustrating the refractive index profile of optical fiber 30.

[0029] The optical fiber 30 has a central rare-earth-doped optical core 32 extending along the propagation axis of the optical fiber 30. An inner cladding, called the pump-guide cladding 34 radially surrounds the core 32. An outer cladding 36 radially surrounds the pump-guide cladding 34. The optical core 32 is generally made of rare-earth-doped silica. The presence of rare-earth ions in the silica matrix increases the refractive index of the core 32. The pump-guide cladding 34 is generally made of undoped silica glass and has a refractive index lower than that of the core 32 for guiding signal light within the core 32. The outer cladding 36 is made of silica glass or polymer and has a refractive index that is lower than that of the pump-guide cladding 34 for guiding pump light within the pump-guide cladding 34.

[0030] In operation, signal light is propagated in core 32 while pump light is propagated in the pump-guide cladding 34. Propagation of pump light in pump-guide cladding 34 is multi- mode. The propagated modes overlap rare-earth-doped core 32 and are absorbed by rare- earth ions as they propagate, which allows amplification of signal light propagating in core 32.

[0031] In one embodiment, the rare-earth-doped core 32 is single-mode but double-clad fibers may also have multi-mode cores or large diameter cores for applications including high power fiber lasers or amplifiers for example.

[0032] In the illustrated embodiment, the pump-guide cladding 34 has a circular cross- section but it is noted that the pump-guide cladding may also have a different cross-sectional shape. For example, an octagonal shape may be used to avoid propagation of helical rays that are not absorbed by the core. [0033] Fig. 4 illustrates an example of a system 40 for inscribing a FBG cladding reflector 42 in a double-clad optical fiber such as double-clad optical fiber 30. The system 40 uses ultrashort pulse light for inscribing the FBG in the cladding instead of standard continuous wave or large pulse ultra-violet light inscription.

[0034] Using ultrashort pulse light allows for inscription in the pump-guide cladding 34 without necessitating the addition of photosensitive dopants. The region where the FBG is inscribed in the optical fiber is determined by the intensity of the ultrashort pulse light incident to the optical fiber instead of being determined by the presence of photosensitive dopants. By focusing ultrashort pulse light on the pump-guide cladding 34, the FBG is inscribed in this region.

[0035] Also, the absorption rate of ultrashort pulse light as it propagates transversally in the optical fiber 30 is quite low compared to absorption rate of ultra-violet light in photosensitive material. The inscription made with ultrashort pulse light may then penetrate deeper in the optical fiber 30, thereby allowing inscription of FBG in a larger cross-sectional area of the optical fiber 30.

[0036] It is noted that Fig.4 is a schematic provided for illustration purposes only and that the relative proportions of the illustrated components may not be representative. The system 40 comprises an ultrashort pulse laser source 44 for generating ultrashort pulse light with ultrashort duration pulses, a cylindrical lens 46 for focusing the ultrashort pulse light to the pump-guide cladding 34 and a diffractive element 48, in this case a phase mask, for generating an interference pattern with a periodic intensity variation along the longitudinal axis of optical fiber 30. The optical fiber 30 is positioned in the interference pattern generated by the diffractive element 48 for FBG inscription.

[0037] The laser source 44 generates ultrashort pulse light with sufficient short pulsewidth and peak power to modify the refractive index in an exposed optical glass to produce an index modulation pattern required to obtain a FBG. A light source 44 with pulse duration in the picosecond range or smaller and with energy in the microjoule range or higher may be used. For instance, the laser source 44 may be a Titanium-Sapphire laser source producing ultrashort pulse light at 800 nm with energy of 1 mJ, duration of about 40 fs with a repetition rate of about 1 kHz. Other source wavelengths, such as 400 nm, may be used. For example, the second harmonic of a Titanium-Sapphire laser generated using a non-linear crystal such as the beta-BaB₂0₄, may be used. Characteristics of the ultrashort pulse light are adjusted to provide the index modulation step, intensity and diffraction order required for the FBG reflector to be produced.

