WO2011032294A1

WO2011032294A1 - Q-switched dfb fiber laser with switchable polarization output

Info

Publication number: WO2011032294A1
Application number: PCT/CA2010/001492
Authority: WO
Inventors: Eric Weynant; Alex Fraser; Martin Bernier; Réal VALLÉE
Original assignee: Phasoptx Inc.; UNIVERSITé LAVAL
Priority date: 2009-09-21
Filing date: 2010-09-21
Publication date: 2011-03-24

Abstract

An optical fiber phase modulator has a body made of a shape memory alloy. The body includes a conduit for receiving an optical fiber. An actuator such as, for example, a piezoelectric transducer varies the constrictive (radially compressive force) applied by the conduit on the optical fiber to modulate the phase of the optical signal carried by the optical fiber. A fiber Bragg grating (FBG) incorporating this novel tunable phase modulator may be used in distributed feedback (DFB) fiber lasers and narrow band pass filters. This invention also provides a tunable birefringent phase modulator, which may be used in fiber polarization switches. The shape memory alloy phase modulator does not require glue to connect the fiber to the actuator and, consequently, this novel modulator does not suffer the optical losses that typical arise with prior-art devices nor is it prone to breakage as are the more fragile bonded devices.

Description

O-SWITCHED DFB FIBER LASER WITH SWITCHABLE

POLARIZATION OUTPUT

TECHNICAL FIELD

[0001] The present invention relates generally to optics and photonics and, more particularly, to techniques and devices for modulating the phase of light within an optical fiber by application of a mechanical force on the optical fiber.

BACKGROUND

[0002] Phase shifted fiber Bragg gratings (PSFBG) are very compact devices that can be used as ultra narrow filters in dense wavelength division multiplexing (DWDM) systems. This is taught by the following references: Xia, L., P. Shum, and C. Lu, Phase- shifted bandpass filter fabrication through C02 laser irradiation. Optics Express, 2005. 13(15): p. 5878., Xu, M.G., et al., Tunable fibre bandpass filter based on a linearly chirped fibre Bragg grating for wavelength demultiplexing. Electronic Letters, 1996. 32(20): p. 1918. The PSFBG is also commonly used as a Fabry-Perot cavity in the design of a distributed feedback (DFB) fiber laser. This is disclosed in the following references: Babin, S.A., et al., Single frequency single polarization DFB fiber laser. Laser physics letters, 2007. 4(6): p. 428-432., Fan, W., et al., Stable single frequency and single polarization DFB fiber lasers operated at 1053 nm. Optics and laser technology, 2007. 39(6): p. 1189-1 192., Zhang, Z., et al., High-power Tm-doped fiber distributed-feedback laser at 1943 nm. Optics letters, 2008. 33(18): p. 2059.

[0003] The distributed feedback (DFB) fiber laser is of great interest for DWDM systems and sensing applications because of the possibility of single polarization and single frequency operation. The PSFBG can be made by post processing, i.e., exposing a small section of the original FBG to additional ultraviolet (UV) pulses (see, e.g., Canning, J. and M.G. Sceats, π-phase-shifted periodic distributed structures in optical fibers by UV post-processing. Electronic Letters, 1994. 30(16): p. 1344, or to additional C0₂ laser pulses (as taught by Xia, L., P. Shum, and C. Lu, Phase-shifted bandpass filter fabrication through C02 laser irradiation. Optics Express, 2005. 13(15): p. 5878.). This post-processing serves to increase the DC refractive index of the irradiated fiber section in order to slow down the optical signal travelling through this section, thus inducing a phase shift in the signal. Another way to make a phase-shifted FBG is to use specially designed phase-shifted phase masks to expose the fiber to an interference pattern with phase-shifted sections (see, e.g., Kashyap, R., P.F. Mckee, and D. Amies, UV written reflection grating structures in photosensitive optical fibres using phase-shifted phase mash. Electronics letters, 1994. 30(23): p. 1977-1978). Another technique uses a standard phase mask that is slightly moved with respect to the fiber during the grating inscription (see, e.g., Cole, M.J., et al., Moving fibre/phase mask-scanning beam technique for enhanced flexibility in producing fibre gratings with uniform phase mash. Electronic Letters, 1995. 31(17): p. 1448.) and another one uses a thermal head to thermally induce a localized permanent phase-shift in the FBG by local heating (as taught by Ngo, N.Q., et al., A phase-shifted linearly chirped fiber Bragg grating with tunable bandwidth. Optics Communications, 2006. 260(2): p. 438-441.). All these techniques induce a permanent and non-tunable phase shift in the optical fiber.

A simple way to introduce a tunable phase shift in a fiber Bragg grating (FBG) is to locally stretches the FBG, as taught by the following references : Barmenkov, Y.O., et al., Threshold of a symmetrically pumped distributed feedback fiber laser with a variable phase shift. IEEE Journal of Quantum Electronics, 2008. 44(8): p. 718., Xu, M.G., et al., Tunable fibre bandpass filter based on a linearly chirped fibre Bragg grating for wavelength demultiplexing. Electronic Letters, 1996. 32(20): p. 1918. These methods introduce a tunable phase shift in the fiber but this phase shift is not birefringent. Also, the fiber needs to be glued to the actuator. This bonding may cause thermal instability when the fiber is highly pumped in a DFB fiber laser design. At the opposite, mechanical stresses imposed transversally on a small section can introduce a birefringent phase shift large enough to create a π optical round-trip phase shift (i.e. for the whole travel back and forth in the cavity) in the FBG without breaking the fiber, as taught by the following references: Michaille, L., et al., Analysis of single and multiple, non-permanent, tunable, birefringent spectral holes in a fibre-Bragg grating stop-band produces via uniaxial pressure. Optics Communications, 2003. 222(1-6): p. 1-8., Torres, P. and L.C.G. Valente, Spectral response of locally pressed fiber Bragg grating. Optics Communications, 2002. 208(4-6): p. 285-291., Matos, C.J.S.d., et al., Fiber bragg grating characterization and shaping by local pressure. Journal of lightwave technology, 2001. 19(8): p. 1206. One author uses a stainless steel wheel to transfer weight to the fiber and another uses a solid frame covered by a rubber layer. These methods are simple but they suffer from a lack of flexibility: they do not allow independent control of many stress points located close to each other on a same FBG and they are not dynamically tunable.

[0004] While it is known in the art to induce a tunable phase shift in an optical fiber by applying mechanical stresses, the prior-art techniques are rudimentary and suffer from a number of drawbacks. The prior-art techniques for mechanically stressing the fiber involve bonding (gluing) an actuator to the fiber. A tensile stress can then be applied on a short section of the fiber using, for example, a magnetostrictive alloy rod or a piezoelectric stack connected by glue to the fiber. However, these prior-art techniques suffer from at least two main drawbacks. Firstly, the glued (bonded or adhesive) connection between the actuator and fiber is prone to fatigue and thermal strain, and thus does not age well. Furthermore, the prior-art techniques have a propensity to induce non- negligible losses in the transmitted signal.

[0005] It is therefore desirable to improve on the prior-art techniques for inducing a phase shift in an optical fiber. Specifically, it remains desirable to provide a tunable all- fiber phase modulator that allows the introduction of several independent tunable birefringent phase shifts on a short section of an optical fiber, without inducing significant optical loss.

SUMMARY OF THE INVENTION

[0006] In general, and as will be elaborated below, the present invention provides an optical fiber phase modulator made of a highly elastically deformable material such as a shape memory alloy. The modulator includes a fiber conduit into which an optical fiber is inserted and held. The modulator is then actuated to expand or contract this fiber conduit to thereby vary the degree of constriction (radial compression) on the fiber in the conduit. This variation in the constriction/compression of the fiber modulates the phase of the optical signal propagating through the fiber (i.e. induces a phase shift). By employing a precisely controllable actuator such as, for example, a piezoelectric transducer stack, the phase shift (phase modulation) can be controlled very precisely by varying the electric field or voltage potential across the piezoelectric stack.

[0007] The present invention overcomes, or at least attenuates, the difficulties and disadvantages of the prior art by providing an optical fiber phase modulator made of a highly elastically deformable material, such as for example a shape memory material, that holds the optical fiber without glue or bond and enables controlled and precise application of external stresses over sections of the fiber.

