WO2000003297A1 - Method and apparatus for manufacturing apodized phase masks and related devices - Google Patents

Abstract

The present invention provides for a system (100) for fabricating apodized phase masks and related methods and devices. A shutter (28) is opened to expose a phase mask substrate (70) to an etch source (31). The depth of a groove formed in the phase mask substrate (70) depends on the time of exposure of that groove to the etch source (31). The shutter (28) is moved according to an apodizing function to produce a series of grooves in the phase mask substrate (70) with depths varying according to the apodized fiber gratings with superior characteristics.

Description

METHOD AND APPARATUS FOR MANUFACTURING APODIZED PHASE MARKS

AND RELATED DEVICES

BACKGROUND OF THE INVENTION The present invention is related to optical mask generation and, more particularly, to systems for producing apodized phase masks as may be used in the production of fiber Bragg gratings with improved characteristics.

Optical fiber waveguides are important components of many optical systems, such as optical fiber communication systems, where they may be used as transmission lines, for example. Optical fiber communication systems typically operate at much higher frequencies than systems based on electrical wires or microwave transmission lines, thus optical fiber transmission lines allow a higher information transfer rate.

The operating frequency range of a transmission ^"line is often split up, or multiplexed, into a number of channels. An example of how a single optical fiber may be multiplexed into a number of channels is by using dense wavelength- division multiplexer (DWDM) techniques. With multiplexed transmissions it is desirable to be able to select a channel without interference to or from other channels carried by the transmission line. One way that channels may be selected is by using filters, such as band-pass or band-stop filters, that transmit or reflect light within a particular band.

Wavelength division multiplex (WDM) systems are quite effective, allowing large amounts of data to be transmitted over a single optical fiber. Data transmission capabilities of optical fibers would benefit significantly if the number of signals transmitted along a fiber could be increased. Fiber data transmission capabilities can be enhanced by increasing the "density" of the multiplexed signals, that is, by decreasing the separation between the discrete frequencies of the multiplexed signals. Higher density of multiplexed signals requires better filters.

In general, better filters have higher discrimination between the desired frequency band and other frequencies. This reduces cross-talk and other types of interference between multiplexed signals. Additionally, it is desirable for a filter to make the transition between the desired frequency band and the other frequencies quickly, that is, within a narrow frequency transition zone. A high performance filter having a narrow transition zone is said to have a steep "skirt" as can be understood by viewing a graph of the transmission response over a range of wavelengths or frequencies .

Several different filter structures have been proposed for use in multiplexed optical transmission systems. One type of filter that has been evaluated is known as a fiber Bragg grating ("FBG") . An FBG typically has a series of alternating regions of different optical characteristics at a selected distance of separation along an optical fiber transmission line. The FBG is reflective at a relatively narrow range of wavelengths, and transmissive to other wavelengths, that are carried on the optical fiber.

Various methods have been developed to produce FBGs . One method uses holographic techniques to photo-induce a periodic change in the index of refraction along an optical fiber made of photo-sensitive glass, such as germanium-doped silica, by shining a light or lights with the appropriate wavelength (s) down the fiber. Various other methods involve shining light from an ultra-violet ("U.V.") source or sources against the side of a photo-sensitive optical fiber. One method uses two beams oriented to create an interference pattern along the length of the fiber. This method involves expensive laser sources with some stringent requirements of long coherent length and very sensitive interferometric setup. What this method does not have are the repeatability and stability of the writing device. Other methods involve shining a U.V. light against the sidewall of the photosensitive optical fiber.

A phase mask can be used in conjunction with a U.V. light source. When light from a U.V. laser shines through a phase mask, it is diffracted into different diffraction orders, which produce an interference pattern inducing a refractive index change to create an FBG within the fiber. A phase mask has a periodic phase shift groove structure on a selected substrate. It diffracts the light transmitted through the phase mask to produce an interference pattern. A photo-sensitive optical fiber is placed next to the mask, opposite the light source, and the interference pattern of light generated by the mask changes the optical characteristics within the core of the fiber. The period of the FBG is determined by the phase mask, and is relatively independent of the wavelength of the U.V. light source used, so the phase mask method has the potential to be quite versatile.

One type of phase mask uses a grating with periodic grooves at a selected spacing and depth. A simple phase mask has groves with equal spacing and non-equal spacing and uniform depth. However, an FBG made with such a simple phase mask typically has a series of side lobes in its reflective response, which can degrade channel-to-channel isolation. The side lobes can be reduced, relative to the main peak, by apodizing the FBG by modifying the diffracted light interference pattern and the residual non-diffracted background light pattern from the phase mask. For example, a filter that varies in a Gaussian fashion from clear in the center to dark on the edges may be placed, in conjunction with the phase mask, between the light source and the optical fiber during exposure to result in an FBG with a Gaussian index modulation. It also needs a second U.V. light exposure with an inverse Gaussian fashion from dark in the center to clear on the edges without using the phase mask. Such an FBG has reduced side lobes.