[0038] The ultrashort pulse light generated by the laser source 44 is aligned to reach the cylindrical lens 46 which is used for focusing the ultrashort pulse light in a direction transversal to the optical fiber 30 and toward the pump-guide cladding 34 in which the FBG cladding reflector 42 is to be inscribed. Light remains unfocused in the longitudinal direction of the optical fiber 30. The cylindrical lens 46 is typically positioned relative to the optical fiber 30 such that the pump-guide cladding 34 of the optical fiber 30 is at the focus of the laser light such that the FBG is specifically inscribed in this region.

[0039] It is noted that the polymer outer cladding can be removed from the optical fiber before inscribing the FGB and replaced after the inscription is completed.

[0040] The diffractive element 48 is placed between the cylindrical lens 46 and the optical fiber 30 and very close to the optical fiber 30 for efficient inscription. The diffractive element 48 is typically a phase mask having a period selected to cause the inscription of a FBG reflector in the pump-guide cladding 34 and which reflects pump light. The period of the phase mask is selected knowing that the central reflectivity wavelength, or Bragg wavelength A_bragg, of the FBG is related to the period of the index modulation pattern as follows: A_bragg = 2 rie_ff Λ where n_eff is the effective index of the modes propagating in the optical fiber 30 and Λ is the period of the index modulation produced in the optical fiber.

[0041 ] It is noted that only the main components of the system 40 are illustrated in Fig.4 and that a complete system for producing FBG reflectors should also include all mechanical components required for mounting and adequately aligning ultrashort pulse laser source 42, cylindrical lens 46, phase mask 48 and optical fiber 30 relative to one another, as well as optical choppers used to interrupt light exposure, etc.

[0042] It is noted that, while FBG inscription using ultrashort pulse light is illustrated herein with an inscription method that uses fiber optic side exposure to an interference pattern, other FBG inscription methods may be used as known in the art. For example, point- to-point inscription may be used, wherein instead of using a diffractive element to create an interference pattern, the ultrashort pulse light is rather translated longitudinally along the fiber to produce the index modulation pattern in the optical fiber.

[0043] It is noted that the pump light to be reflected by the FBG cladding reflector 42 comprises multiple modes. Each mode corresponds to a specific effective index in the optical fiber 30. The FBG cladding reflector 42 should then be designed to reflect all or most of these modes. Because the Bragg wavelength A_bragg of the FBG varies with the effective index of the modes, each mode of the pump light should be considered. In one embodiment, FBG cladding reflector 42 is a chirped FBG inscribed using a chirped phase mask for example. The chirped FBG then covers the multiple modes of the pump light. Other FBG designs may also be used to provide a FBG with a wide reflection bandwidth.

[0044] For example, considering a reflection wavelength of 915 nm, a refractive index of silica of about 1 .45 at 915 nm, a cladding numerical aperture of 0.22 and an index change due to FBG inscription of about 16.5E-3, the effective indices of all modes propagating in the pump-guide cladding vary between 1 .451 and 1 .433. As given by A_bragg = 2 n_eff Λ, in order to reflect modes with effective index of 1 .451 , the period Λ of the index modulation should be around 315 nm while reflecting modes with effective index of 1 .433, the period Λ of the index modulation should be around 319 nm. Accordingly, using a chirped modulation period extending from 315 nm to 320 nm, all propagating modes of pump light at 915 nm can be significantly reflected.

[0045] Additionally, in one embodiment, ultrashort pulse light is scanned transversally to the optical fiber using a piezo-electric element, in order for the index modulation to be inscribed to cover the entire cross-section of the pump-guide cladding so as to ensure a cross-sectional index profile which allows reflection of all propagation modes.

