WO2003077386A1 - Filtre d'interference a faisceau en couche mince variable et procede d'utilisation pour accordage de laser - Google Patents

Filtre d'interference a faisceau en couche mince variable et procede d'utilisation pour accordage de laser Download PDF

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Publication number
WO2003077386A1
WO2003077386A1 PCT/US2002/008566 US0208566W WO03077386A1 WO 2003077386 A1 WO2003077386 A1 WO 2003077386A1 US 0208566 W US0208566 W US 0208566W WO 03077386 A1 WO03077386 A1 WO 03077386A1
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WIPO (PCT)
Prior art keywords
tapered
spacer layer
external cavity
thin film
interference filter
Prior art date
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PCT/US2002/008566
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English (en)
Inventor
George W. Hopkins
Nathan Lee Shou
Mark E. Mcdonald
Original Assignee
Intel Corporation
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Filing date
Publication date
Priority claimed from US09/814,464 external-priority patent/US6816516B2/en
Priority claimed from US09/900,412 external-priority patent/US6717965B2/en
Application filed by Intel Corporation filed Critical Intel Corporation
Priority to AU2002367576A priority Critical patent/AU2002367576A1/en
Publication of WO2003077386A1 publication Critical patent/WO2003077386A1/fr

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/14—External cavity lasers
    • H01S5/141—External cavity lasers using a wavelength selective device, e.g. a grating or etalon
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/0121—Operation of devices; Circuit arrangements, not otherwise provided for in this subclass
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/21—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  by interference
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29346—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by wave or beam interference
    • G02B6/29358—Multiple beam interferometer external to a light guide, e.g. Fabry-Pérot, etalon, VIPA plate, OTDL plate, continuous interferometer, parallel plate resonator
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29346—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by wave or beam interference
    • G02B6/29361—Interference filters, e.g. multilayer coatings, thin film filters, dichroic splitters or mirrors based on multilayers, WDM filters
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29379—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device
    • G02B6/29395—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device configurable, e.g. tunable or reconfigurable
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/02—Structural details or components not essential to laser action
    • H01S5/022—Mountings; Housings
    • H01S5/02208—Mountings; Housings characterised by the shape of the housings
    • H01S5/02216—Butterfly-type, i.e. with electrode pins extending horizontally from the housings
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/02—Structural details or components not essential to laser action
    • H01S5/022—Mountings; Housings
    • H01S5/023—Mount members, e.g. sub-mount members
    • H01S5/02325—Mechanically integrated components on mount members or optical micro-benches
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
    • H01S5/0617—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium using memorised or pre-programmed laser characteristics
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
    • H01S5/068—Stabilisation of laser output parameters
    • H01S5/06804—Stabilisation of laser output parameters by monitoring an external parameter, e.g. temperature
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
    • H01S5/068—Stabilisation of laser output parameters
    • H01S5/0683—Stabilisation of laser output parameters by monitoring the optical output parameters

