WO2003001262A1

WO2003001262A1 - Apparatus and method for variable optical output control

Info

Publication number: WO2003001262A1
Application number: PCT/US2002/019420
Authority: WO
Inventors: Andrei N. Starodoumov; Andrey A. Demidov; Xiaojun Li; Leonard Wan; Ian Lee; Joseph F. Deck; Richard S. Ferreira; Robert J. Hannemann; Philip R. Norris
Original assignee: Optical Power Systems Inc
Current assignee: Optical Power Systems Inc
Priority date: 2001-06-22
Filing date: 2002-06-19
Publication date: 2003-01-03
Anticipated expiration: 2003-12-22

Abstract

This invention relates to aaparatus and methods for variable optical output control and systems containing such controls. The apparatus comprises an optical fiber (10) having two identical Bragg gratings (G1, G2). The fiber (1) is attached to a housing (12) at three points (F1, F2, F3) and a central portion (14) of the housing (12) is movable. The wavelength of the optical signal propagating in the fiber is varied by applying a force to the fiber (10). The apparatus can be also used with a fiber laser.

Description

Apparatus and Method for Variable Optical Output Control

TECHNICAL FIELD

This invention relates to apparatus and methods for variable optical output control and systems containing such controls.

BACKGROUND

Optical fibers are essentially thin strands of glass capable of transmitting information-containing optical signals over long distances with low loss. In essence, an optical fiber is a small diameter waveguide comprising a core having a first index of refraction surrounded by a cladding having a second (lower) index of refraction. As long as the refractive index of the core exceeds that of the cladding, a light beam propagated along the core exhibits total internal reflection, and is guided along the length of the core. Typical optical fibers are made of high purity silica. Various concentrations of dopants may be added to control the index of refraction.

It is known in the art of optical fibers to impress one or more Bragg gratings in the core of an optical fiber. A typical Bragg grating comprises a length of optical waveguide, such as optical fiber, in which a plurality of perturbations in the index of refraction are substantially equally-spaced along the waveguide length. A Bragg grating of this kind reflects the light launched into the fiber core for guided propagation. Only that light having a wavelength within a very narrow range dependent on the grating element periodicity is reflected back along the fiber axis opposite to the original propagation direction. The grating is substantially transparent to light at wavelengths outside this narrow band and therefore does not adversely affect the further propagation of such light. In effect, this type of grating creates a narrow notch in the transmission spectrum, and by the same token a similarly narrow peak in the reflection spectrum. Bragg gratings may be conveniently fabricated by doping a waveguide core with one or more dopants sensitive to ultraviolet light, e.g., germanium or phosphorous, and exposing the waveguide at spatially periodic intervals to a high intensity ultraviolet light source, e.g., an excimer laser. The ultraviolet light interacts with the photosensitive dopant to produce long-term perturbations in the local index of refraction. The appropriate periodic spacing of perturbations can be obtained by use of a physical mask, a phase mask, or a pair of interfering beams. Bragg gratings have many uses such as for sensor devices and as components for fiber communications. They provide a wavelength-tunable reflective element which can be used as transducer elements in fiber sensors, as wavelength control devices for fiber, semiconductor, and solid state lasers, as wavelength division multiplexing components in communication systems, as wavelength analyzers, as components in signal processing systems, and for other uses.

In many applications, it would be desirable to have an optical controller that can quickly alter the output intensities of one or more fiber laser wavelengths without affecting other wavelengths the fiber laser(s) generate. The present invention provides such an optical output control.

SUMMARY

In general, the invention relates to apparatus and methods for variable optical output control and systems containing such controls. In general, one aspect of the invention features a method for varying the transmission characteristics of an optical signal propagating in an optical medium that includes a first and second Bragg grating, the method comprising applying a force to the optical medium to change the peak reflection wavelength of the first Bragg grating from a first wavelength to a second wavelength and to change the peak reflection wavelength of the second Bragg grating from a third wavelength to a fourth wavelength, thereby varying the transmission characteristics of the optical signal propagating in the optical medium.

In another aspect, the applying force simultaneously changes the reflection peak of the first and second Bragg grating. In a further aspect, the optical medium is an optical fiber.

In a further aspect, the first wavelength may be equal to the third wavelength, while the second wavelength is less than the first wavelength, and the fourth wavelength is greater than the third wavelength.