[0008] The present invention provides an optical fiber phase modulator, which is capable of locally modulating the phase of transmitted or reflected signals in optical fibers, which enables inducing of birefringence in a short section of an optical fiber and controlling the amount of birefringence induced in said optical fiber and which allows a user to induce several independent localized and tunable phase shifts in fiber Bragg gratings and to induce localized birefringence in fiber Bragg gratings. In main implementations of this technology, the modulator has a body made of a shape memory alloy (SMA) or other shape memory material although other highly elastically deformable materials may be used for the body of the modulator.

[0009] Furthermore, it is also a part of the present invention to provide a technique to precisely and independently control the force (or stress) applied on each localized fiber section. A controllable actuator transfers precise movement to the SMA phase modulator, for example, by using an opening lever or by any other suitable mechanical linkage. A piezoelectric actuator, or any other kind of actuator suitable for this application, may be used to control the lever. Multiple modulators may be connected to the same optical fiber to produce phase shifts at various locations along the fiber.

[0010] Accordingly, one main aspect of the present invention is an optical fiber phase modulator comprising a body made of a highly elastically deformable material, the body having a first end and a second end. The modulator comprises a fiber conduit formed in the body and extending from the first end of the body to the second end of the body and a slot from an outer surface of the body into the fiber conduit, wherein the fiber conduit is dimensioned to receive an optical fiber and wherein the fiber conduit may be expanded and contracted to vary a compressive stress on the optical fiber to thereby modulate a phase of an optical signal carried by the optical fiber.

[001 1] Another main aspect of the present invention is a method of modulating a phase of an optical signal carried in an optical fiber. The method entails inserting the optical fiber into a fiber conduit formed in a highly elastically deformable body of an optical fiber phase modulator, transmitting the optical signal through the optical fiber, and exerting an external force on the highly elastically deformable body of the optical fiber phase modulator to cause the fiber conduit in the body of the optical fiber phase modulator to vary a compressive stress on the optical fiber to thereby modulate the phase of the optical signal carried in the optical fiber.

[0012] Yet another aspect of the present invention is a polarization switchable Q- switched DFB fiber laser comprising an optical fiber phase modulator having a highly elastically deformable body and a fiber conduit formed in the body, a piezoelectric actuator for exerting a compressive force on the body of the optical fiber phase modulator to thereby induce a birefringent phase shift in the optical fiber for two orthogonal polarizations, and means for applying an offset voltage while varying the compressive force to cause pulse-to-pulse switching between the two orthogonal polarizations.

[0013] A method of Q-switching an optical signal using a distributed feedback laser, the method comprising inserting the optical fiber into a fiber conduit formed in a highly elastically deformable body of an optical fiber phase modulator, transmitting the optical signal through the optical fiber, exerting an external force on the highly elastically deformable body of the optical fiber phase modulator to cause the fiber conduit in the body of the optical fiber phase modulator to vary a compressive stress on the optical fiber to thereby modulate the phase of the optical signal carried in the optical fiber and to induce a birefringent phase shift in the optical fiber for two orthogonal polarizations to generate two distinct transmission peaks for the optical signal propagating in the optical fiber, and applying an offset voltage while varying the external force to cause pulse-to- pulse switching between the two orthogonal polarizations.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order that the invention may be more readily understood, currently preferred embodiments will now be further described by way of example with reference to the accompanying drawings in which:

[0015] Figure 1 is an isometric view of a shape memory alloy (SMA) ferrule used as an optical fiber phase modulator in accordance with one embodiment of the present invention. The two slots serve to open the ferrule to enable the fiber conduit to embrace the fiber and induce mechanical stresses in the fiber without damaging the fiber.

[0016] Figure 2 is a front view of the SMA ferrule depicted in Figure 1. The values Lfl and Lf2 represent the widths of the large and small slot, respectively. The values Pfl , Pf2 and Pt represent the depths of the slots and the position of the hole with respect to the center of the ferrule, respectively.

[0017] Figure 3 is a side view of the SMA ferrule depicted in Figure 1. Dl and

D2 represent the diameters of the fiber conduit and of the funnel, respectively. De and Le represent the diameter and the length of the SMA ferrule, respectively. Lp represents the length of the fiber conduit that embraces the fiber.

[0018] Figure 4 is a front view of the SMA ferrule into which a moving lever and a fixed anchor have been inserted in order to enable the conduit to be expanded by applying a transverse force on the moving lever.

[0019] Figure 5 is a front view of the SMA ferrule, fixed anchor and moving lever wherein the lever is connected to, and actuated by, a controllable actuator such as, for example, a piezoelectric transducer (PZT). [0020] Figure 6 illustrates the possibility of placing multiples SMA ferrules on an optical fiber or a fiber Bragg grating.

[0021] Figure 7 is a graph of the measured induced phase shift with respect to the voltage applied to the piezoelectric actuator.

[0022] Figure 8 is a graph of the measured loss induced by the phase modulator with respect to the voltage applied to the piezoelectric actuator.

[0023] Figure 9 is a graph showing the resulting transmission spectra of a FBG with a phase shift induced by the SMA ferrule used as a phase modulator.

[0024] Figure 10 is a graph showing the resulting reflection spectra of a FBG with a phase shift induced by the SMA ferrule used as a phase modulator.

[0025] Figure 1 1 is a graph showing the resulting transmission spectra of a FBG for three different phase shift amplitude induced by the SMA ferrule used as a phase modulator.

[0026] Figure 12 is a graph showing the calculated residual stresses in x and y directions at the fiber center (in the fiber core).

[0027] Figure 13 is a schematic depiction of a phase modulator actuated by a piezoelectric actuator for inducing mechanical stresses in the phase modulator.

[0028] Figure 14 depicts an experimental set-up of the Q-switched DFB fiber laser and measurement apparatus.

[0029] Figure 15 depicts a continuous wave (CW) laser power against the operating wavelength, in which squares represent pump power of 31 mW and circles represent pump power of 40 mW, empty symbols represent one PM and dark symbols represent the other.

[0030] Figure 16 depicts a CW laser power against the voltage applied to the piezoelectric actuator for a pump power of 40 mW: Letters A, B and C correspond to the three modes of operation shown on Figure 17, empty circles are for one PM and dots for the other.

[0031] Figure 17 depicts the pulse shape for a) x polarization (mode A, pump power = 65 mW), b) y polarization (mode B, pump power = 65 mW) and c) two PM's regime (mode C, pump power = 40 mW).

[0032] Figure 18 are isometric and sectional views of a variant of the device shown in Figure 13.

[0033] Figure 19 is a graph showing experimental transmission spectra of the uncompressed FBG (black curve) along with the spectra of the phase shifted FBG for x polarisation (red) and y polarisation (blue) at voltage of 40 V (dotted curve) and 50 V (solid curve).

[0034] Figure 20 is a graph showing phase shift induced by the SMA phase modulator against the applied voltage to the piezoelectric actuator, in which circles are for decreasing stress and squares for increasing stress, filled symbols are for one PM and empty symbols for the other PM.

[0035] Figure 21 are graphs showing example of two laser regimes, namely a Q- switched regime and a power modulation regime (in which the region inside the grey rectangle shows the range where lasing is obtained for a given PM).

[0036] Figure 22 are graphs showing a) laser power vs. the applied voltage to the piezoelectric actuator, b) Lasing wavelength vs. the applied voltage to the piezoelectric actuator and c) Laser power vs. the lasing wavelength. White circles are for one SOP and black circles for the other SOP (pump power = 45 mW.

[0037] Figure 23 are graphs showing laser power vs. applied voltage to the piezoelectric actuator for a triangular waveform at 60 V peak-to-peak voltage and at a 10- Hz modulation frequency (pump power = 29 mW). [0038] Figure 24 are graphs of a) signal obtained at positions 1, 3, 4 and 6 and b) signal obtained at positions 2 and 5 (pump power = 43 mW and modulation frequency = 7 kHz).

[0039] Figure 25 are graphs of a) signal obtained for positions 1, 3, 4 and 6

(pump power = 29 mW) and b) signal obtained for positions 2 and 5 (pump power = 57 mW) (modulation frequency = 208 kHz in both cases).