Other methods have been used to create Gaussian apodized FBGs, such as variable . scanning of the exposure beam, holographic interference techniques, ion implanting substrate blanks, and variable-thickness coating techniques; however, conventional methods are somewhat limited in that they do not offer precise control, and/or are cumbersome, time consuming, and not easily adapted to create a wide variety of FBGs with either different stop bands or other types of filter characteristics. Other types of apodized FBGs may have superior characteristics, such as lower side lobes and improved channel isolation. Therefore, it would be desirable to provide improved phase masks, systems for making phase masks, and devices, such as FBGs made from the improved phase masks. It would be particularly desirable to provide improved FBGs and related devices, such as distributed Bragg reflectors ("DBRs") for use in dense wavelength division multiplexed systems and optical resonators, and to economically provide various FBGs with a wide variety of stop bands. It would further be desirable if these improved structures, methods, and devices provided steep skirts for wavelength signal multiplexing and de-multiplexing, wavelength routing, switching, connecting, and other multiple wavelength operations. These improved techniques should ideally exhibit accurate operation wavelengths and a skirt sufficiently sharp to minimize channel cross-talk when the wavelength separation between optical signals is decreased below that of existing dense wavelength division multiplexed systems .

Toward this end, the present invention is directed toward improved phase masks, methods and systems for making improved phase masks, and FBGs and related or similar devices made using the improved phase masks .

SUMMARY OF THE INVENTION The present invention provides for apparatus and methods for fabricating apodized phase masks and apodized fiber gratings and related devices, apparatus, and methods. In one embodiment, an apodized phase mask is fabricated by opening a shutter to expose a phase mask substrate to an etch source. The depths of the grooves formed in a surface of the phase mask substrate vary according to a function relating to the speed at which the shutter is opened. In another embodiment, multiple etch mask patterns on a single phase mask substrate are apodized to form a phase mask with multiple phase mask patterns.

In another embodiment, a phase mask with multiple phase mask patterns is used in a fiber grating fabrication work station. A light beam is scanned across the phase mask to expose a light-sensitive optical fiber to the light beam and create an optical fibers with multiple fiber gratings. In a further embodiment, an optical spectrum analyzer monitors the response of the optical fiber during exposure to the light source to control the exposure. In yet a further embodiment, an apodized phase mask is used in the fiber grating fabrication workstation to produce fiber Bragg gratings where a modulated index of refraction varies according to a non- Gaussian apodizing function. The resulting fiber Bragg grating has improved out-of-band rejection compared to conventional apodized fiber Bragg gratings

BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A-1F are simplified cross sections of a substrate undergoing an exemplary phase mask fabrication sequence;

Figs. 2A-2C are simplified cross sections of a substrate during a variable etch process according to an embodiment of the present invention;

Fig. 3 is a simplified representation of a light beam incident on a phase mask;

Fig. 4A is a simplified top view of an apparatus for variable etching of phase masks; Fig. 4B is a simplified block diagram of a stepping motor controller and computer;

Fig. 4C is a simplified block diagram of a phase mask fabrication apparatus;

Fig. 4D is a simplified diagram illustrating various configurations of stepping motors, controllers, and chambers;

Fig. 5A is a simplified block diagram of a fiber Bragg grating fabrication work station;

Fig. 5B is a simplified view of a fiber plate; Fig. 5C is an exemplary graph of index modulation versus distance for an apodized fiber Bragg grating;

Fig. 6 is a simplified cross section of a interactive fiber laser with multiple gratings; Fig. 7A is a simplified cross section of a fiber with multiple fiber gratings;

Fig. 7B is a simplified graph of the index modulation along a fiber with multiple apodized fiber gratings;

Fig. 7C is a graph of the reflectivity versus wavelength of a fiber with multiple apodized fiber gratings;

Fig. 8A is a graph showing the reflectivity of a fiber Bragg grating with and without apodization; and Figs. 8B-8L are graphs showing the reflectivities of fiber Bragg gratings apodized according to various functions.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS The present invention relates to apparatus and techniques for making improved fiber Bragg gratings ("FBGs"), distributed feedback ("DFB") devices, distributed Bragg reflectors ("DBRs"), and related devices. The apparatus includes phase masks and systems for exposing light-sensitive optical fibers, and the devices include optical fiber devices. Optical fiber devices with improved Q-factor, transmission loss, channel isolation, and other characteristics are described. The devices include optical fibers with multiple grating structures, such as interactive lasers.

I. PHASE MASK PROCESS SEQUENCES

Figs. 1A-1F illustrate simplified cross sections of a phase mask 10 during a fabrication sequence. In Fig. 1A, an anti-reflective layer 12 of chromium or other material is formed on a substrate 14, such as a fused silica substrate. Fused silica is a desirable substrate material because it is transparent at the frequencies of interest, has a low thermal expansion coefficient, and can operate at relatively high temperatures, compared to other substrate materials. However, other substrate materials, such as boro-silicate glass, may be appropriate, depending on the application. A resist layer 16 is formed over the anti-reflective chromium layer 12 and is patterned by exposing selected portions 18 of the resist layer 16 and by not exposing other portions 20 of the resist layer. In this instance, the resist layer is a resist material suitable for use with an electron beam ("E-beam"), and a writing E-beam is used to expose the selected portions, as is known in the art.