In a still another aspect, the applying force comprises applying a force to shorten the first Bragg grating and a force to lengthen the second Bragg grating.

In yet another aspect, applying the force changes the peak reflection wavelength of the first Bragg grating by changing a first grating period of the first Bragg grating, while applying the force changes the peak reflection wavelength of the second Bragg grating by changing a second grating period of the second Bragg grating.

In still another aspect, the first grating period changes from a first period to a second period less than the first period, and the second grating period changes from a third period to a fourth period greater than the fourth period.

In still further aspects, the force is a mechanical force, e.g. thermo-mechanical, electro-mechanical, magneto-mechanical, electro-strictive, magneto-strictive, etc.

In general, another aspect of the invention features an optical device comprising an optical medium having first and second Bragg gratings; and a device configured to apply a force on the optical medium between the first and second Bragg gratings to simultaneously change the peak reflection wavelength of the first Bragg grating from a first wavelength to a second wavelength, and to change the peak reflection wavelength of the second Bragg grating from a third wavelength to a fourth wavelength.

In another aspect, the device is configured to indirectly apply the force to the optical medium.

In a further aspect, the optical medium is an optical fiber.

In yet another aspect, the device comprises a coupler attached to the optical medium between the first and second Bragg gratings, wherein the device further comprises an actuator configured to apply a force to the coupler. In still a further aspect, the device comprises a piezo-electric element.

In still another aspect, the first Bragg grating has a first grating period and the second Bragg grating has a second grating period equal to the first grating period, wherein the device is configured to cause a period of the first grating to change from a first value to a second value less than the first value, and a period of the second grating to change from a third value to a fourth value greater than the third value.

In another aspect, the first Bragg grating is a chirped Bragg grating, or the second Bragg grating is a chirped Bragg grating, or both the first and second Bragg gratings are chirped Bragg gratings.

In still another aspect, the device is attached to the optical medium at a first and second attachment point, wherein the first and second Bragg gratings are located between the first and second attachment points.

In yet another aspect, the first wavelength is between 0.3 and 3 microns. In general, another aspect of the invention features an optical system further comprising a source configured to transmit an optical signal to the optical device; and a receiver configured to receive an optical signal transmitted by the optical device.

In another aspect, the optical fiber is configured to guide an optical signal from the transmitter to the optical device, and an optical signal from the optical device to the receiver.

In another aspect, the optical system source is a laser.

In general, another aspect of the invention features an optical device comprising an optical fiber; and a housing attached to the optical fiber at first, second and third attachment points, the third attachment point being between the first and second, and the housing being configured to apply a force to the optical fiber at the second attachment point.

In another aspect, the housing comprises an actuator configured to apply the force to the third attachment point, wherein the applied force results in shortening the optical fiber between the first attachment point and the third attachment point, and lengthening the optical fiber between the second attachment point and the third attachment point.

In a further aspect, the optical fiber includes a first reflector between the first attachment point and the third attachment point, and a second reflector between the second attachment point and the third attachment point, wherein the first and second reflectors comprise Bragg gratings.

In still another aspect, the first reflector has a first grating period and the second reflector has a second grating period equal to the first grating period.

In general, another aspect of the invention features an optical cavity comprising a variable reflector comprising an optical medium having first and second Bragg gratings; and a device configured to apply a force on the optical medium between the first and second Bragg gratings to simultaneously change the peak reflection wavelength of the first Bragg grating from a first wavelength to a second wavelength and to change the peak reflection wavelength of the second Bragg grating from a third wavelength to a fourth wavelength; a second reflector; and an optical medium located between the variable reflector and the second reflector. In another aspect, an optical signal at the first wavelength propagating in the optical medium oscillates between the variable reflector and the second reflector in operation.

In yet another aspect, the optical medium comprises an active material that provides gain to the optical signal propagating in the optical medium and the optical cavity further comprises an energy source for exciting the active material.