[0040] Figure 26 are graphs of a) pulse width (FWHM) of the first cycle of the relaxation oscillations at 7-kHz modulation frequency (white circles) and of the single pulse emitted at 207-kHz modulation frequency (black circles) vs. pump power, b) inverse of the period between the two first relaxation oscillations vs. pump power, circles are for the single SOP (position 1) and squares are for the SOP switching mode (position 2).

[0041] Figure 27 are graphs of a) output power modulation measured at 40-mW pump power for different offset voltages applied to the piezoelectric actuator and for a 330-mV peak-to-peak voltage at 7-kHz frequency, black curve: 2.35 V, red curve: 3.61 V and blue curve: 4.53 V, b) Zoom on the power oscillation for the 4.53 V offset voltage.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0042] In general, an optical fiber phase modulator made of a highly elastically deformable material such as a shape memory alloy can be actuated by applying a force to the body of the modulator to cause the fiber conduit holding the optical fiber to vary its degree of constriction (i.e. to vary its radially compressive pressure). This variation in the radially compressive pressure on the section of the optical fiber contained within the fiber conduit modulates the phase of the optical signal in the fiber (i.e. shifts the phase). The embodiments of the present invention also provide a tunable optical fiber phase modulator that may be used to induce phase shifts at precise locations in a fiber Bragg grating (FBG). A fiber Bragg grating incorporating this novel tunable phase modulator may be used in DFB fiber lasers and narrow band pass filters. Some embodiments of the present invention also relate to a tunable birefringent phase modulator, which may be used in fiber polarization switches.

[0043] In the specific embodiment depicted by way of example in Figures 1-3, the optical fiber phase modulator device, which is generally designated by reference numeral 10, has a longitudinally extending body 12 which may be generally cylindrical. However, although the optical fiber phase modulator device is shown here as having a cylindrical body, the body of the modulator may be square, rectangular or any other suitable shape. The body of the modulator has at least one fiber conduit 14 that has a substantially circular cross-section for receiving (embracing) an optical fiber. As depicted by way of example in these three figures, the body of the optical fiber phase modulator device has a first end 16 and a second end 18. The body has at least one conduit, but it can have two concentric conduits 20, 22, as shown in Figure 3. The larger conduit 20 has a diameter D2 while the smaller conduit 22 has a diameter Dl . The larger conduit may act as a funnel to facilitate insertion of the fiber into the smaller conduit, when the ferrule is opened by the application of an external force. A flared or tapered portion 23 may be provided to guide the fiber from the larger conduit into the smaller conduit. The smaller conduit has a diameter slightly smaller than the optical fiber, and is used to embrace (gently constrict) the optical fiber, i.e. apply radially inwardly compressive pressure on this fiber, while preventing it from breaking. The length of the fiber conduit, denoted Lp on Figure 3, is chosen to provide the desired amount of phase retardation for a given application: a longer conduit will result in a higher phase retardation. In one embodiment, the modulator body has at least one longitudinal slot 24 extending from the first end to the second end and from the outer surface of the modulator body to the fiber conduit to permit expansion of the fiber conduit for insertion of an optical fiber.

[0044] In the specific embodiment illustrated in Figures 1-3, the longitudinal slot

24 is a shouldered slot (or stepped slot) composed of a first narrow portion 25 (narrow slot or small slot) and a second wide portion 26 (or wide slot or large slot). Between the narrow and wide portions of the stepped/shoulder slot are a pair of steps or shoulders 28. Extending orthogonally to these steps or shoulders are inwardly facing walls (inner walls) 29. The large slot (wide slot) is designed to receive the separator arms of an opening tool that is inserted into the wide slot to facilitate the opening of the fiber conduit. The separator arms (lever and anchor) engage the surfaces of the inner walls 29 and exert equal and opposite outward forces on the slot via these inner walls. The depth of the narrow slot and the position of the hole, which are respectively denoted Pf2 and Pt on Figure 2, can be adjusted to apply the desired amount of stress on the fiber: a deeper slot and hole will result in smaller stress applied to the fiber.

[0045] The optical fiber phase modulator of the present invention is preferably made from shape memory material (SMM), such as a shape memory alloy (SMA), including those materials that may be used for manufacturing the optical fiber connector devices of the type described in PCT International Patent Application WO 2008/151445, published December 18, 2008. The optical fiber phase modulator may also be made of any suitable highly elastically deformable material. The modulator of the present invention may, for example, be made from a polymeric material such as isostatic polybutene, shape ceramics such as zirconium with some addition of Cerium, Beryllium or Molybdenum, copper alloys including binary and ternary alloys, such as Copper - Aluminum alloys, Copper - Zinc alloys, Copper - Aluminum - Beryllium alloys, Copper - Aluminum - Zinc alloys and Copper - Aluminum - Nickel alloys, Nickel alloys such as Nickel - Titanium alloys and Nickel - Titanium - Cobalt alloys, Iron alloys such as Iron - Manganese alloys, Iron - Manganese - Silicon alloys, Iron - Chromium - Manganese alloys and Iron - Chromium - Silicon alloys, Aluminum alloys, and high elasticity composites which may optionally have metallic or polymeric reinforcement.

[0046] To insert an optical fiber into the optical fiber phase modulator device of the present invention, the fiber conduit of the phase modulator is enlarged or expanded by deforming the body of the modulator. The shape memory alloy body may be deformed by applying one or more external forces, by application of heat or by a combination thereof. For example, an external force (pressure) may be applied to the body of the optical fiber phase modulator device so as to expand the fiber conduit. Once the conduit has been expanded, the optical fiber is inserted into the expanded conduit. As depicted in Figure 4, an opening lever 30 can be inserted in the slot to cause the modulator device (SMA ferrule) to open. A fixed anchor 32 is provided to hold the ferrule in place when the opening lever 30 is actuated. The fixed anchor thus exerts an equal and opposite force on the wide portion of the slot so that the slot expands without rotating the ferrule.

[0047] Once the optical fiber is fully inserted into the conduit of the phase modulator, the force is released to enable the phase modulator to return to an initial shape due to the elasticity of the shape memory alloy from which the body of the phase modulator is constructed. Upon release of the force on the phase modulator, the phase modulator will apply a radial pressure (i.e. compressive radial force) along a length of the fiber that is constricted within the small diameter portion of the conduit of the phase modulator. By further varying the constriction of the fiber, the optical fiber phase modulator can exert a controllable compressive force on the optical fiber, sufficiently strong to control the phase of the optical signal transmitted through the fiber, but small enough not to damage the optical fibers by over-compression. The configuration of the shape memory alloy body presented by way of example in Figures 1-3 is but one of many possible embodiments. The geometry, shape and configuration may be varied provided the expansion and contraction of the conduit (degree of constriction) can be controlled so as to enable precise modulation of the phase. A shape memory alloy makes for an ideal body for the modulator because of its super-elasticity, which thus enables highly precise, controlled actuation of the modulator.

[0048] The advantages of using the optical fiber phase modulator of the present invention for inducing phase shifts in fiber Bragg gratings, over, for example, a phase shifted FBG fabricated by post-processing with UV or C0₂ lasers or with special π-phase shifted phase mask, are its simplicity, versatility, reversibility and cost relative the prior- art techniques.

[0049] Referring to Figure 5, the optical fiber phase modulator may be controlled by using one or more controllable actuators, for example, a piezoelectric stack 40. The reverse piezoelectric effect may be exploited to produce stress and strain in the lever arm when an electric field is applied to the piezoelectric stack. Other types of actuators may be substituted (e.g. a thermal actuator, a magnetostrictive element, etc.). Irrespective of its type, this controllable actuator is meant to deform the body of the modulator to thereby modulate the phase of the optical signal in the optical fiber. In this particular embodiment, the deformation of the modulator body is accomplished by transferring the strain of the PZT stack to a substantially linear (lateral or transverse) motion of the opening lever that, in turn, causes the conduit in the body of the modulator (i.e. the ferrule) to expand or contract in a precisely controlled manner. This expansion and contraction (opening and closing) of the conduit, in turn, causes a variation in the constriction of the fiber held within the conduit of the modulator, which thus changes (increases or decreases) the amount of stress applied to the optical fiber over the section in contact with the conduit. With this method, it is possible to precisely control the amplitude of the phase shift induced in the optical fiber, or in the FBG, simply by changing the applied voltage (or electric field) on the piezoelectric stack.