[0046] Fig. 5 shows an amplification portion of a fiber amplifier which uses a FBG cladding reflector. As in the fiber laser of Fig. 1 , the amplification fiber is a double-clad optical fiber. Signal 52 is injected to propagate in the core 57 of the amplification fiber. Pump light 50 is injected to propagate in the pump-guide cladding 58 using a pump coupler 51 . Accordingly, pump propagation 56 occurs in the pump-guide cladding 58 of the amplification fiber. In this illustrated example, pump light is injected in co-propagation with the signal 52. The FBG cladding reflector 53 is inscribed near the end 54 of the amplification fiber using ultrashort pulse as described herein above. The FBG cladding reflector 53 has an index modulation period selected to reflect light having a wavelength near or at the pump wavelength such that the signal 52 remains generally unaffected by its propagation through the FBG cladding reflector 53 and exits the amplification fiber at 54. The strength and length of the FBG cladding reflector 53 are selected to maximize the amount of reflected pump light. [0047] Pump light first is absorbed to generate gain in the amplification fiber while co- propagating with the signal 52. Pump light remaining at the end of the amplification fiber, i.e. at the FBG cladding reflector 53, is then reflected at 55 and propagates back in counter- propagation in the amplification fiber. Reflected pump light which is also absorbed by the amplification fiber in counter-propagation and generates more gain for a given length.

[0048] The pump coupler 51 used is in the embodiment of Fig. 5 is a side coupler but it is noted that any other type of pump coupler may be used such as a fiber tapered bundle for instance.

[0049] In one example, the optical fiber is an Ytterbium-doped double-clad optical fiber pumped at 900-980 nm to produce a laser signal light at 1000-1200 nm but various other dopants are possible pumped with pump light with a different wavelength to produce laser light signal at other wavelengths.

[0050] A variant of Fig. 5 is shown in Fig. 10 where a passive extension 94 is provided at an end of the active optical fiber 90, connected by a fusion splice or the like. The FBG cladding reflector can be provided in the passive extension 94 with comparable results.

[0051] It is also noted that FBG cladding reflectors may find other applications such as in Amplified Spontaneous Emission light sources for example.

[0052] It is noted the embodiments described and illustrated herein use double-clad rare- earth-doped optical fibers but it is noted that FBG cladding reflectors may also be inscribed in triple-clad or other multi-clad optical fiber. It is also noted that the system and method described herein for inscribing a FBG cladding reflector in a double-clad optical fiber may also be used to inscribe FBG reflectors in large-core multimode optical fiber. Such optical fibers are used for example for carrying high power pump light from a pump source to a fiber laser or to an amplifier for example.

[0053] Laser diodes are commonly used as pump light sources for fiber lasers. Laser diodes have a large emission bandwidth of several nanometers. The central wavelength of the emission also significantly drifts with temperature with a typical drift of 0.33 nm/°C. FBG reflectors may be used to lock the emission wavelength of such laser diodes which are widely used in telecom applications. Generally, laser diodes are pigtailed with a single-clad multimode fiber of pure silica. Accordingly, the pigtail fiber has large diameter and non- photosensitive composition that does not allow the inscription of a FBG reflector using standard ultraviolet light. However, there is a need to control precisely the emission wavelength of laser diode in order for pumping rare-earth doped amplification fibers in fiber laser or fiber amplifiers for example. For instance, Ytterbium-doped amplification fibers have a narrow absorption band at 976 nm that requires precise tuning of the pump wavelength for efficient absorption when pumping fiber lasers or fiber amplifiers with laser diodes.

[0054] Accordingly, Fig. 6 illustrates another example embodiment wherein a FBG reflector 59 is used to lock the wavelength of a high power laser diode 60 used to pump an amplification fiber 63 amplifying a signal 49. Ultrashort pulse light may be used to inscribe a FBG wavelength locking reflector 59 in the large-core multimode optical fiber pigtail of the laser diode 60 which is directly used as the pump injection fiber 62. Such a FBG wavelength locking reflector 59 may be inscribed in the pump injection fiber 62 with ultrashort pulse light even though the pump injection fiber 62 has a large core diameter and is made of pure silica, i.e. is not photosensitive. In order to provide a wavelength locking reflector, the FBG reflector

59 is inscribed with a partial reflectivity, typically less than 50%, to provide a feedback 61 to the laser diode 60. The feedback allows to lock the emission wavelength of the laser diode