Fig. IB shows the phase mask after E-beam exposure and development, leaving the areas not exposed to the E-beam 20 covering portions of the anti-reflective layer 12. Fig. 1C shows the phase mask after the anti-reflective layer 12 has been etched. In the case of a chromium layer, a wet etch process is used. After etching the anti-reflective layer 12, the remaining resist is removed using a resist lift-off technique .

Fig. ID shows the phase mask with a patterned anti- reflective layer 12 on the substrate 14. A reactive ion etching method is used to etch the portions of the substrate 22 that are not covered by the patterned anti-reflective layer 12. The remaining portions of the anti-reflective layer serve as a mask during the etch process; however, the reactive ion etch removes some of the anti-reflective layer, in addition to the substrate material, thus thinning the antireflective material 12 and forming grooves 24 in the substrate, as shown in Fig. IE. Fig. IF shows the phase mask 10 after the remaining anti-reflective material/etch mask has been removed using a chrome liftoff method. As an example, the depth 26 of the grooves is equal to about 244 nm for use with a 248 nm U.V. laser. A typical phase mask that might be used to fabricate an FBG band-stop filter might include about 1,000 grooves/mm and be about about 10-50 mm long overall, typically about 15 mm.

Figs. 2A-2C illustrate simplified cross sections of a phase mask during another type of fabrication process. Fig. 2A shows a substrate 14 with a patterned layer of etch mask 26. The substrate could be the substrate described in relation to Fig. ID with an anti-reflective chromium layer used as the etch mask, or could be patterned with a different etch masking material, or be a different substrate material. A variable aperture is formed between two shutters 28, 29. The shutters are positioned between the substrate 14 and a reactive ion source 30. As is known in the art, reactive ion etching may be done in the presence of an alternating electronic field, which can produce an anisotropic etching effect. This enhances etching in the direction normal to the surface of the substrate with relatively little lateral etching, producing well-defined grooves in the substrate. It is understood that it is not strictly necessary to use reactive ions in an anisotropic etch process, and that non- reactive ions, such as argon ions, may be used.

The variable shutters 28, 29 are moved during the etch process in the direction of the arrows 32 to open an aperture and expose different portions of the masked substrate to the ion etch for different periods of time, the center portion being exposed for the longest period of time. Fig. 2B is a simplified representation of a substrate 14 after ion etching as the variable aperture was retracted from the center of the substrate. The grooves 36 are of varying depths 38, 40, 42, a greater depth 42 indicating a longer exposure period to the etch. Fig. 2C shows the resulting phase mask 44. If the shutters are moved in the proper fashion, a phase mask with a particular characteristic may be fabricated, resulting in a FBG with desirable properties. In particular, if the aperture is moved according to specific functions, the resulting phase mask will 5 have an envelope profile of groove depths that may be used to make apodized phase masks. The envelope profile of groove depth can be derived from the required apodization profile.

II. APODIZING FUNCTIONS FOR FIBER BRAGG GRATINGS 10 Fig. 3 is a simplified representation of a light beam 50 incident on a phase mask 52. The phase mask 52 diffracts the light beam into a zero order beam 54, two first order beams, +1 56 and -1 58, and two second order beams 60, 62. The intensity of the various beams may be approximated 15 (ignoring reflective and absorptive losses of the phase mask) as :

-Lo⁼ -1-zero-order ~*~ ^-'-first-order ^"*^" ^^■'-second-order

20 Where I₀ is the intensity of the light beam from the U.V. source. For an ideal uniform phase mask, zero-order light power will be canceled out, each first-order beam will be about 45% of the total power, and each second-order beam will be about 5% of the total power. Assuming that the index

25 modulation of the photo-sensitive fiber core is proportional to the laser power irradiating that point, then the following index modulation distribution will result along the fiber core :

■5 U Δnpgajς. eajς¹* 1 first-order

or

Δ f χ_rst -order ~ Q " zero-order

Since the second-order power is less than 10% of the total power, its effect is neglected in this approximation. The zero order power of a phase mask can be expressed analytically: z_ero_or_der= Io ( η² + ( 1 - η ) ²+2η ( 1 -η ) cosΨ )

Where η=η(z) is the duty cycle and Ψ= (2πΔnd (z) ) /λ, Δn being the difference in the index of refraction between fused silica and air at the U.V. laser wavelength, λ, and d(z) being the etch depth of the phase mask. From theoretical calculations, an apodized FBG should have index modulation as shown in Fig. 5C. To obtain an apodized FBG, the duty cycle and etch depth of the phase mask must obey: l-η²-(l-η)²-2η(l-η)cosΨ = f(z)

According to the above equation, there are two ways to make an apodized phase mask. The first approach is to vary the duty cycle. The second approach is to vary the etch depth. In some instances, it may be desirable to combine approaches, either by varying etch depth concurrently with a variable duty cycle or by varying the etch depth along various portions of the phase mask that have fixed, but different duty cycles. For the duty-cycle approach, a uniform etch depth is chosen such that Ψ=π, therefore:

4η²-4η+ f(z)= 0 The solution for the above equation is:

l±Jl -f ( z ) η=η(z)= ^y—- For the etch-depth approach, a uniform duty cycle is chosen, such as η=η(z)=50%, therefore:

1-cosΨ ., . =f(z)

2

The solution for the above equation is:

d(z) = λ—arccos (l-2jf(z) )

2πΔn

The etch-depth approach is easier to realize than the duty cycle approach when making a variety of phase masks for the production of a variety of FBGs with different characteristics. The following apodization functions, representing the variation, or profile along the axis of an optical fiber, of the index modulation, may be used in an FBG:

Z-0.5L , Cosine function: cos ( π)

(Z-0.5L)

Gaussian function: exp αL" α: constant parameter, typical values are 0.111, 0.08 etc

Double cosine function: 0.796 cos ( ^'■ π) + 0.204 cos ( ^' 3π)

L L

2 Z — L 2 Z — L

Sine function: (sin( π) / ( π)

L L

2.Z — L Hamming function: 0.543 + 0.457 cos ( π)

2. Z —∑i 2 Z ^~~L

Blackman function: 0.427 + 0.497 cos ( π) + 0.076 cos ( 2π)

L L

Triple cosine function: 0.6905 cos( ^Z'°'^5Lπ) + 0.2771 cos ( ^Z~°^,5L3π) + 0.0324 cos ( ^Z~°-^5L5π) H41: 0.36295+0.48860cos ( ^Z °^'5L2π) +0.1373 Ocos

L

( ^^"°'^5£4π) +0.01115cos( ^Z~°-^5L6π) L L

H42: 0.61835cos ( ^Z~0'5Lπ) +0.30873cos ( ^Z~°'^5L3π) +

L L

0.06836cos ( ^Z'°'^5L5π) +0.00456cos ( ^Z~°^'5Llπ) L L

H51: 0.32120+0.47006cos( ^Z~°'⁵¹2π) +0.17732cos ( ^Z~°'^5L4π) +

L L

0.02992cos ( ^Z~°^'5L6π) +0.0015Ocos ( ^Z~°^'5L8π) L L

H52: 0.5651Ocos ( ^Z~°^'5Lπ) +0.3219cos ( ^Z~°^{' L} 3π) +0.09852cos

( ^Z °-^5L5π) +0.01388cos( ^Z °^'5L7π) +0.00059cos( ^{Z 0'5L}9π)

For convenience, these functions will be referred to as " f ( z ) " .

III. A SYSTEM AND METHOD FOR MAKING APODIZED PHASE MASKS The term "apodized phase mask" is used in this application to mean a phase mask suitable for making an apodized FBG or similar structure. As discussed above in relation to Figs. 2A-2C, the etch depth of grooves fabricated in a substrate with a patterned etch mask can be varied by opening a shutter during the etch process.

Fig. 4A is a simplified top view of a shutter mechanism used to make apodized phase masks in a processing chamber. A phase mask substrate 70, such as the substrate shown in Fig. ID or Fig. 2A, lies behind a pair of shutters 28, 29 mechanically coupled to a shutter controller 73. The shutters are coupled to a lead screw 72 that has left-hand threads on one portion of it, and has right-hand threads on another portion, so that the shutters can be closed or opened by rotating the lead screw in one direction or another. The shutters attached to the lead screw have corresponding left- hand and right-hand inner threads, as appropriate. A slider 74 supports the shutters opposite the lead screw. A first stepping motor 76 is controlled through control lines 77 by a stepping motor controller (not shown) that is outside the processing chamber. The stepping motor 76 is coupled to the lead screw 72 with an axles connector 78. In this case, the stepping motor is a vacuum stepping motor, such as a model VSS26.200 manufactured by PHYTRON-ELE TRONIK, GmbH of Germany, but could be another type of stepping motor, such as an electronic stepper motor, or other motive means, such as a voice coil or piezoelectric transducer. In a particular embodiment, the shutter controller could be a combination of a motor system and a piezoelectric transducer to provide precision control to the aperture opened between the shutters and a wide range of apertures to accommodate phase masks that are relatively long compared to the displacement range of a piezoelectric transducer.

A second stepping motor 82 is attached to a phase mask stage 80 with a axles coupler 84. The second stepping motor is actuated by control lines 86 from a stepping motor controller (not shown) . The second stepping motor controller can move the phase mask substrate 70 in relation to the center of the aperture formed by the shutters 28, 29. In one embodiment, an etch process is performed on one portion of the phase mask substrate 70 with a first etch mask pattern, and then the shutters are closed and the phase mask substrate is moved to align a second portion of the phase mask substrate with a second etch mask pattern under the shutters, and a second etch process is performed.