[0050] Referring to Figure 6, another aspect of the present invention involves the use of multiple phase-modulating devices (e.g. multiple SMA ferrules) to induce multiple phase shifts at different locations along the same optical fiber. Analogously, multiple phase-modulating devices (e.g. SMA ferrule) may be placed at different locations along a fiber Bragg grating to induce phase shifts. The position of each phase-modulating ferrule along the fiber (or along the FBG) may be varied to provide specific results for a given application. The amount of stress applied at each section may be controlled independently by individually actuating each modulator device.

[0051] In the phase modulators shown by way of example in Figures 4-6, there is shown a fixed anchor and a moving lever. The fixed anchor exerts an equal and opposite force on the body of the modulating device when the moving lever is actuated. In an alternative embodiment, the modulator may have two moving levers. The moving levers (or transverse separators) may be driven independently but concurrently by two separate piezoelectric transducers (or separate PZT stacks) energized either by a common electric field or by separately applied voltage potentials. Alternatively, the two opening levers (transversely displaceable separators) may be actuated concurrently by using a single transducer that is coupled to a linkage mechanism that converts the unidirectional linear motion (strain) of the piezoelectric transducer stack into equal and opposite traverse movement of the two separators (i.e. of the two opening levers).

[0052] In another embodiment, the modulator may be actuated by an actuator cluster or actuator assembly having a plurality of actuators (e.g. piezoelectric transducers, magnetostrictive transducers, etc.) that are connected in series or in parallel (or in any combination thereof) to provide the actuation for the moving lever.

[0053] In the foregoing, it should be appreciated that the moving lever and fixed anchor (or pair of moving levers) may be replaced by any equivalent mechanism having jaws, grips, hooks, claws, separators, etc., which shall be collectively referred to herein as "separator arms". These L-shaped separator arms may be made of any suitable material and need not be made of shape memory alloy.

[0054] It should also be appreciated that the L-shaped anchor and lever (the reshaped separator arms) are so designed to fit the shouldered slot (stepped slot) formed in the body of the phase-modulating ferrule. The shouldered (stepped) slot has a wide portion (large slot) and a narrow portion (small slot), as depicted in Figures 1-2. The dimensions and geometry of this anchor and lever are complementary to those of the shouldered slot. Therefore, the anchor, lever and slot may of course be varied in shape and configuration provided the anchor and lever may be fitted within the shouldered slot for the purposes of exerting equal and opposite forces on the inwardly facing walls of the wide portion of the slot.

[0055] Figures 7 and 8 depict the effects of the optical fiber phase modulator of the present invention on the signal transmitted through a standard SMF-28 optical fiber (i.e. the Corning® SMF-28™ Optical Fiber). Figure 7 illustrates the stress-induced phase shift experienced by the two orthogonal polarizations with respect to the voltage applied to the piezoelectric actuator. Figure 8 shows that the corresponding losses are very low, which represents a radical improvement over the prior art. The benefits of this novel technology would be obtained with any other type of optical fiber be it made of glass, including fused silica, fluoride glass, chalcogenide glass, or any other such material. [0056] Figures 9 and 10 depict the effect of the optical fiber phase modulator when the modulator is placed in the middle of a FBG. Figure 9 shows the transmission spectra while Figure 10 shows the reflected spectra. In this case, the Bragg grating was fabricated with a UV laser. Without the SMA ferrule, in this specific instance, the FBG has a reflectivity of 21 dB and a FWHM of approximately 56 pm. When the SMA ferrule constricts the optical fiber, it acts as an optical fiber phase modulator and creates a localized phase shift in the FBG at the location of the applied stress. The magnitude of the phase shift is not the same for the two orthogonal polarizations because the stress applied by the SMA ferrule is, in practice, not necessarily uniform around the fiber: in such as case, the ferrule induces a birefringent phase shift in the optical fiber. These two different phase shifts create two distinct transmission peaks in the FBG spectra. By controlling the closed state (degree of constriction) of the SMA ferrule, a user can adjust the position of the transmission peak with respect to the wavelength and place one of the two peaks in the middle of the FBG reflected peak: it will correspond to a round-trip phase shift of π radians. Because the lasing threshold of a DFB fiber laser is a function of the value of the round-trip phase shift in the FBG, and is minimum when the round-trip phase shift is equal to π, a single polarization output can be obtained from a DFB fiber laser (e.g. a silica based DFB fiber laser) using the optical fiber phase modulator of the present invention placed at the right place on the FBG written in doped fiber. On Figures 9 and 10, it is to be noted that the two dashed-line curves correspond to the response of the device for two linear polarizations directed in orthogonal directions. For these two figures, one polarization experiences a round-trip phase shift less than π while the other experiences a round trip phase shift greater than π, as shown in the figures.

[0057] Figure 1 1 clearly shows the two peaks corresponding to the two orthogonal polarization states for three different stresses applied to the optical fiber. It may be observed that the difference of wavelength between the two peaks of the same stress level increases when the stress level increases. This observation can be understood with reference to Figure 12, which shows the simulated stress in x and y directions in the fiber core. The stress difference between these x and y directions increases when the SMA ferrule closes on the optical fiber. [0058] Another innovative aspect of the present invention is a polarization- switchable Q-switched DFB (distributed feedback) fiber laser. This further innovative aspect of the present invention is illustrated in Figures 13-17. In broad terms, this Q- switched DFB fiber laser has a switchable polarization output. This new Q-switching method allows one to obtain linearly polarized laser emission in either the X or Y direction. The principle underlying this method is based on a variable birefringent phase shift induced by a lateral stress applied at a precise location along a fiber Bragg grating by a specially designed hyper-elastic device. A piezoelectric actuator controls the amount of stress delivered to the fiber, thus allowing a precise and rapid tuning of the cavity Q-factor.

[0059] Distributed feedback (DFB) fiber lasers are very compact optical devices that have received attention over the past few years. Their narrow linewidth combined with robust single-mode operation make them ideal optical sources for a broad range of applications, including coherent communications, optical fiber sensing and high resolution spectroscopy.

[0060] These lasers are realized by optically pumping an active cavity consisting of a phase shifted fiber Bragg grating (PSFBG) written in a rare earth doped fiber. Robust CW single polarization operation of these lasers requires the breaking of the degeneracy between the two orthogonal polarization modes (PM). This has been demonstrated in silica fiber by writing a permanent birefringent phase shift with UV light, by adding a small lateral stress on the phase shifted section of a PSFBG or by simply using the intrinsic birefringence of the FBG itself combined to non-uniform fiber heating. However, these methods do not allow rapid changing of the phase shift amplitude that is required for polarization switching operation.

[0061] Recently, Q-switched DFB fiber lasers have been demonstrated in Er3+ doped fiber by introducing an axially propagating acoustic wave into the fiber. Although single polarization operation has been achieved with this method, it relies only on the intrinsic birefringence of the FBG, which makes polarization switching impossible. Q- switched single frequency fiber laser have also been demonstrated in the distributed Bragg reflector (DBR) configuration. In this case, a small section of active fiber is spliced between two FBG's, one of which is made in polarization maintaining fiber. The polarization is then modulated inside the cavity by applying a lateral stress to the fiber with a piezoelectric actuator to produce Q-switched operation.

[0062] A novel Q-switched DFB fiber laser with switchable PM output enables a pulsed regime with pulse-to-pulse switching between the two orthogonal PM's. The laser is based on the new method of inducing a rapidly tunable birefringent phase shift in a FBG disclosed herein. The method uses a specially designed device using hyper-elastic (or super-elastic) material, e.g. a shape memory alloy, that embraces the fiber and generates mechanical stress in the fiber. The amount of stress imposed on the fiber is precisely controlled by a piezoelectric actuator and can be rapidly changed at frequencies approaching 400 kHz. Moreover, the hyper-elastic fixture allows the fiber to expand almost freely in response to the heat generated by the signal and the pump power, thus preventing the fiber from breaking.