In general, another aspect of the invention features a method for varying the reflectance at a first wavelength of an optical signal in an optical medium including first and second Bragg gratings, the method comprising applying a force to the optical medium to reduce the reflectance of the first and second Bragg gratings at the first wavelength, wherein the peak reflectance wavelength of the first Bragg grating shifts from a first initial wavelength to a longer wavelength and the peak reflectance wavelength of the second Bragg grating shifts from a second initial wavelength to a shorter wavelength. In general, another aspect of the invention features an optical device, comprising an optical medium having first and second Bragg gratings; and a device configured to apply a force to the optical medium between the first and second Bragg gratings to simultaneously reduce the reflectance of the first and second Bragg gratings at a first wavelength, wherein a peak reflectance wavelength of the first Bragg grating shifts from a first initial wavelength to a longer wavelength and a peak reflectance wavelength of the second Bragg grating shifts from a second initial wavelength to a shorter wavelength.

In another aspect, the optical device further comprises a sheath surrounding a portion of the optical medium between the first and second attachment points, wherein the sheath is hermetically sealed to the optical medium.

In general, another aspect of the invention features a fiber laser, comprising a variable reflector, comprising an optical medium having first and second Bragg gratings; and a device configured to change the peak reflection wavelength of the first Bragg grating from a first wavelength to a second wavelength and to change the peak reflection wavelength of the second Bragg grating from a third wavelength to a fourth wavelength; a second reflector; and an optical medium located between the variable reflector and the second reflector, the optical medium comprising an active material. In another aspect, the peak reflection wavelengths of the fiber laser are changed by applying a force to the optical medium between the first and second Bragg gratings.

In still another aspect, the peak reflection wavelengths of the fiber laser may be changed simultaneously. In a further aspect, the fiber laser active material comprises a rare earth element.

In another aspect, the peak reflection wavelength of the first Bragg grating may be changed by decreasing the period of the first Bragg grating, while the peak reflection wavelength of the second Bragg grating may be changed by increasing the period of the first Bragg grating. In another aspect, the fiber laser is configured to indirectly apply the force to the optical medium.

In a further aspect, the fiber laser includes a coupler attached to the optical medium between the first and second Bragg gratings, and an actuator configured to apply a force to the coupler. In a further aspect, the fiber laser includes a piezo-electric element.

In a further aspect, the first Bragg grating has a first grating period and the second Bragg grating has a second grating period equal to the first grating period, wherein the fiber laser is configured to cause a period of the first grating to change from a first value to a second value less than the first value, while a period of the second grating changes from a third value to a fourth value greater than the third value.

In yet a further aspect, the first Bragg grating is a chirped Bragg grating.

In another aspect, the fiber laser includes an optical device that is attached to the optical medium at a first and second attachment point, wherein the first and second Bragg gratings are located between the first and second attachment points. In a further aspect, the fiber laser generates a first wavelength that is between

0.3 and 3 microns.

In general, another aspect of the invention features a fiber laser, a waveguide configured to transmit an optical signal from the fiber laser; and a receiver configured to receive an optical signal transmitted by the waveguide.

Features, objects and advantages of the invention are in the description, drawings and claims. DESCRIPTION OF DRAWINGS

The above and other objects and features advantages of the present invention will become apparent from the following description given in conjunction with the accompanying drawings, wherein: 5 FIG. 1 is a schematic view of an embodiment of an output coupler according to the present invention.

FIG. 2 is a graph showing experimental results obtained from an embodiment of the present invention.

FIG. 3 is a schematic view of another embodiment of an output coupler o according to the present invention.

FIG. 4 is a graph showing experimental results obtained from an embodiment of the present invention.

FIG. 5 is a schematic view of another embodiment of an output coupler according to the present invention. 5 FIGS. 6 A, 6B, 6C and 6D are graphs showing simulated experimental results obtained from embodiments of the present invention.

DETAILED DESCRIPTION

In general, if grating reflectivities are Ri(λ) and R₂(λ) respectively, the total 0 reflectivity of two gratings is as follows:

l - R₁{λ)R₂^) (a)

5 Equation (a) is derived for an incoherent light wave. In the case of coherent light, the reflectance is defined by a more complicated formula according to the Fabry- Perot interferometric effect. When the distance between gratings is large compared with the wavelength λ and the wave is partially incoherent, however, the interferometric phenomena can be neglected. In a Fabry-Perot interferometer with essentially ideal 0 coherent light, for example, the distance between two neighboring fringes is about: δλ = , where n is the fiber refractive index.

2Ln

If L = 2 cm and λ = 1300 nm, the spacing δλ = 0.03 nm. Thus, light sources with relative coherence less than δλ/λ = 2x10^~5 may be treated as incoherent sources to which Equation (a) is applicable.