60 to a given value and to narrow its emission bandwidth. For instance, a laser diode 60 having a wavelength of 976 nm with spectral width of 6 nm and temperature drift of 0.33 nm/°C may be locked at 976 nm over tens of degrees Celsius with a spectral width of 3 nm by inscribing a FBG reflector in a pump delivery fiber having a core diameter of 105 μιη and an of 0.15 NA. It is noted that the FBG wavelength locking reflector 59 should have a small reflectivity in order to not decrease the efficiency of the laser diode by allowing a high pump power to be transmitted through the FBG wavelength locking reflector 59 for injection in the amplification fiber 63 and amplification of the signal 49. In one embodiment, a wavelength locking at 976 nm allows to tune the laser diode 60 to the high absorption and narrow peak of Ytterbium at this specific wavelength. FBG cladding reflectors are described herein above for reflecting pump light propagating in the cladding. It is further noted that FBG reflectors may also find applications in reflecting signal light propagating in large core optical fibers and/or multimode core optical fibers.

[0055] In a variation of the embodiment of Fig. 6, the index modulation is inscribed mainly in a central region of the large core area of the optical fiber. This is made by adapting the ultrashort pulse writing system to control the focusing of the ultrashort pulse light to properly focus ultrashort pulse light to this region. This is used to maximize the reflection coupling coefficient of the FBG with part of the propagating modes. The central location of the FBG inside the fiber core then benefits lower order modes reflection and reduces the effective bandwidth of the reflective spectrum. Such a FBG has less efficiency in terms of total reflected power but has a benefit by narrowing the spectral bandwidth which gives better locking performance for high power laser diodes. It is noted that the same principle may be applied to different regions of the cladding such as the outer part of the cladding so as to select modes propagating along the cladding boundary.

[0056] The pump-guide cladding of a double-clad optical fiber is highly multimode in order to propagate a large pump power. The core may also be multimode when a large core diameter or high core Numerical Aperture (NA) is required. Such large multimode claddings or cores make it difficult to inscribe efficient FBG with standard ultraviolet inscription methods due to the low penetration of ultraviolet light in photosensitive glass and to the non- photosensitivity of pure silica as mentioned above. However, FBG inscription in such multimode cores and claddings is highly sought for use in fiber laser applications. A large core or a multimode core is required for example to obtain a large output power while mitigating non-linear effects. A large core also provides better overlap with the pump and may be useful to obtain a high inversion level. This allows lasing at shorter wavelengths in Ytterbium-doped fiber lasers for instance.

[0057] Fig. 7 illustrates another example embodiment wherein FBG signal reflectors are inscribed in the large and highly multimode core 64 of an amplification fiber. Similarly to what is being described herein above, ultrashort pulse light is now used to write FBG signal reflectors 65, 66 in the large diameter and/or multimode core 64 of an amplification fiber. Standard ultraviolet inscription techniques are unable to write FBGs in such large diameter cores because of the short penetration of ultraviolet light in photosensitive glass. Accordingly, in the example embodiment of Fig. 7, FBG signal reflectors 65, 66 are inscribed in the core 64 of the amplification fiber using ultrashort pulse light as described herein above. The FBG signal reflectors 65, 66 are used to reflect the signal propagating in the core 64, and not the pump 67 propagating in the cladding 68. This makes a fiber laser cavity where the signal oscillates within the two FBG reflectors 65 and 66. The output power then exits through one or the two FBG reflectors 65, 66. In fact, it exits through the FBG reflector that is made with lower reflectivity and in this case FBG reflector 65. For instance, if FBG reflector 66 has reflectivity of 99% and FBG reflector 65 has a reflectivity of 10% at the signal wavelength, then almost all signal power will exit through FBG reflector 65 located near end 69 of the amplification fiber. The use of the large core allows a higher inversion level in the amplification fiber and the use of large diameter core that emits high output power.

[0058] It should be understood that all the embodiments described herein are not mutually exclusive and may be combined as illustrated in Fig. 8 for example. Fig. 8 illustrates another example embodiment wherein a fiber laser combines a FBG wavelength locking reflector 73 used in the pump delivery fiber to lock the emission wavelength of the pump source 74, FBG signal reflectors 71 , 72 inscribed in the large and highly multimode core 78 of the amplification fiber and defining a laser cavity, and a FBG cladding reflector 70 for reflecting pump light at the end of the laser cavity and thereby reduce the amplification length of the fiber laser. It is noted that signal light goes through FBG cladding reflector 70 substantially unaffected because the index modulation of FBG cladding reflector 70 is selected to reflect light spectrally located around the pump wavelength and not at the signal wavelength.