[0063] In one specific example implementation, the hyper-elastic material used in the phase modulator may be a monocristalline CuAlBe alloy. This material can support elastic deformation as high as 10% when a phase transformation is activated inside the material via the application of a stress or by changing its temperature. Other shape memory alloys or shape memory materials may be substituted, as will be appreciated by those of ordinary skill in the art. For the present application, the phase transformation (from the martensitic phase to the austenitic phase) is induced by applying mechanical stress to the device with a piezoelectric actuator. Moreover, the thermally induced phase transformation may be set to appear between -80 °C and -120 °C, thus preventing the device from undesirable thermally induced phase transformation. The body of the optical phase modulator may be made by drilling a hole and two slots in a rectangular block of 1.5 mm x 1.5 mm x 3 mm hyper-elastic material, as shown in Figure 13, although a body having a different size and shape may be used as will be appreciated by those of ordinary skill in the art. The hole may be tapered or flared (like funnels) at both ends up to a diameter of, for example, 400 μπι to facilitate fiber insertion from either side, thus providing a central cylindrical section in contact with the fiber of approximately 900 μηι. Again, the lengths of the funnel sections 23 and of central cylindrical section may be varied, as will be appreciated by those of ordinary skill in the art. The two slots (i.e. deformation cavity 70 and dividing slit 80) allow the diameter of the hole to be increased for easy fiber insertion into the device: the moving part of the actuator is directly linked to the bendable portion of the hyper-elastic phase modulator, thus allowing a rapid and precise control of the stress induced in the fiber (as shown by way of example in Figure 13).

[0064] The hyper-elastic phase modulator is very easy-to-use and efficient for applying a high compressive lateral stress onto an optical fiber without damaging it. With the simple system presented in Figure 13, round-trip phase retardations of the order of 14 rad as well as relative round-trip phase shift between the two orthogonal PM's of 5 rad have been measured. The transmission losses increase steadily with the induced phase shift but are below 0.01 dB for the round-trip phase shift range of interest, i.e. between 0 and 2π. While the phase modulator depicted by way of example in Figure 13 is generally rectangular (blockish) in shape, it is to be appreciated that the shape of the phase modulator may be varied (e.g., cylindrical) without departing from the inventive concept presented herein.

[0065] With reference to Figure 13 and Figure 18, this polarization switchable Q- switched DFB fiber laser has an optical fiber phase modulator 10 having a highly elastically deformable body 12 and a fiber conduit 14 formed in the body 12. The modulator 10 includes a piezoelectric actuator 60 (or any equivalent actuator) for exerting a compressive force on the body 12 of the optical fiber phase modulator 10 to thereby induce a birefringent phase shift in the optical fiber 50 for two orthogonal polarizations. The modulator 10 includes means for applying an offset voltage while varying the compressive force to cause pulse-to-pulse switching between the two orthogonal polarizations. These means may include a voltage control circuit. As depicted in Figures 13 and 18, the body 12 of the modulator 10 may be a rectangular block made of a shape memory alloy. The block may have a slit 80 (or dividing slit) extending from the fiber conduit 14 to an upper surface 15 of the block upon which the piezoelectric actuator 60 acts. The dividing slit divides the upper portion of the block into a movable/bendable portion and a generally immovable portion. The movable/bendable portion is deformed under the load applied by the piezoelectric actuator. The block (body 12) may have a deformation cavity 70 extending orthogonally to the slit 80 to facilitate limited deformation of the movable/bendable portion of the block. In other words, when a load is applied to the upper surface of the block, the movable/bendable portion of the block deforms. The deformation cavity allows the block to deform until the bottom surface of the deformation cavity abuts the top surface of the deformation cavity. At that point, further deformation is restricted. Deformation of the block causes a change in the compressive stress exerted on the fiber in the fiber conduit. The change in compressive stress (and its commensurate strain) on the fiber causes the phase shift.

[0066] Figure 14 is a schematic depiction of one example of the DFB fiber laser and measurement apparatus. A 50-mm FBG is written in the middle of a 55-mm piece of highly doped Er-Yb single mode fiber (SMF) (3.21% Yb and 0.33% Er). The doped SMF has a photosensitive Ge ring surrounding the core and was deuterium loaded in order to allow efficient FBG writing. The FBG was written by standard technique with a uniform phase mask and 244-nm writing beam. This produced a FBG with a FWHM bandwidth of approximately 120 pm and a reflectivity peak in excess of 50 dB at the Bragg wavelength of 1552 nm. The FBG is then inserted inside the hyper-elastic phase modulator, in order to apply a lateral stress over a short FBG section. The phase modulator is placed 6 mm away from the FBG center, on the launched pump side, in order to get unidirectional operation in the counter pumping configuration. This configuration has been selected because it is known to be the most efficient for this type of laser. The precise position of the phase modulator was actually optimized by measuring the maximum output power achievable at different positions for a fixed pump power level. The fiber sections on both sides of the phase modulator were mounted in Cu sleeves for efficient heat dissipation. The foregoing numerical parameters are presented solely by way of example and it is to be appreciated that these may be varied without departing from the inventive concept.

[0067] Figure 15 shows the CW output power from the DFB as a function of wavelength for two different pump power levels. The voltage applied on the piezoelectric actuator provides the fine tuning on the operation wavelength for both linear PM's. This arises from the fact that lasing occurs when the induced round-trip phase shift crosses the value of π as the tension on the piezo is varied. Because the amount of phase retardation is different, for a given tension, for both PM's, the π- round-trip phase shift will occur for different values of the applied tension, as can be seen in Figure 16 where it is shown that the DFB fiber laser can operate in CW on either X or Y linear PM. Actually, the switches from one PM to the other PM happens when the applied voltage to the piezoelectric actuator crosses 14.8 V. The polarization extinction ratios were precisely measured to be of the order of 25 dB for both PMs. The corresponding spectral linewidth of 31 kHz was also obtained by measuring its beating with a frequency comb for 48 μ8. This value is in agreement with a previous report obtained with similar DFB fiber lasers. Again, the numeric values presented above are intended to be exemplary only and are offered solely for the purposes of illustration.

[0068] By applying a periodic square-wave signal to the piezoelectric actuator, pulsed laser emission can be obtained in either of the two PM's, by simply changing the offset of the signal (modes A and B). It is also possible to obtain pulse-to-pulse switching between the two orthogonal PM's by choosing the offset voltage in the neighbourhood of 14.8 V (mode C). Real-time monitoring of the pulsed emission on an oscilloscope allows easy and rapid fine tuning of the offset voltage, in order to get the desired laser operation. Figure 17 shows the results for the three types of operation A, B and C, at 184-kHz modulation frequency. For this modulation frequency, only one pulse is generated during the modulation period. At lower modulation frequencies, multiple pulses are visible during one modulation period. This multi-pulse regime appears when the modulation frequency of the piezoelectric actuator is smaller than the relaxation oscillations frequency of the gain medium. In this multi-pulse regime, the pulse duration and relaxation oscillation frequency are dependent on the pump power: higher pump power leading to shorter pulses at higher frequency. For the pulse-to-pulse PM switching corresponding to an offset voltage of 14.8 V (type C), the laser power did not reach zero between the two pulses as can be seen from figure 5. At the highest modulation frequency of 356 kHz that could be reached with this exemplary set-up, the spectral width of one pulse, measured as before with a frequency comb, was 685 kHz which corresponds to the transform limited pulse of 0.63 μ8 that was measured. Finally, it must be noted that for high pump power, i.e. above 70 m W, CW lasing is obtained for any value of the phase shift, thus preventing Q-switching operation.

[0069] To summarize, this polarization switchable Q-switched DFB fiber laser is based on a rapidly tuneable birefringent phase shift induced in a FBG by mechanical stresses. Modulation frequencies as high as 356 kHz have been achieved, with extinction ratio higher than 25 dB. A new regime of pulse-to-pulse switching between orthogonal PM's has also been demonstrated.

[0070] A further embodiment of the present invention is depicted by way of example in Figure 18. This figure shows a rectangular-shaped phase modulator (similar to the one introduced in Figure 13 but with a longer central channel) and also made of a hyper-elastic material. The hyper-elastic material used for the phase modulation in this particular embodiment is a monocrystalline CuAlBe shape memory alloy. This material was chosen for its hyper-elastic behaviour, which originates from a phase transformation from the martensitic phase to the austenitic phase. This phase transformation can be activated via the application of a stress or by changing the temperature. For this application, it is induced by applying mechanical stress to the alloy with a piezoelectric actuator. Moreover, the thermally induced phase transformation is set to appear between - 80 °C and -120 °C for preserving the alloy from any undesirable thermally induced phase transformation.