When an external force is applied and spectral shift Δλ is introduced, Equation (a) takes the following form:

₌ R_l(A₎ - Δλ) + R₂ {Λ₀ + ΔΛ) - 2R, { - Aλ)R₂ (Λp + ΔΛ)

R_lnl(Λ,AΛ) 1 - ^CΛo - ΔΛ)R₂ ( , + ΔΛ)

When the device is used together with another reflector to form a laser cavity, the spectral selectivity of the laser cavity may be achieved by the spectral selectivity of the other reflector, R_ϋR(λ) (e.g., a Bragg grating), located at the opposite side of the fiber laser cavity. Preferably, the center of the R_HRW reflectivity curve is positioned at λ₀ as well. The resulting spectral function that controls the spectral properties and energy balance of the fiber laser is defined by the following equation:

R = R_tot{λ,Aλ)R_HR{λ)

Referring now to the drawings, and more particularly to Fig. 1, an embodiment of a variable output coupler is disclosed. As shown in Fig. 1, optical fiber 10 contains two identical Bragg gratings Gl and G2. Fiber 10 is attached to housing 12 at three points (FI, F2, F3). Central portion 14 of housing 12 is movable as shown by the arrows. This movement compresses grating G2 and expands grating Gl by equal amounts ΔL (or vice versa). The resulting strain ε = j ΔL | LI = | ΔL | /L2 causes the change of the central Bragg wavelength by Δλ:

= εd-P.) (1) > where λ₀ is the position of the grating reflectance spectrum when it is relaxed; P_ε is a photoelastic coefficient which takes into account the dependence of the fiber refractive index on strain. For a silica based fiber P_ε = 0.22. Strain is related to applied force F as follows: ε = {F I S)l E , where S is a fiber cross-section and E is a Young's constant. For a silica based fiber E = 8 • 10¹⁰ Pascal .

Equation 1 yields the following values of ΔL and F when fiber 10 has a 125 μm diameter, λ₀ = 1300 nm and Δλ = 1.3 nm:

Λ 1 1

ΔL = LI = (25000 μm) • 10^"3 - 1.28 ≡ 32μm

ΔL^ ₌ 32 3.14(125 - 10^-6)² F = 2εSE = 2—^—E = 2 ' 8 - 10¹⁰ = 2 - 1..26 N ^' » (260 g) fore II 4 25000 4

A factor of two in the above equation accounts for simultaneous compression and stretching of both gratings. Thus, one needs to apply approximately 260 g force and move central portion 14 of housing 12 by approximately 32 μm to obtain about 1.3 nm tuning of the Bragg wavelengths. The spectrum of grating G2 moves to a shorter range by Δλ (compression), while the Bragg wavelength of grating Gl moves to a longer range by approximately the same amount Δλ (expansion). The equality of these changes is secured by a small induced strain (~ 10^"3).

The driver for central portion 14 of housing 12 is preferably a multi-layer piezo- ceramic.

Fig. 2 shows actual results obtained from experiments with a variable output coupler such as that shown in Fig. 1.

Referring now to Fig. 3, another embodiment of a variable output coupler is disclosed. As shown in Fig. 3, optical fiber 10 is attached to flexible member 14 on two sides. Flexible member 14 is configured so that it can bend as shown in Fig. 3 by an angle ±ΔΘ. This bending causes first grating Gl to compress while second grating G2 is stretched by substantially equal displacement amounts Δx (or vice versa).

Fig. 4 shows the spectral response of a variable output coupler such as that shown in Fig. 3. Temperature control also can be used as a driver in the present invention. If temperature is used as the driving force for the variable output coupler of the present invention, the dependence of Δλ on ΔT in a silica based fiber is defined by the following formula:

Aλ ≡ 0.0071 - AT

For example, a 100° C change in temperature causes about 0.7 nm spectral shift. Such a temperature control is preferably to be used in combination with temperature sensitive housing materials (e.g. Ti-Ni, Aluminum, etc.)