Fig. 4B is a simplified schematic of the stepping motor controller 90 coupled to a controller 92, such as a personal computer, with an interface 94, such as a serial bus, that provides the stepping motor controller 90 with signals from the controller 92. The stepping motor controller may be an IMS PANTHER LE2 manufactured by INTELLIGENT MOTION SYSTEMS, Inc. of Marlborough, CT, for example.

The phase mask 70 with the patterned etch mask(s) is placed under closed shutters so that no part of the mask is exposed when the ion etch conditions are established. As known to those skilled in the art, it is desirable to establish and maintain the ion density, exhaust flow, chamber pressure, and other process parameters to achieve a predictable etch characteristic. The center of the mask is aligned with the center of the aperture, that is, the common edge of the closed shutters, so that when the shutters are opened the center of the phase mask has the longest exposure time. The opening speed, and hence exposure times, can be varied as the shutters are opened by varying the speed of the stepping motor. The appropriate shutter speed along the length of the phase mask is determined by the desired etch depth. The shutter speed is determined by the stepping motor speed, which is controlled by the stepping motor controller according to the signals received from the controller. The controller may be programmed to configure the phase-mask etching system to produce a variety of phase masks.

The exposure time needed to create a particular etch depth, d(z) , is:

time(z) = d(z)/R

where R is the etching rate (e.g. in A/s) . Since the aperture opening is symmetrical to the center of the phase mask substrate, the center point is chosen at L/2 point, we only need to consider the part of the phase mask with z> L/2. Then, the speed of the shutter (variable along the length of the phase mask) is:

The negative sign is added because the exposure time decreases with increasing z when z>L/2, where ΔZ is an incremental distance along the phase mask. If the lead screw has N threads per inch, then the proper rotational speed of the stepping motor is:

v { z) (μm/sec) *N ω(z) ( revol u tions / second) =

2.54xl0⁴(μm)

This equation is the basic equation used for making apodized phase masks according to one embodiment of the present invention. The controller 92 may be programmed with an apodizing function, such as the ones given as examples above, to modulate the basic lead screw rotational speed equation to produce phase masks with groove depth envelopes according to the apodizing function.

The controller 92 determines the proper control signals to send to the stepping motor controller 90 according to the apodizing function selected and grating period. The stepping motor controller 90 then provides signals to the stepping motor 76 to control the rotational speed of the lead screw 78 and hence control the shutter speed. This results in an array of grooves in the phase mask with a nominally centered depth profile envelope according to the selected apodizing function. A single apodizing function may be applied to a variety of phase masks with different groove spacings and maximum depths, as the profile envelope may be applicable to modulate different gratings in different phase masks with similar overall lengths. In some instances, it may be desirable to adapt the apodizing function to a longer or shorter overall length (L) of a phase mask grating.

Fig. 4C is a simplified block diagram of an apparatus for fabricating phase masks 100. A pair of shutters 28, 29 are connected to a shutter driver 73, which can move the shutters together and apart. The shutter driver is shown coupled to only one shutter 29 for clarity of illustration, but can control both shutters. In some embodiments, only a single shutter is used. In this instance, the shutter driver 73 is a stepping motor and lead screw. The shutter driver 73 within a processing chamber 118 is coupled to a stepping motor controller 108 outside of the chamber via control lines 77. The stepping motor controller would not be required if other types of shutter controllers are used. A phase mask substrate 70 lies on a phase mask stage

80, that is moved by a stepping motor 82, also within the chamber, that is controlled via control lines 86 by a second stepping motor controller 109. Both stepping motor controllers 108, 109 are coupled to the controller 102 by control lines 94, 95.

An etch source 31 lies on the other side of the shutters -28, 29 from the phase mask substrate 70 within the chamber 118. The etch source can be a reactive ion etch source, as described above, or can be an ion etch source, a plasma etch source, an e-beam etch source, or another so- called "dry" etch source. Fig. 4D shows a single controller 102 coupled to a plurality of stepping motor controllers 104, 106, 108, which in turn control a plurality of stepping motors 110, 112, 114, in a plurality of etch chambers 116, 118 or within a single etch chamber. Thus, a single controller may be used to simultaneously produce a number of apodized phase masks. The apodized phase masks may be similar with respect to the apodizing function, or may be different with respect to the apodizing function, depending on the processing capabilities of the controller and rate of the control signals, among other factors.

IV. A FIBER BRAGG GRATING WORK STATION

Fig. 5A is a simplified block diagram of an FBG workstation 500. A fiber plate 502 holds up to eight optical fibers 504. Typically, each optical fiber has a sheath cladding and a core. At least a portion of the core is photosensitive, meaning that the index of refraction of the core may be changed by exposure to the correct type of light, such as from a U.V. excimer laser 506. Fig. 5B is a simplified top view of the optical fibers 504 on the fiber plate 502, showing an x-direction 508 and a y-direction 510 labeled for illustrative purposes. The fiber plate 502 may be controllably moved in the x-direction and/or the y-direction, as well as rotated in the θ direction (about the z-axis)512 and/or φ direction (about the y-axis) 514 to align the fiber with a phase mask. A phase mask 516 or a plurality of phase masks is mounted on a phase mask plate 518, which may also be moved in the x-direction or the y-direction, relative to the fiber plate. Additionally, each phase mask may include several grating structures, so that an optical fiber may be exposed to different gratings on the same phase mask by scanning the UV laser source.