[0071] By way of example, the particular phase modulator shown in Figure 18 can be fabricated by drilling a hole and two slots in a 1.5 mm X 1.5 mm X 3 mm alloy block. The hole may be funneled at both ends up to a diameter of 400 μπι to ensure easy fiber insertion from either side, thus providing a central 900-μιη region in direct contact with the fiber. The two slots are located so as to enlarge the hole diameter for the dual purpose of easing fiber insertion and providing a means of adjusting the holding force applied on it. The size of the hole can thus be controlled by a piezoelectric actuator directly acting on the bending portion of the ferrule, which is in turn allows for a rapid and precise control of the pinching force applied on the fiber. The hyper-elastic alloy further allows the fiber to expand almost freely in response to the heat generated by the signal and pump power at the pinching location, thus preventing the fiber from breaking.

[0072] The phase shifting performance of the hyper-elastic alloy based miniature phase modulator was investigated by placing it in the middle of a 5 cm long FBG written in a hydrogenated single-mode optical fiber (SMF) of the ITU-T recommendation G.652 category. An optical vector analyzer (e.g. an OVA, LUNA technologies, model OVA/CTE) was used for recording the modulator transfer function -consisting of its 4 Jones-matrix elements- for various voltages. High-resolution FBG transmission versus wavelength plots were inferred from such transfer function measurements. By numerically changing the input state of polarization (SOP), the eigenstates of the SMA- PM transfer function can be found, including the two fiber segments on both sides of the FBG. These eigenstates correspond to the SOPs that would lie along the birefringence axis of the stressed FBG after propagating in the fiber segment located before the FBG. Fig. 19 shows the FBG transmission spectra for these two SOPs for two different voltages (40V and 50 V) applied to the piezoelectric actuator.

[0073] The locally applied phase-shift is opening up a bandgap within the FBG reflection bandwidth, creating a narrow transmission window. The induced round-trip phase shift amplitudes can thus be derived from the peak transmission wavelengths. Induced round-trip phase shifts for each SOP are plotted against the applied voltage in Figure 20. The round-trip phase shift amplitude can be as high as 4.5π rad and the round- trip phase difference between orthogonal SOPs as high as 1.5π rad. The induced loss is below 0.02 dB for the entire piezo-voltage range.

[0074] Standard DFB-FL (distributed feedback fiber laser) operating in CW mode typically involve FBGs with a permanent π round-trip phase shift induced either by post processing with UV light, by C02 laser pulses or by shifting the phase mask during the FBG fabrication. Alternatively, the introduction of a tuneable phase-shift inside the active fiber grating can lead to Q-switched operation of the DFB-FL, wavelength tuning and output power modulation. [0075] DFB-FL threshold strongly depends on the phase-shift amplitude and has a minimum value for a π- round-trip phase shift. As a result, Q-switched operation can be obtained by rapidly changing the phase shift amplitude between values below and over the threshold value for a specific pump power. If the shift is birefringent enough, only one SOP is experiencing a phase shift within the lasing range so that a linear SOP output is obtained. At the opposite, when the induced phase shift is not sufficiently birefringent, both SOP experience approximately the same phase shift and the emitted SOP is then prescribed by the intrinsic birefringence of the FBG. This behavior is shown in Fig. 21 where the grey rectangle represents the lasing range i.e. the range where lasing threshold is reached for a specific pump power. Q-switched operation range is represented with an arrow in Fig. 21a, where the transmission peak for one SOP is voltage-tuned from one position located inside to another one located outside the lasing range. In Fig. 21b, the principles of wavelength tuning and power modulation are shown. By changing the transmission peak position within a given lasing range, output power modulation is obtained, which is generally accompanied by a wavelength tuning. At large pump power, lasing is obtained for any value of the phase-shift. In this regime, any phase shift modulation leads to a modulation of the output power and wavelength tuning whereas Q- switching is prevented.

[0076] One example of a laser set-up is shown in Fig. 14, already described above. Pump light at, for example, 976nm coming from a pigtailed DFB laser diode is launched into the Er-Yb doped single mode fiber through a WDM coupler. The high concentration of Yb (3.21 %) and Er (0.33%) in the doped fiber provides a very high absorption over 500 dB/m at that wavelength. The active fiber also has a photosensitive Ge ring surrounding the core and was deuterium loaded in order to allow efficient FBG writing. A 5-cm long uniform FBG was written in a 5.5-cm long segment of that active fiber by standard writing technique using 244-nm UV light and a phase mask. A highly reflective FBG with peak reflectivity of about 50 dB was produced. The FBG was then heated to 1 10-120 °C for 3 days, to remove the remaining deuterium and to stabilize it. In order to get unidirectional output in the counter propagating direction, the phase modulator was offset by 7.5 mm from the FBG mid-point, on the launched pump side. This configuration is known to be the most efficient for this type of laser. However, launching the pump from only one side of the doped fiber results in a non-uniform pump absorption and thus to a thermally induced FBG chirp. In addition, the strong signal power at the modulator location can also lead to a thermally induced chirp and instabilities. To minimize these side effects, the fiber was placed in Cu sleeves on both sides of the modulator to act as heat sinks. Note that, in this particular implementation, 3 mm segments of fiber on both sides between the modulator and the Cu sleeves remained uncovered. This partial heat removal around the modulator location was found to somehow limit the maximum achievable power. Tuning the pump wavelength to 980 nm, for example, strongly decreases the pump non-uniformity and induced chirp, but the overall laser efficiency decreases as well. The 976-nm pump wavelength was selected even if thermal instability occurred for high launched pump power.

[0077] The DFB fiber laser presented here allowed for operating in three different modes: CW, pulsed and power modulation. In addition, as the induced phase shift is highly birefringent, single SOP output is easily obtained on either polarization mode with extinction ratio in excess of 25 dB, simply by changing the applied voltage to the piezoelectric actuator.

[0078] In the CW operating mode, the lasing wavelength, output power and SOP are controlled by the voltage applied to the piezo. Fig. 22 shows the results obtained when the voltage is varied between 0 V and 40 V. On Fig. 22a, it can be seen that CW laser operation occurs for values of round-trip phase shift in the vicinity of π or 3π, on either x or y linear SOP, simply by changing the offset voltage. As the lasing threshold is minimum for round-trip phase shifts of π or 3π, maximum laser output power is observed for these values. The corresponding lasing wavelength ranges are shown on Fig. 22b, where sudden changes occur when the laser shifts from one SOP to the other. On Fig. 22c, the laser power is shown against the wavelength for both SOPs. It can then be noted that the wavelength where maximum power is reached is different for both SOPs. This wavelength difference arises from the intrinsic FBG birefringence that leads to different Bragg wavelengths for both SOPs. The difference in the laser power between SOPs is attributed to the different laser thresholds caused by a FBG polarization dependant reflectivity. [0079] The laser linewidth has been investigated with a heterodyne measurement set-up. The RF spectrum of the beating between the laser and a stabilized frequency comb was measured. For a 48-μ8 measurement, for example, a narrow linewidth of 31 kHz was obtained, which is in agreement with a previous report for similar DFB-FL. The central laser frequency was not stable because of the minimum acoustic insulation of the system. Nevertheless, this frequency drift was almost completely removed by using a feedback loop to tune the pump diode current according to the frequency difference between the laser and one frequency from the comb.

[0080] For pulsed operation, applying a 60-V peak-to-peak voltage modulation at a 10-Hz frequency yield laser emission as shown in Fig. 23. For this low repetition rate, high peak-to-peak voltage can be applied to the piezoelectric actuator and the resulting behaviour is reminiscent of the CW plot previously shown in Fig. 22.