[0059] Another application of FBG reflectors inscribed in large cross-sectional areas of non-photosensitive optical fibers is found in filtering signal leaks. It occurs that signal propagating in the core of an optical fiber partially leaks from the core to the cladding. The leaking signal light may then propagate to the pump diodes and may sometimes damage the pump diode, especially if the signal is pulsed and the peak power is high.

[0060] Fig. 9 illustrates another application of FBG reflectors inscribed in the cladding of an optical fiber. In this embodiment, an FBG reflector 86 with a high reflectivity at the signal wavelength is inscribed in the cladding 81 of the amplification fiber which is a multimode non-photosensitive cladding 81.

[0061] For example, when a splice or a junction 83 is made in a fiber amplifier or fiber laser, some signal 82 can leak from the core 85 and propagate in the cladding 81. The FBG 86 then reflects any signal propagating in the cladding 81 and avoids the signal to reach and potentially destroy the pump source 84. In the embodiment of Fig. 9, signal light 79 propagates in the core in the backward direction and thus in counter-propagation with respect to pump power 80 propagating in the cladding 81. The FBG reflector 86 is located between the pump source and the amplification fiber wherein signal light propagates, and is inscribed using ultrashort pulse light. The FBG reflector 86 has a reflectivity with a central wavelength at the signal wavelength and is only inscribed in the cladding 81 of the optical fiber such that signal propagating in the core 85 is not reflected by FBF reflector 86. With ultrashort pulse light it is possible to inscribe the FBG reflector 86 only in the cladding 81 since the pulses can be focused on a well defined section of the fiber without intersecting the core 85. In another embodiment wherein such a FBG reflector 86 is used in a fiber laser cavity, the FBG reflector 86 may be written in both the core 85 and the cladding 81 such that it serves as a signal reflector to provide the laser cavity as well as to block signal propagating in the cladding to protect the pump source 84. [0062] It is noted that some of the embodiments described and illustrated herein use double-clad rare-earth-doped optical fibers but it is noted that FBG reflectors may also be inscribed in double-clad undoped optical fibers, in triple-clad or in other multi-clad optical fiber. FBG reflectors may also be inscribed in other types of waveguides such as planar waveguides, Photonics Crystal Fibers (PCF), Photonics bandgap fibers, holey fibers and multicore fibers.

[0063] It is noted that while in some of the embodiments described and illustrated herein the FBG is written in an active, i.e. rare-earth doped, optical fiber, similar FBGs may also be written in passive, i.e. undoped, optical fibers. For example, a FBG reflector inscribed in a passive optical fiber may be spliced or otherwise joint at an end of an active optical fiber in order to perform for example the functions performed by FBG reflector 22 of Fig. 1 , FBG reflector 53 of Fig. 5, FBG reflectors 65 and 66 of Fig. 7, FGB reflector 70 of Fig. 8 or FBG reflector 86 of Fig. 9. This may be done for instance to decrease the cost of the manufacturing process since undoped passive fiber is typically less expensive than rare- earth doped optical fiber. Since the FBG inscription process typically generate a certain amount of spoilage, the overall cost can be lowered by inscribing the FBGs in passive optical fibers.

[0064] The ultra-short inscription technique is not dependent upon doping with UV- sensitive dopants, and is not specifically dependent upon the depth at which the inscription is done inside the optical fiber, or of inscription previously done in the optical fiber. Therefore, we predict that the ultra-short inscription technique can be extended over a larger surface that what was previously done while maintaining satisfactory reflectivity. For instance, we believe that reflectivity above 50%, preferably above 65% and more preferably above 80% will be achieved on cross-sectional areas above 500 μιη², preferably above 1000 μιη², more preferably above 5000 μιη², and even more preferably above 10 000 μιη².

[0065] The embodiments described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the appended claims.

Claims

Claims

1 . A method of making an active optical device component comprising inscribing a Bragg grating on a cross-sectional area of at least 500 μιη² οί a longitudinally-extending waveguide section of the active optical device component by exposing the waveguide section to pulses of light in the picosecond or smaller range.