Referring again to Fig. 5A, the laser 506 produces light at 248 nm or 193 nm. The laser is chosen according to the wavelength needed to convert the intended photo-sensitive fiber blanks and the power consistent with efficient photo conversion and system capabilities. The laser is controlled by a computer 520 over an instrument bus 522, such as an IEEE488 bus, which also controls the movement of the fiber plate 502, phase mask plate 518, reflector stage 523 and other elements of the system. However, other control means may be used, such as a serial bus coupled to the computer 520 or individual controllers, including analog controllers, for some system elements A movable reflector element 521, such as a front-surface U.V. mirror, reflects light from the laser 506 through an optional cylindrical lens 524 and then through the phase mask 516. As discussed above, the phase mask produces an optical interference pattern that creates the index pattern in the optical fiber that results in a fiber optical grating. Fig. 5C is a simplified representation of the index modulation in arbitrary units (a.u.) versus the distance along the fiber grating. The actual index modulation 550 varies within an envelope 552 that represents the apodizing function used to fabricate the phase mask that will be used to fabricate an FBG. The zero value position of the index modulation axis that corresponds to the original index of the fiber core. The index offset is caused by zero order diffraction of light from the phase mask. The index modulation is contributed to by the interference pattern of the two first order diffraction light from the phase mask.

For a simple (or uniform) phase mask, the zero order light is canceled out. Hence the index offset is zero. The fiber plate and/or phase mask plate is manipulated to bring usually a single fiber under the desired phase mask. In some embodiments, a number of fibers held on the fiber plate may be aligned under a single phase mask to expose, or "write", several fibers at once. Referring again to Fig. 5A, the selected fiber is multiplexed to a light source 526 through a first optical switch 528, which includes a three port circulator or similar device, such as a fiber coupler (not separately shown), and the output of the optical fiber is multiplexed to an optical spectrum analyzer ("OSA") 530 through a second optical switch 532. In this instance, 1x8 switches are used in conjunction with the eight-fiber fiber plate 502.

The light source 526 provides a test signal of light over a band of wavelengths that includes the desired response or measurement band of the fiber grating being fabricated, or a harmonic thereof. During exposure of the selected fiber, the transmission and/or reflection characteristics of the fiber are measured with the OSA to provide real-time monitoring of the exposure process. For example, a reflectivity peak may be analyzed to determine when the insertion loss of the peak stops changing, thus indicating the end of the exposure process, or the transmission may be monitored to produce a distributed Bragg reflector with a selected transmissivity.

V. FABRICATION OF MULTIPLE-GRATING FIBERS

The FBG fabrication workstation described above in section IV can be used to create a variety of multi-grating optical fiber devices. For example, an interactive fiber laser can be fabricated using the above workstation. Interactive fiber lasers can be fused from separate fibers containing different gratings, or the fiber lasers can be fabricated in a single fiber using an exposure sequence of the FBG fabrication workstation.

Fig. 6 is a simplified diagram of an interactive fiber laser 600. A "left" FBG 602 and a "right" FBG 604 are on either side of a "center" FBG 606 with a phase shift formed in the core 608 of an optical fiber 610. The center FBG 606 includes an active optical element 612, such as erbium-doped glass. In one embodiment, the left FBG, right FBG, and center FBG gratings are written into three different fiber segments, and then the fiber segments are fused together to form the interactive fiber laser. In another embodiment, all three fiber gratings are written into a single fiber using the FBG fabrication workstation described above. A single phase mask with a plurality of different gratings or a plurality of phase masks with different gratings may be used. Referring to Fig. 5A, in the first instance, a multiple-grating phase mask 516 is first aligned to a fiber on the fiber plate 502. The U.V. reflector 522 is scanned along a selected length of the fiber and associated portion of the phase mask by moving the reflector stage 523 according to control signals provided to the reflector stage 523 over the interface bus 522 by the computer 520. The computer is programmed to move the reflector at a selected rate that ensures proper exposure of the fiber blank, or may be controlled according to the optical characteristics of the fiber blank measured with the OSA 530. Alternatively, the reflector may be moved step-wise from one portion of the phase mask with Moving the reflector relative to the phase mask and fiber allows the light from the U.V. laser 506 to expose different portions of photo-sensitive fiber to the light, thus writing multiple grating patterns into the fiber from the phase mask. Each section of the phase mask may be optimized for the intended resulting fiber grating. For example, the reflectivity of the left FBG may transmit light at the pump frequency and reflect almost all light at the lasing frequency, while the right FBG reflects light at the pump frequency and transmits a portion of light at the lasing frequency. The OSA 530 , in conjunction with the light source 526 and optical switches 528, 532 may be used to monitor the optical characteristics of the fibers the gratings are being written, and to provide feedback to the controller to control the scanning rate and/or to indicate the end of the exposure process .