[0081] For modulation frequencies in the kHz range though, the effective voltage applied to the piezoelectric actuator is highly damped, resulting in low peak-to-peak voltage and small phase shift modulation. By adjusting the offset of the electric modulation to the positions denoted 1 to 6 on Fig. 23, Q-switched operation can be obtained on either x polarization (positions 1 and 4) or y polarization (positions 3 and 6). In addition, pulse-to-pulse switching from one SOP to the other SOP can be achieved by setting the offset voltage to the positions 2 or 5. The resulting pulse shapes are shown on Fig. 24 for the 7 kHz modulation frequency and on Fig. 25 for the 208 kHz modulation frequency. The fact that the laser power does not return to zero in the polarization switching mode (Fig. 24b and 25b) is readily understood by inspection of Fig. 23, where the laser power is shown to not return to zero between the two SOPs for positions 2 and 5. Nevertheless, total extinction of the laser power between the two SOPs is obtained by pumping just above threshold, but with a corresponding much lower peak power.

[0082] For the 7 kHz repetition rate, relaxation oscillations are visible. Laser theory predicts that the angular frequency of these relaxation oscillations is given by Equation 1 : [0083]

(Equation 1)

[0084] Where r is the ratio of the pump power to the threshold power, TC is the photon cavity lifetime and τ2 is the excited state lifetime of Er (1 1 ms).

[0085] The pulse width and frequency of these oscillations are strongly related to the pump power, as shown on Fig. 26 (white circles), where it can be seen that the pulse width of the first spike decreases and that the inverse of the period between the two first spikes increases when the pump power is increased. The inverse of the period between the first and second oscillations reaches higher value for the SOP switching mode (squares in Fig. 26b) because the switching occurs at round-trip phase shift amplitudes closer to π, i.e. at a lower laser threshold, thus increasing the r parameter in Equation (1). For the 208-kHz modulation frequency, for example, the switching is too fast to allow for relaxation oscillations so that only one laser pulse is produced. As for the relaxation oscillations measured at 7 kHz, the pulse width decreases when the pump power is increased (black circles). The results of Fig. 26b (white circles) and Equation (1) were used to calculate the cavity photon lifetime xc, which gives a result of approximately 0.29 ns for a threshold pump power of 4.5 mW (for this particular implementation).

[0086] Interestingly, one notes that the pulse width reaches slightly smaller values for the 208-kHz frequency (~ 500 ns) than for the 7-kHz frequency (~ 800 ns). For both frequencies, the peak power is strongly limited by the phase shift modulation, which is restricted to values close to the lasing threshold, because of the strong damping of the electrical modulation at high frequency. The Q-switched operation of the DFB-FL comes with wavelength sweeping, leading to chirped laser pulses. This wavelength modulation is explained by the wavelength tuning of the transmission peak observed when the voltage is changed, as shown on Fig. 19. This wavelength sweeping is much higher for the 7-kHz modulation frequency (~ 300 MHz) compared to the 208-kHz modulation frequency (^¾ 1 MHz), because of the much more damped electric modulation in the latter case. The amplitude of the wavelength scanning range was obtained from the measurement of the laser beating with a frequency comb. Note that for the 208-kHz modulation frequency, the measured pulse width of 1 MHz is very close to the transform limited spectral width of the 435 ns pulse that was measured, thus giving an almost unchirped pulse for that high-modulation frequency.

[0087] The long-term reliability of the Q-switched DFB-FL device was investigated by applying a 50-mV peak-to-peak voltage at a 108-kHz modulation frequency while pumping the laser at 18 mW. The pulsed laser could be operated without interruption over 650 hours, provided the small drift of the appropriate off-set voltage to apply to the piezoelectric actuator was compensated periodically. From a practical viewpoint, long-term pulsed operation would be readily feasible by using a feedback loop to maintain this offset voltage at the appropriate value.

[0088] Power modulation is obtained when the voltage is applied so as to scan the transmission peak position within the lasing range, as shown on Fig. 21b. Similarly to the pulsed operation case, the power modulation mode is also accompanied with wavelength tuning and the corresponding spectral range is directly related to the peak-to-peak voltage modulation actually applied to the piezo actuator in agreement with Fig 22.

[0089] Fig. 27 shows examples of the power modulation observed for a 7-kHz modulation frequency and for three different offset voltages. At that frequency, the damped peak-to-peak voltage is about 33 mV. The power modulation peak-to-peak amplitude depends on the offset voltage applied to the piezoelectric actuator; a voltage value corresponding to a round-trip phase shift closer to π would lead to a smaller power modulation. The rapid power oscillations shown on Fig. 27b are attributed to the change in wavelength that disturbs the laser dynamics. The frequency of these oscillations is 571 kHz. This frequency is higher than the relaxation oscillation frequency observed for the SOP switching mode at the same pump power of 40 mW, in agreement with the fact that the round-trip phase shift is even closer to π in this case. As the wavelength scanning range is maximum around the π round-trip phase shift, it could explain why these oscillations are weaker for the smaller voltages.

[0090] The peak-to-peak modulation voltage, which decreases with the modulation frequency because of the damping of the electric signal, also has an impact on the power modulation; a higher voltage modulation leads to a higher power modulation of the laser output.

[0091] The effective cavity length of the DFB-FL can be approximated from

Equation 2, where p and κ are the amplitude reflectivity coefficient the coupling coefficient of each sub-grating, respectively. In our case, the coupling coefficient is constant over the whole FBG so κΐ = κ2= κ.

[0092] ^ef{ 2x_t 2K, ~ 2K (Equation 2)

[0093] The FBG considered for this article has a reflectivity of 50 dB and a length of 5 cm. As the phase shift is located 7.5 mm away from the center of the FBG, it generates two FBG of length 17.5 mm and 32.5 mm. Using κ =130 m^"1 (obtained from simulations) and R=tanh2(KL), corresponding transmission (loss) of 4.2% and 0.1% were calculated. These losses give values of 0.98 and 1.0, respectively, for pi and p2. Substituting these values into Equation 2 gives a cavity length of 7.7 mm and a cavity round-trip time tRt of 17.5 ps, which is related to the cavity loss by Equation 3 and gives cavity round trip loss of .27 dB (6.0%). δ =—

[0094] ^c c (Equation 3)

[0095] As mentioned earlier, the one-way loss generated by the phase modulator is below 0.02 dB for the entire piezo-voltage range. For the π round-trip phase shift, one can use 0.1 dB as the one-way loss and consider 0.2 dB (0.5 %) for the cavity round-trip loss. By using Deuterium only a very small background loss is induced from the FBG writing and it is difficult to measure. It is estimated that these one-way losses are around 0.05 dB so one could consider a 0.1 dB (0.1 %) round-trip loss. The sum of these losses gives a total round-trip loss of 0.26 dB (5.8%), in accordance with loss of the cavity calculated with the relaxation oscillations frequency.

[0096] This novel DFB-FL design possesses several advantages. In fact, the possibility of operating the laser on either a CW or a Q-switched regime is not only novel but also very promising from a practical viewpoint. An important characteristic to achieve this behaviour is the introduction of a high-speed tuneable phase shifter inside the laser cavity. Another interesting property of the laser is the possibility of changing the output SOP. The birefringent phase shift introduced inside the laser cavity actually breaks the degeneracy between the two orthogonal linear SOPs, thus allowing the laser to emit on either x or y linear SOP. The wavelength scanning is another particularity of the laser that comes from the fundamental behaviour of phase-shifted FBG. The tuning of the phase shift actually allows a precise tuning of the laser emission wavelength and is accompanied by a power modulation.

[0097] The performance of the laser either in the Q-switching regime or in the power modulation regime is strongly dependant on the modulation frequency applied to the piezoelectric actuator. The voltage applied to the piezoelectric actuator is highly damped for frequency higher than the cut-off frequency of the piezoelectric actuator. As a result, peak-to-peak phase modulation decreases with the increase of the modulation frequency. Nevertheless, some frequencies well over the cut-off frequency of the piezoelectric actuator exhibit resonance. This resonance leads to higher phase modulation and enables laser Q-switching at resonant frequencies. The highest frequency where laser Q-switching was obtained is 356 kHz. However, optimization of the piezoelectric actuator could allow laser Q-switching on a larger frequency range and at higher maximum frequency.

[0098] Thermal effects are another inherent aspect of the Q-switched DFB-FL.