Referring now to Fig. 5, another embodiment of a variable output coupler is disclosed in which temperature is the driving force. As shown in Fig. 5, optical fiber 10 contains two Bragg gratings Gl and G2. Fiber 10 is attached to housing 12 at three points (FI, F2, F3). Housing 12 contains temperature sensitive portions 14 and 16, which respond differently to temperature variations. Preferably, temperature sensitive portion 14 is comprised of a material that compresses when heated (e.g., Ti-Ni), while temperature sensitive portion 16 is comprised of a material that expands when heated (e.g., Aluminum), or vice versa. Portions 18 and 20 adjacent to temperature sensitive portions 14 and 16, respectively, are provided to allow for such compression and expansion to occur. Portions 18 and 20 may be fitted with flexible members, e.g. springs. The compression and expansion of temperature sensitive portions 14 and 16 causes first grating Gl to compress while second grating G2 is stretched by substantially equal displacement amounts Δx (or vice versa).

Fabry-Perot Effect

In the configuration shown in Fig.l, the effect of Fabry-Perot interference can be neglected if the coherence of Raman light is less than approximately 0.06 nm. Indeed, the distance between the Fabry-Perot fringes is about:

Although improvement can be obtained by increasing the length of separation between gratings Gl and G2, this increase in length L requires an increase in ΔL in order to achieve the desired tuning. As a result, L = 2 cm was selected as a reasonable compromise.

In the situation when the Fabry-Perot effect can be neglected, the reflectance of the pair of gratings Gl and G2 is described by the following equation:

^{totai )} \ - R\{λ - λ)R2{λ + Aλ))

The effective spectral profile {Sel{λ,Δλ)) of the generated wave is defined by the product of the high reflectance (HR) mirror (grating) R_HROO located on the other side of a fiber (not shown in Fig.1) and R_totaι (see Equation 2):

Sel{λ, ΔΛ) = R_total {Λ, ΔΛ) • R_HR (Λ) (3)

Figs. 6A, 6B, 6C and 6D contains calculations of RI, R2, R_totai and Sel at different Δλ. Bragg wavelength λ₀ = 1300 nm; FWHM of RI (λ) equals FWHM of

R2(λ) (R1=R2), which are equal to 1.7 nm; while the FWHM of R_HR is about 0.8 nm.

Figs. 6A-6D indicate that a tuning range of Sel{λ,Aλ) from 60-90% to 8-15% can be achieved within the strain range cited above (ΔL ~ 30-35 μm).

An important consideration in producing a variable output coupler according to the present invention is the tendency of a compressed fiber to bend under strain. This tendency can be reduced by reinforcing at least a portion of the fiber (e.g., encapsulating the fiber in a glass capillary or body of epoxy). For example, the whole assembly can be epoxy sealed. Preferably, the fiber is teflon recoated before epoxying the assembly. Alternatively, both gratings can be put under tensile stress statically. A force applied at a point between the gratings would then increase the tensile stress in one grating while reducing the tensile stress in the other. The effect would then be to lengthen one grating while shortening the other.

Preferably, the output coupler is hermetically sealed. For example, the active parts of the output coupler may be assembled with the fiber bonded under tension to two mounting zones on a base (e.g. quartz) and a ferrule bonded to the fiber at the exit and entrance points. The coupler may then be mounted in a sealable housing, preferably made from a low thermal expansion metal such as Invar. The ferrules may be bonded to sealing pieces made from a higher expansion metal such as brass or stainless steel. Preferably, these two junctions are well inboard of the secondary junctions of the sealing pieces to the housing such that the thermal expansion of the sealing pieces can compensate for the difference between the thermal expansion of the quartz fiber and the housing. The length of exposed fiber may be treated with a reinforcing material (e.g. a ring of gold flashing) near the exit and entry zones of the control volume before it is attached to the bending device, adjusted and bonded (e.g. with curable adhesive) to ground mounting zones on the mounting base. Preferably, the fiber is treated in selective zones with at least one layer of reinforcing material (e.g. metal flash, gold, solder, etc.) During the fiber attaching process, a cap piece may be used to sandwich the fiber ends with the base near, but not touching, each gold ring. This cap piece is preferably made from thin (e.g. 1mm) metalized quartz with side edges precisely ground to match the width of the base in order to help distribute the adhesive on the fiber in a more controlled manner.

While certain embodiments of the invention have been disclosed herein, the invention is not limited to these embodiments. A number of other embodiments are possible. Any two reflectors separated by a distance such that their reflection profiles can be simultaneously shifted in opposite directions by an equal amount Δλ (nm) from their initial spectral position at λ₀, where Δλ « λ₀ can be used. The driving force could be a temperature change, a mechanical force, a magnetic force, an electro- and/or magnetostriction, etc. Other fiber arrangements and configurations are possible within the spirit of the current invention. Other embodiments are in the claims.