Another advantage of using the OSA 530 to manufacture multiple-grating fibers that is not available when using fuse-splicing to join multiple fiber segments to form a single fiber is that the OSA can directly measure optical characteristics of the entire multiple-grating fiber as it is being fabricated (written) . Thus, the OSA can account for interactive effects between the gratings that are difficult or impossible to control when assembling a multi-grating fiber using fuse-splicing.

Alternatively, the U.V. reflector 521 may be held stationary while the phase mask 516 and fiber plate 502 are scanned beneath it, or the U.V. reflector may be scanned concurrently with the phase mask and fiber plate. The fiber plate and phase mask may be synchronously scanned, or may be physically attached to each other, to maintain the proper registration between the phase mask and the optical fiber. Similarly as above, the OSA may provide feedback to control the rate of scanning and/or to indicate the end of the exposure process . Fig. 7A is a simplified representation of an optical fiber 700 with multiple FBG reflectors 702, 704, 706 written into the fiber using an FBG fabrication workstation. The spaces 708 shown between the fiber gratings are for illustration purposes only, as the gratings may adjoin, or in some instances partially or completely overlap. As described above in relation to the FBG interactive laser, the multiple- filter fiber is made by scanning the light from the U.V. laser 506 in relation to the fiber and phase mask. The phase mask used in this instance has three different sets of gratings that produce three FBGs, each with a different stop band, in the optical fiber. Each set of gratings may be apodized with the same or different apodization functions using the system for making apodized phase masks described above . Fig. 7B is a representation of index modulation along a single optical fiber with multiple apodized fiber gratings. A first region 710 forms a grating responsive to a first wavelength λ_1# a second region 712 forms a grating responsive to a second wavelength λ₂, and a third region 714 forms a grating responsive to a third wavelength λ₃. In an alternative embodiment, the gratings may partially or completely overlap, depending on the wavelengths of interest and processing constraints.

Fig. 7C is a graph showing the measured reflectivity versus wavelength of an optical fiber having three FBGs fabricated with an apodized phase mask. Three stop bands 716, 718, 720 were fabricated in a single fiber. A single fiber with multiple FBG reflectors provides a compact structure that can replace several independent fibers with FBGs and their associated hardware, and thus reduce costs. Fabricating a single multiple-filter fiber rather than fuse-splicing several fibers together to form a similar structure provides a more compact assembly with less insertion loss between filter sections. Furthermore, handling multiple fibers to fuse- splice can be difficult, whereas scanning the light source along a single fiber is repeatably controlled by the computer, thus a scanned multiple-filter fiber is easier to manufacture and less expensive.

VI. EXPERIMENTAL RESULTS

Figs. 8A-8L show the calculated reflectivity versus wavelength for various FBGs fabricated with various phase masks according to the methods and apparatus described above . Fig. 8A shows the difference between the frequency response of an unapodized FBG 802 and the frequency response of a similar apodized FBG 804. The phase mask used to write the apodized phase mask is fabricated using a cosine function to control the opening of the shutters during the etch process .

Fig. 8B shows the calculated reflectivity 808 of an FBG apodized by a Gaussian function. As discussed above, various other methods have been used to fabricate Gaussian- apodized FBGs, but the method described above can be used to make apodized phase masks of varying length with ease. The complex systems and controls typical of prior systems may result in FBGs with inferior performance.

Fig. 8C shows the reflectivity 810 of a uniform fiber grating apodized by a cosine function.

Fig. 8D shows the reflectivity 812 of a uniform fiber grating apodized by a double-cosine function.

Fig. 8E shows the reflectivity 814 of a fiber Bragg grating apodized by a sine function. Fig. 8F shows the reflectivity 816 of a fiber Bragg grating apodized by a Hamming function. Fig. 8G shows the reflectivity 818 of a fiber Bragg grating apodized by a Blackman function. Note that the highest side lobes 819 are approximately -57 dB below the peak reflectivity. Fig. 8H shows the reflectivity 820 of a fiber Bragg grating apodized by a triple-cosine function.

Fig. 81 shows the reflectivity 822 of a fiber Bragg grating apodized by the H41 function described above in section II. Fig. 8J shows the reflectivity 824 of a fiber Bragg grating apodized by the H42 function described above in section II.

Fig. 8K shows the reflectivity 826 of a fiber Bragg grating apodized by the H51 function described above in section II.

Fig. 8L shows the reflectivity 828 of a fiber Bragg grating apodized by the H52 function described above in section II.

While the description above provides a full and complete disclosure of the preferred embodiments of the present invention, various modifications, alternate constructions, and equivalents will be obvious to those with skill in the art. For example, it should be evident that while the specific embodiment of the present invention was described with a scanned U.V. laser source, other light sources may be applicable. Furthermore, some light sources may not need to be scanned, but might provide sufficient light output to expose a series of gratings on a phase mask, or the physical size of the phase mask may be small enough to allow exposure of multiple gratings without scanning using a conventional light source. Thus, the scope of the present invention is not limited by the above exemplary embodiments, but is limited solely by the appended claims.