The high absorption of the doped fiber and the non-radiative decay from the pump level to the upper laser level lead to a significant heating of the doped fiber. Without efficient heat removal, the FBG reflectivity band is strongly broaden by a thermally induced chirp. Although heat removal can be achieved by placing the fiber inside copper (Cu) sleeves on both sides of the shape-memory alloy phase modulator (SMA-PM), it is not possible to encapsulate the fiber along its whole length. As a result, thermal chirp still occurs at high launched pump power and laser emission becomes unstable. [0099] The FBG reflectivity is also an important parameter that affects the heating and many characteristics of the laser. An important effect of the FBG reflectivity is on the effective cavity length: a stronger FBG would lead to a shorter effective cavity length and higher confinement of the signal power around the PS location. It was determined experimentally that a stronger FBG (with reflectivity approaching 80 dB) will be more affected by thermal effects. In this case, lasing was only possible with encapsulation of the fiber in Cu sleeves. For a weaker FBG, like the 50 dB FBG described herein, lasing was obtained even without fiber encapsulation for pump powers as high as 50 mW and more, but laser power saturates for pump powers over 50 mW. The higher thermal sensitivity of the stronger FBG may be attributed to the higher signal power present at the phase shift location, which causes distortion of the PS and the FBG.

[00100] In conclusion, the SOP switchable Q-switched DFB-FL can operate in continuous wave (CW), pulsed or power modulation modes, because of the tuneable phase shift (PS) induced in the FBG. Relaxation oscillations are obtained when the PS is modulated at frequencies lower that the laser natural relaxation oscillation frequency. As predicted theoretically, the frequency of the oscillation increases with the pump power.

[00101] The embodiments of the present invention described above are meant to be exemplary and illustrative only. The embodiments and examples presented herein are not meant to limit the scope of the invention. Persons of ordinary skill in the art, having regard to the present disclosure, will readily appreciate that the embodiments described above may be modified, varied, refined and altered without departing from the spirit and scope of the present invention as defined by the appended claims.

Claims

1. An optical fiber phase modulator comprising:

a body made of a highly elastically deformable material, the body having a first end and a second end; and

a fiber conduit formed in the body and extending from the first end of the body to the second end of the body and a slot extending from an outer surface of the body into the fiber conduit, wherein the fiber conduit is dimensioned to receive an optical fiber and wherein the fiber conduit may be expanded and contracted to vary a compressive stress on the optical fiber to thereby modulate a phase of an optical signal carried by the optical fiber.

2. The optical fiber phase modulator as claimed in claim 1 wherein the body is made of a shape memory alloy.

3. The optical fiber phase modulator as claimed in claims 1 or 2 wherein the body is a rectangular block having:

a slit extending from the fiber conduit to an upper surface of the block upon which the piezoelectric actuator acts; and

a deformation cavity extending orthogonally to the slit to facilitate limited deformation of the block.

4. The optical fiber phase modulator as claimed in claims 1 or 2 wherein the slot comprises a wide portion and a narrow portion, the wide portion having inwardly facing walls against which outwardly acting transverse forces are applied by an actuator to expand the fiber conduit.

5. The optical fiber phase modulator as claimed in claims 4 further comprising L-shaped separator arms to exert the forces on the inwardly facing walls of the wide portion of the slot.

6. The optical fiber phase modulator as claimed in claim 5 wherein the actuator is a piezoelectric stack connected to at least one of the L-shaped separator arms.

7. The optical fiber phase modulator as claimed in any one of claims 1 to 6 wherein an induced phase shift produced by the modulator is different for two orthogonal components of light, thus inducing birefringent sections in said optical fiber.

8. The optical fiber phase modulator as claimed in any one of claims 1 to 7 wherein the optical loss induced in the optical fiber is less than 0.02 dB.

9. The optical fiber phase modulator as claimed in any one of claims 1 to 8 wherein the optical fiber phase modulator is used as a part of a polarization scrambler.

10. The optical fiber phase modulator as claimed in any one of claims 1 to 8 wherein the optical fiber phase modulator is used as a polarization switch.

11. The optical fiber phase modulator as claimed in any one of claims 1 to 8 wherein said optical fiber comprises a fiber Bragg grating (FBG) written on a section of the optical fiber.

12. The optical fiber phase modulator as claimed in any one of claims 1 to 8 wherein said optical fiber phase modulator is used to induce one or more phase shifts in a FBG, to thereby provide a phase-shifted FBG.

13. The optical fiber phase modulator as claimed in claim 12 wherein said phase-shifted FBG is used as a narrowband tunable filter.

14. The optical fiber phase modulator as claimed in claim 12 wherein said phase-shifted FBG is used in a distributed feedback (DFB) fiber laser.

15. The optical fiber phase modulator as claimed in claim 14 wherein said optical fiber phase modulator is use to induce Q-switching in the DFB fiber laser.

16. A method of modulating a phase of an optical signal carried in an optical fiber, the method comprising:

inserting the optical fiber into a fiber conduit formed in a highly elastically deformable body of an optical fiber phase modulator;

transmitting the optical signal through the optical fiber; and

exerting an external force on the highly elastically deformable body of the optical fiber phase modulator to cause the fiber conduit in the body of the optical fiber phase modulator to vary a compressive stress on the optical fiber to thereby modulate the phase of the optical signal carried in the optical fiber.

17. The method as claimed in claim 16 wherein exerting the external force comprises causing a pair of separator arms to exert outwardly acting transverse forces on inwardly facing walls of a wide portion of a shouldered slot formed in the body of the optical fiber phase modulator.

18. The method as claimed in claim 17 wherein the separator arms are actuated by a piezoelectric stack.

19. The method as claimed in any one of claims 16 to 18 wherein a plurality of optical fiber phase modulators that are connected serially to a common optical fiber are independently actuated to individually modulate the phase of the optical signal at different locations along the optical fiber.

20. The method as claimed in claim 19 wherein the optical fiber phase modulator is used to induce phase shifts at precise locations in a fiber Bragg grating (FBG).

21. The method as claimed in claim 20 wherein the fiber Bragg grating incorporating the optical fiber phase modulator is used as part of a DFB fiber laser.

22. The method as claimed in claim 20 wherein the phase modulator is use to switch the DFB fiber laser operation mode between the two orthogonal polarisations modes.

23. The method as claimed in claim 20 wherein the fiber Bragg grating incorporating the optical fiber phase modulator is used as a narrow band pass filter.

24. The method as claimed in any one of claims 16 to 19 further comprising:

inducing a birefringent phase shift in the optical fiber for two orthogonal polarizations to generate two distinct transmission peaks for the optical signal propagating in the optical fiber; and

adjusting a location of one of the two transmission peaks by exerting the external force on the body of the modulator to thereby provide a tunable birefringent phase modulator.

25. The method as claimed in claim 24 wherein the tunable birefringent phase modulator is used in a fiber polarization switch.

26. The method as claimed in claim 24 wherein the tunable birefringent phase modulator is used in a polarization switchable DFB fiber laser.

27. The method as claimed in claim 24 wherein the tunable birefringent phase modulator is used in a polarization switchable Q-switched DFB fiber laser.

28. A polarization switchable Q-switched DFB fiber laser comprising:

an optical fiber phase modulator having a highly elastically deformable body and a fiber conduit formed in the body;

a piezoelectric actuator for exerting a compressive force on the body of the optical fiber phase modulator to thereby induce a birefringent phase shift in the optical fiber for two orthogonal polarizations; and means for applying an offset voltage while varying the compressive force to cause pulse-to-pulse switching between the two orthogonal polarizations.

29. The DFB fiber laser as claimed in claim 27 wherein the modulator is a rectangular block made of a shape memory alloy, the block having:

30. A method of Q-switching a distributed feedback laser, the method comprising:

transmitting the optical signal through the optical fiber;

exerting an external force on the highly elastically deformable body of the optical fiber phase modulator to cause the fiber conduit in the body of the optical fiber phase modulator to vary a compressive stress on the optical fiber to thereby modulate the phase of the optical signal carried in the optical fiber and to induce a birefringent phase shift in the optical fiber for two orthogonal polarizations to generate two distinct transmission peaks for the optical signal propagating in the optical fiber; and

applying an offset voltage while varying the external force to cause pulse-to-pulse switching between the two orthogonal polarizations.