Claims

1. A method for varying the transmission characteristics of an optical signal propagating in an optical medium including a first and a second Bragg grating, the method comprising: applying a force to the optical medium to change the peak reflection wavelength of the first Bragg grating from a first wavelength to a second wavelength and to change the peak reflection wavelength of the second Bragg grating from a third wavelength to a fourth wavelength, thereby varying the transmission characteristics of the optical signal propagating in the optical medium.

2. The method of claim 1, wherein applying the force simultaneously changes the peak reflection wavelength of the first Bragg grating and the second Bragg grating.

3. The method of claim 1, wherein the optical medium comprises an optical fiber.

4. The method of claim 1, wherein the first wavelength is equal to the third wavelength.

5. The method of claim 1, wherein the second wavelength is less than the first wavelength.

6. The method of claim 5, wherein the fourth wavelength is greater than the third wavelength.

7. The method of claim 1, wherein applying the force decreases a length of the first Bragg grating.

8. The method of claim 1, wherein applying the force increases a length of the second Bragg grating.

9. The method of claim 1, wherein applying the force changes the peak reflection wavelength of the first Bragg grating by changing a first grating period of the first Bragg grating.

10. The method of claim 9, wherein the first grating period changes from a first period to a second period less than the first period.

5 11. The method of claim 9, wherein applying the force changes the peak reflection wavelength of the second Bragg grating by changing a second grating period of the second Bragg grating.

12. The method of claim 11, wherein the second grating period changes from a o third period to a fourth period greater than the fourth period.

13. The method of claim 1, wherein the force comprises a mechanical force.

14. The method of claim 13, wherein the mechanical force comprises a thermo- 5 mechanical force.

15. The method of claim 13, wherein the mechanical force comprises an electro-mechanical force.

0 16. The method of claim 13, wherein the mechanical force comprises a magneto-mechanical force.

17. The method of claim 1, wherein the mechanical force comprises an electro- strictive force. 5

18. The method of claim 1, wherein the mechanical force comprises a magneto- strictive force.

19. An optical device, comprising: 0 an optical medium having first and second Bragg gratings; and a device configured to apply a force on the optical medium between the first and second Bragg gratings to simultaneously change the peak reflection wavelength of the first Bragg grating from a first wavelength to a second wavelength and to change the peak reflection wavelength of the second Bragg grating from a third wavelength to a fourth wavelength.

20. The optical device of claim 19, wherein the device is configured to indirectly apply the force to the optical medium.

21. The optical device of claim 19, wherein the optical medium is an optical fiber.

22. The optical device of claim 19, wherein the device comprises a coupler attached to the optical medium between the first and second Bragg gratings.

23. The optical device of claim 22, wherein the device further comprises an actuator configured to apply a force to the coupler.

24. The optical device of claim 19, wherein the device comprises a piezoelectric element.

25. The optical device of claim 19, wherein the first Bragg grating has a first grating period and the second Bragg grating has a second grating period equal to the first grating period.

26. The optical device of claim 19, wherein the device is configured to cause a period of the first grating to change from a first value to a second value less than the first value.

27. The optical device of claim 26, wherein the device is configured to causes a period of the second grating to change from a third value to a fourth value greater than the third value.

28. The optical device of claim 19, wherein the first Bragg grating is a chirped Bragg grating.

29. The optical device of claim 19, wherein the device is attached to the optical medium at a first and second attachment point.

30. The optical device of claim 29, wherein the first and second Bragg gratings are located between the first and second attachment points.

31. The optical device of claim 19, wherein the first wavelength is between 0.3 and 3 microns.

32. An optical system, comprising: the optical device of claim 19; a source, configured to transmit an optical signal to the optical device; and a receiver, configured to receive an optical signal transmitted by the optical device.

33. The optical system of claim 32, further comprising an optical fiber configured to guide an optical signal from the transmitter to the optical device.

34. The optical system of claim 32, further comprising an optical fiber configured to guide an optical signal from the optical device to the receiver.