Claims

WHAT IS CLATM n TS:

1. An apparatus for fabricating phase masks of a type used to manufacture optical fiber gratings, the apparatus comprising: a shutter configured to be between a phase mask substrate and an etch source; a shutter driver capable of controllably moving the shutter to expose a greater portion of the substrate to the etch source.

2. The apparatus of claim 1 further comprising a second shutter disposed opposite the shutter, wherein the shutter driver is capable of moving the second shutter in an opposite direction from the shutter.

3. The apparatus of claim 2 wherein the shutter driver comprises a lead screw coupled to a stepping motor, the lead screw having a first portion of left-hand threads and a second portion of right-hand threads, the shutter being coupled to the first portion of the lead screw and the second shutter being coupled to the second portion of the lead screw.

4. The apparatus of claim 3 further comprising a stepping motor controller electrically coupled to the stepping motor and to a computer, the computer being configured to control a rotational speed of the stepping motor according to an apodizing function.

5. The apparatus of claim 4 further comprising a second stepping motor coupled to a phase mask substrate stage, the second stepping motor being capable of moving the phase mask substrate stage relative to the first shutter and the second shutter in a selected manner.

6. The apparatus of claim 1 wherein the etch source is a reactive ion etch source.

7. An apparatus for fabricating phase masks, the apparatus comprising: a processing chamber; a reactive ion etch source within the processing chamber; a shutter system within the processing chamber, the shutter system including a first shutter and a second shutter configured to be between a phase mask substrate and the reactive ion etch source, a lead screw having a first portion of left- hand threads and a second portion of right-hand threads, the first shutter being coupled to the first portion of the lead screw and the second shutter being coupled to the second portion of the lead screw, and a stepping motor coupled to the lead screw, the stepping motor being capable of controllably rotating the lead screw to move the first shutter and the second shutter to expose a greater portion of the substrate to the reactive ion etch source; a stepping motor controller electrically coupled to the stepping motor; and a computer electrically coupled to the stepping motor controller, the computer being configured to control a rotational speed of the stepping motor according to an apodizing function.

8. A method of fabricating a phase mask, the method comprising: (a) placing a phase mask substrate behind a shutter; and (b) exposing the phase mask substrate to an etch source while moving the shutter relative to the phase mask substrate to expose a greater portion of the phase mask substrate to the etch source.

9. A phase mask comprising a substrate with a plurality of grooves, each of the plurality of grooves being essentially normal to an axis in a surface of the silica substrate, the plurality of grooves defining a non- Gaussian envelope profile of groove depths.

10. The phase mask of claim 9 wherein the non-Gaussian envelope profile of groove depths varies according to a non- Gaussian apodizing function.

11. An optical fiber comprising a section wherein an index of refraction modulation varies according to a non- Gaussian apodizing function to form an apodized fiber grating.

12. An apparatus for fabricating fiber gratings, the apparatus comprising: a light source providing a light beam; a phase mask plate configured to hold a phase mask; and a fiber plate, the fiber plate configured to hold a photo-sensitive optical fiber to expose a first portion of the photo-sensitive optical fiber to the light beam passing through a first portion of the phase mask, wherein the fiber plate and light beam are configured to move relative to each other to expose a second portion of the optical fiber to the light beam through a second portion of the phase mask.

13. The apparatus of claim 12 further comprising: a reflector disposed between the light source and the fiber plate to reflect the light beam from the light source to a selected portion of the phase mask, the reflector being mounted on a reflector stage capable of scanning the light beam across the phase mask.

14. A method for fabricating an optical fiber with multiple fiber gratings, the method comprising: (a) exposing a first portion of a photo-sensitive optical fiber to a light beam from a light source through a first portion of a phase mask to form a first fiber grating; (b) moving the light beam relative to the phase mask and to the photo-sensitive optical fiber; and (c) exposing a second portion of the photo- sensitive optical fiber to a second portion of the phase mask to form a second fiber grating.

15. An apparatus for fabricating fiber gratings, the apparatus comprising: a laser providing a light beam; a phase mask plate configured to hold a phase mask; a fiber plate, the fiber plate configured to hold a photo-sensitive optical fiber to expose the photo-sensitive optical fiber to the light beam passing through the phase mask; a light source coupled to the photo-sensitive optical fiber to provide a test signal to the photo-sensitive optical fiber; and an optical spectrum analyzer coupled to the photo- sensitive optical fiber to measure a response of the photo- sensitive optical fiber to the test signal during exposure of the photo-sensitive optical fiber to the light beam.

16. The apparatus of claim 15 further comprising a computer coupled to the optical spectrum analyzer and to the laser, the computer controlling the laser according to a signal from the optical spectrum analyzer.

17. The apparatus of claim 15 further comprising a first optical switch and a second optical switch wherein the fiber plate is configured to hold a plurality of photo-sensitive optical fibers, the first optical switch being disposed between the light source and the fiber plate to couple the light source to a selected one of the plurality of photo- sensitive optical fibers and the second optical switch being disposed between the optical spectrum analyzer and an output of the selected one of the plurality of photo-sensitive optical fibers.