Definitions

  • the present invention relates generally to the field of optical lasers, and more particularly to the tuning of wavelength emissions from an optical laser.
  • Fiberoptic telecommunications are continually subject to demand for increased bandwidth.
  • WDM wavelength division multiplexing
  • Each data stream is modulated onto the output beam of a corresponding semiconductor transmitter laser operating at a specific channel wavelength, and the modulated outputs from the semiconductor lasers are combined onto a single fiber for transmission in their respective channels.
  • ITU International Telecommunications Union
  • ITU International Telecommunications Union
  • channel separations 50 GHz, or approximately 0.4 nanometers. This channel separation allows up to 128 channels to be carried by a single fiber within the bandwidth range of currently available fibers and fiber amplifiers. Improvements in fiber technology together with the ever-increasing demand for greater bandwidth will likely result in smaller channel separation in the future.
  • Transmitter lasers used in WDM systems have typically been based on distributed feedback (DFB) lasers operating with a reference etalon associated in a feedback control loop, with the reference etalon defining the ITU wavelength grid.
  • DFB distributed feedback
  • Statistical variation associated with the manufacture of individual DFB lasers results in a distribution of channel center wavelengths across the wavelength grid, and thus individual DFB transmitters are usable only for a single channel or a small number of adjacent channels.
  • Continuously tunable external cavity lasers have been developed to overcome this problem.
  • the trend towards smaller channel separation and the advent of channel selectivity in transmitter lasers has given rise to a need for greater accuracy and control in the positioning of tunable elements associated with transmitter lasers. As tunable elements are configured for narrower channel separation, decreasing component tolerances and thermal fluctuation become increasingly important. Non-optimal positioning of tunable elements results in spatial losses and reduced transmitter output power.
  • U.S. Patent No. 6,108,355 discloses the use of an air spaced etalon fortuning an external cavity laser.
  • Such an etalon is constructed from two pieces of glass each having a high- reflectance (HR) coated surface and an anti-reflection (AR) coated surface.
  • HR high- reflectance
  • AR anti-reflection
  • the high reflectance surfaces of the two pieces are placed facing one another and are separated by a spacer.
  • the spacer is formed using tiny drops of epoxy filled with precision glass beads.
  • This type of etalon is not dimensionally stable, even over short time spans, making it difficult to use the laser for any extended period of time without recalibrating it.
  • An external cavity laser incorporates a gain medium, an external reflective element, a grid generator element, a collimator, and a tapered, thin film interference filter.
  • the grid generator element, collimator and tapered, thin film interference filter are aligned in an optical path between the gain medium and the external reflective element.
  • a driver is operably connected to the tapered, thin film interference filter and adapted to adjustably position it in the optical path.
  • a preferred method of adjusting the tuning element or tuning the laser includes linearly translating the thin film interference filter in directions normal to the optical path, although other methods of tuning include rotating a circular variable filter in the optical path, or tilting a filter in the optical path.
  • An example of a tapered, thin film interference filter according to the present invention includes a tapered half wave spacer layer, a tapered first quarter wave layer stack positioned adjacent a first side of said half wave spacer layer, and a tapered second quarter wave layer stack positioned adjacent a second side of said half wave spacer layer.
  • a tapered, thin film interference filter according to the present invention may include more than one spacer layer. Each spacer layer, whether one or more is employed, has an optical wavelength which is an odd integral multiple of a half wavelength of light to be passed therethrough, and each side thereof is adjacent a tapered layer stack.
  • a tuning element for an external cavity laser which includes a tapered thin film interference filter including a tapered half wave spacer layer, a tapered first quarter wave layer stack positioned adjacent a first side of said half wave spacer layer, and a tapered second quarter wave layer stack positioned adjacent a second side of said half wave spacer layer.
  • a method for tuning an external cavity laser includes providing a tapered thin film interference filter including a tapered half wave spacer layer, a tapered first quarter wave layer stack positioned adjacent a first side of said half wave spacer layer, and a tapered second quarter wave layer stack positioned adjacent a second side of said half wave spacer layer ; and adjustably positioning the tapered thin film interference filter in an optical path defined by a beam associated with the external cavity laser.
  • Adjustment of the position preferably includes translating the thin film interference filter in at least one direction substantially normal to the optical path.