35. The optical system of claim 32, wherein the source is a laser.

36. An optical device, comprising: an optical fiber; and a housing attached to the optical fiber at first, second and third attachment points, the third attachment point being between the first and second, and the housing being configured to apply a force to the optical fiber at the second attachment point.

37. The optical device of claim 36, wherein the housing comprises an actuator configured to apply the force to the third attachment point.

38. The optical device of claim 36, wherein the applied force decreases a length of a portion of the optical fiber between the first attachment point and the third attachment point.

5 39. The optical device of claim 36, wherein the applied force increases a length of a portion of the optical fiber between the second attachment point and the third attachment point.

40. The optical device of claim 36, wherein the optical fiber includes a first o reflector between the first attachment point and the third attachment point.

41. The optical device of claim 40, wherein the optical fiber includes a second reflector between the second attachment point and the third attachment point.

5 42. The optical device of claim 41, wherein the first and second reflectors comprise Bragg gratings.

43. The optical device of claim 42, wherein the first reflector has a first grating period and the second reflector has a second grating period equal to the first grating 0 period.

44. An optical system, comprising: the optical device of claim 36; a source, configured to transmit an optical signal to the optical device; and 5 a receiver, configured to receive an optical signal transmitted by the optical device.

45. The optical system of claim 44, wherein the source is a laser.

0 46. An optical cavity, comprising: a variable reflector, comprising: an optical medium having first and second Bragg gratings; and a device configured to apply a force on the optical medium between the first and second Bragg gratings to simultaneously change the peak reflection wavelength of the first Bragg grating from a first wavelength to a second wavelength and to change the peak reflection wavelength of the second Bragg grating from a third wavelength to a fourth wavelength; a second reflector; and an optical medium located between the variable reflector and the second reflector.

47. The optical cavity of claim 46, wherein during operation an optical signal at the first wavelength propagating in the optical medium oscillates between the variable reflector and the second reflector.

48. The optical cavity of claim 46, wherein the optical medium comprises an active material that provides gain to the optical signal propagating in the optical medium.

49. The optical cavity of claim 48, further comprising an energy source for exciting the active material.

50. The optical cavity of claim 46, wherein the optical medium is an optical fiber.

51. A method for varying the reflectance at a first wavelength of an optical signal in an optical medium including first and second Bragg gratings, the method comprising: applying a force to the optical medium to reduce the reflectance of the first and second Bragg gratings at the first wavelength, wherein the peak reflectance wavelength of the first Bragg grating shifts from a first initial wavelength to a longer wavelength and the peak reflectance wavelength of the second Bragg grating shifts from a second initial wavelength to a shorter wavelength.

52. The method of claim 51, wherein the first initial wavelength equals the second initial wavelength.

53. The method of claim 51, wherein applying the force simultaneously changes the peak reflection wavelength of the first Bragg grating and the second Bragg grating.

54. The method of claim 51, wherein the optical medium comprises an optical fiber.

55. The method of claim 51, wherein applying the force decreases the length of the first Bragg grating.

56. The method of claim 51, wherein applying the force increases the length of the second Bragg grating.

57. The method of claim 51, wherein applying the force changes the peak reflection wavelength of the first Bragg grating by changing a first grating period of the first Bragg grating.

58. The method of claim 57, wherein the first grating period changes from a first period to a second period less than the first period.

59. The method of claim 57, wherein applying the force changes the peak reflection wavelength of the second Bragg grating by changing a second grating period of the second Bragg grating.

60. The method of claim 59, wherein the second grating period changes from a third period to a fourth period greater than the fourth period.

61. An optical device, comprising: an optical medium having first and second Bragg gratings; and a device configured to apply a force to the optical medium between the first and second Bragg gratings to simultaneously reduce the reflectance of the first and second Bragg gratings at a first wavelength, wherein a peak reflectance wavelength of the first Bragg grating shifts from a first initial wavelength to a longer wavelength and a peak reflectance wavelength of the second Bragg grating shifts from a second initial wavelength to a shorter wavelength.

62. The optical device of claim 61, wherein the device is configured to indirectly apply the force to the optical medium.

63. The optical device of claim 61, wherein the optical medium is an optical fiber.

64. The optical device of claim 61, wherein the device comprises a coupler attached to the optical medium between the first and second Bragg gratings.

65. The optical device of claim 64, wherein the device further comprises an actuator configured to apply a force to the coupler.