2. The method of claim 1 wherein the active optical device component has an optical fiber with a core for propagating a light signal and a first cladding surrounding the core.

3. The method of claim 2 wherein the optical fiber further comprises a second cladding surrounding the first cladding for guiding pump light to be propagated in the first cladding, wherein the longitudinally-extending waveguide section is the first cladding.

4. The method of claim 3 wherein the Bragg grating is longitudinally opposite a light injection point of the optical fiber; wherein the Bragg grating has a central wavelength corresponding to a wavelength of the pump light.

5. The method of claim 3 wherein the Bragg grating is adjacent a light injection point of the optical fiber, wherein the Bragg grating has a central wavelength corresponding to a wavelength of the light signal.

6. The method of claim 2 wherein the longitudinally-extending waveguide section is the core, the Bragg grating has a central wavelength corresponding to a wavelength of the light signal and is provided at a first end of a laser cavity, further comprising inscribing an other Bragg grating at a second end of the laser cavity.

7. The method of claim 2 wherein the optical fiber is to be connected to convey light between a light source and a gain medium of the active optical device, wherein the longitudinally-extending waveguide section is the core, further comprising identifying a locking wavelength for a pump light source, wherein the Bragg grating has a central wavelength corresponding to the locking wavelength.

8. The method of claim 1 wherein the Bragg grating on a cross-sectional area of at least 1000 μιη² οί a longitudinally-extending waveguide section, preferably at least 5000 μιη², and more preferably at least 10 000 μιη².

9. The method of claim 1 wherein the inscribing includes focalizing laser light of an energy at least in the microjoule range, preferably in the millijoule range, over the cross-sectional area to generate the pulses of light.

10. The method of claim 1 wherein the inscribing includes side-exposing the waveguide section to an interference pattern of the pulses of light.

1 1. The method of claim 1 further comprising determining an effective indicia range for modes in the waveguide section, determining a period of the index modulation range based on the effective indicia range, and wherein the inscribing includes using a chirped modulation period extending along the period of the index modulation range.

12. An active optical device component comprising a longitudinally-extending waveguide section, having a Bragg grating having a reflectivity of at least 50% at a central wavelength, over at least 500 μιη² cross-sectional area of the waveguide section.

13. The active optical device component of claim 12 wherein the active optical device component has an optical fiber with a core for propagating a light signal, a first cladding surrounding the core for guiding the light signal, and a second cladding surrounding the first cladding for guiding pump light to be propagated in the first cladding.

14. The active optical device of claim 13 wherein the longitudinally-extending waveguide section is the first cladding.

15. The active optical device component of claim 14 wherein the Bragg grating is longitudinally opposite a light injection point of the optical fiber and the Bragg grating has a central wavelength corresponding to a wavelength of the pump light.

16. The active optical device component of claim 14 wherein the Bragg grating is adjacent a light injection point of the optical fiber, and wherein the Bragg grating has a central wavelength corresponding to a wavelength of the light signal.

17. The active optical device component of claim 13 wherein the longitudinally-extending waveguide section is the core, wherein the Bragg grating has a central wavelength corresponding to a wavelength of the light signal and is provided at a first end of a laser cavity, further comprising an other Bragg grating at a second end of the laser cavity.

18. The active optical device component of claim 12 wherein the waveguide section is multi- mode, wherein the Bragg grating has a reflectivity higher than 65%, preferably higher than 80%, on a cross-sectional area of at least 1000 μιη², preferably at least 5000 μιη², and more preferably at least 10 000 μιη².

19. An active optical device component comprising a pump light source optically coupled to a longitudinally-extending waveguide section having a Bragg grating over a cross-sectional area of at least 500 μιη² of the waveguide section wherein the Bragg grating has a central wavelength and a reflectivity configured and adapted to lock and narrow a spectral emission of the pump light source.

20. The active optical device component of claim 19 wherein the active optical device component is an optical fiber with a core for propagating a light signal and a first cladding surrounding the core for guiding the light signal, wherein the longitudinally-extending waveguide section is the core.