66. The optical device of claim 61, wherein the device comprises a piezoelectric element.

67. The optical device of claim 61, wherein the first Bragg grating has a first grating period and the second Bragg grating has a second grating period equal to the first grating period.

68. The optical device of claim 61, wherein the device is configured to cause a period of the first grating to change from a first value to a second value less than the first value.

69. The optical device of claim 68, wherein the device is configured to causes a period of the second grating to change from a third value to a fourth value greater than the third value.

70. The optical device of claim 61, wherein the first Bragg grating is a chirped Bragg grating.

71. The optical device of claim 61, wherein the device is attached to the optical medium at a first and second attachment point.

72. The optical device of claim 71, wherein the first and second Bragg gratings are located between the first and second attachment points.

73. The optical device of claim 61, wherein the first wavelength is between 0.9 and 2 microns.

74. An optical system, comprising: the optical device of claim 61 ; a source, configured to transmit an optical signal to the optical device; and a receiver, configured to receive an optical signal transmitted by the optical device.

75. The optical device of claim 71, further comprising a sheath surrounding a portion of the optical medium between the first and second attachment points.

76. The optical device of claim 75, wherein the sheath is hermetically sealed to the optical medium.

77. The optical device of claim 29, further comprising a sheath surrounding a portion of the optical medium between the first and second attachment points.

78. The optical device of claim 77, wherein the sheath is hermetically sealed to the optical medium.

79. A fiber laser, comprising: a variable reflector, comprising: an optical medium having first and second Bragg gratings; and a device configured to change the peak reflection wavelength of the first Bragg grating from a first wavelength to a second wavelength and to change the peak reflection wavelength of the second Bragg grating from a third wavelength to a fourth wavelength; a second reflector; and an optical medium located between the variable reflector and the second reflector, the optical medium comprising an active material.

80. The fiber laser of claim 79, wherein the device changes the peak reflection wavelengths by applying a force to the optical medium between the first and second Bragg gratings.

81. The fiber laser of claim 79, wherein the device simultaneously changes the peak reflection wavelengths.

82. The fiber laser of claim 79, wherein the active material comprises a rare earth element.

83. The fiber laser of claim 79, wherein the device changes the peak reflection wavelength of the first Bragg grating by decreasing the period of the first Bragg grating.

84. The fiber laser of claim 83, wherein the device changes the peak reflection wavelength of the second Bragg grating by increasing the period of the first Bragg grating.

85. The fiber laser of claim 79, wherein the device is configured to indirectly apply the force to the optical medium.

86. The fiber laser of claim 79, wherein the device comprises a coupler attached to the optical medium between the first and second Bragg gratings.

87. The fiber laser of claim 86, wherein the device further comprises an actuator configured to apply a force to the coupler.

88. The fiber laser of claim 79, wherein the device comprises a piezo-electric element.

89. The fiber laser of claim 79, wherein the first Bragg grating has a first grating period and the second Bragg grating has a second grating period equal to the first grating period.

90. The fiber laser of claim 79, wherein the device is configured to cause a period of the first grating to change from a first value to a second value less than the first value.

91. The fiber laser of claim 90, wherein the device is configured to cause a period of the second grating to change from a third value to a fourth value greater than the third value.

92. The fiber laser of claim 79, wherein the first Bragg grating is a chirped Bragg grating.

93. The fiber laser of claim 79, wherein the device is attached to the optical medium at a first and second attachment point.

94. The fiber laser of claim 93, wherein the first and second Bragg gratings are located between the first and second attachment points.

95. The fiber laser of claim 79, wherein the first wavelength is between 0.3 and 3 microns.

96. An optical system, comprising: the fiber laser of claim 79; a waveguide, configured to transmit an optical signal from the fiber laser; and a receiver, configured to receive an optical signal transmitted by the waveguide.

97. The method of claim 1, wherein applying the force changes the output of a fiber laser.

98. A fiber laser comprising the optical device of claim 19.

99. The optical device of claim 19, wherein the optical medium is hermetically sealed inside a housing.

100. The optical device of claim 19, further comprising a heating element configured to heat the device sufficient to cause the device to apply the force to the optical medium.

101. The optical device of claim 100, wherein the device comprises a material that expands when heated by the heating element.

102. The optical device of claim 100, wherein the device comprises a material that contracts when heated by the heating element.