  • adjustment of the positioning of the filter may include rotating the thin film interference filter in a plane substantially normal to the optical path, or tilting the thin film interference filter by rotating it about an axis substantially normal to the optical path.
  • a method for tuning an external cavity laser to account for thermal wavelength drift includes generating a first table of position locations, at a baseline temperature, to move a tuning element to for respective desired wavelengths to be emitted from the external cavity laser and storing data values representative of said first table in non-volatile memory; generating a second table of position location adjustments to be made, relative to a range of variations in temperature surrounding said baseline temperature to account for thermal wavelength drift of the tuning element and storing data values representative of said second table in non-volatile memory; obtaining a gross position of the tuning element in accordance with a wavelength desired to be emitted from the laser by accessing said first table and selecting the position location stored for the desired wavelength; measuring a temperature of the environment surrounding the tuning element; obtaining a position location adjustment value from said second table that corresponds to the temperature measured; and determining position correction to finely adjust said gross position, and moving the tuning element to the finely adjusted position, thereby accurately tuning the laser to emit the desired wavelength.
  • FIG. 1 is a schematic diagram of an external cavity laser apparatus utilizing a thin film wedge etalon as a tunable element.
  • FIG. 2A-2C are graphical illustrations of pass band characteristics of the external cavity laser of FIG. 1 for the wedge etalon, grid etalon and external cavity with respect to a selected channel in a wavelength grid.
  • FIG. 3A-3C are graphical illustrations of gain response to tuning of the external cavity laser of FIG. 1 for a plurality of channels in a wavelength grid.
  • FIG. 4 is an exaggerated schematic view in cross section of a thin film wedge etalon usable with the present invention.
  • FIG. 5 is a perspective view of a mount to which an etalon (a portion of which is shown in phantom) may be mounted.
  • FIG. 6 is a plan view of an external cavity laser showing a preferred adaptation of an etalon mount according to the present invention.
  • FIG. 7 A is a graphical representation of wavelength/linear position data obtained from testing of a thin film etalon according to the present invention.
  • FIG. 1 there is shown external cavity laser apparatus 10 of the type described in U.S. Patent No. 6,108,355.
  • the apparatus 10 includes a gain medium 12 and an end or external reflective element 14.
  • Gain medium 12 may comprise a conventional Fabry-Perot diode emitter chip and has an anti-reflection (AR) coated front facet 16 and a partially reflective rear facet 18.
  • Retroreflective element 14 may comprise an end mirror.
  • the external laser cavity is delineated by rear facet 18 and end mirror 14.
  • the external cavity laser 10 includes a grid generator element and a tunable element, which are respectively shown in FIG. 1 as a grid etalon 24 and a wedge etalon 26 positioned in optical path 22 between gain medium 12 and end mirror 14.
  • Grid etalon 24 typically is positioned in optical path 22 before tunable element 26, and has parallel reflective faces 28, 30.
  • Grid etalon 24 operates as an interference filter, and the refractive index of grid etalon 24 and the optical thickness of grid etalon 24 as defined by the spacing of faces 28, 30 give rise to a multiplicity of peaks of maximum transmission within the communication band at wavelengths which coincide with the center wavelengths of a selected wavelength grid which may comprise, for example, the ITU (International
  • Grid etalon has a free spectral range (FSR) which corresponds to the spacing between the grid lines of the ITU grid, and the grid etalon 24 thus operates to provide a plurality of pass bands centered on each of the gridlines of the wavelength grid.
  • FSR free spectral range
  • Grid etalon 24 has a finesse (free spectral range divided by full width half maximum or FWHM) which suppresses neighboring modes of the external cavity laser between each channel of the wavelength grid.
  • Wedge etalon 26 also acts as an interference filter, with non-parallel reflective faces 32, 34 providing a tapered shape.
  • Wedge etalon 26 may be a Fabry-Perot etalon filter constructed from a solid spacer layer having a stack of thin, dielectric films on each side as discussed in more detail below with reference to Fig. 4.
  • the wedge etalon 26 is only one example of a tunable element which may be used in accordance with the invention in an external cavity laser.
  • a circular variable etalon filter may also be used to practice the methods according to the present invention.
  • Such a circular variable etalon filter may be made from a solid spacer layer and stacks of thin films on each side.
  • variation in thickness of the device change circumferentially, as opposed to linearly with the wedge etalon 26.
  • the pass bands defined by the wedge etalon 26 are substantially broader than the pass bands of the grid etalon 24, with the broader pass bands of the wedge etalon 26 having a periodicity substantially corresponding to the separation between the shortest and longest wavelength channels defined by the grid etalon 24.
  • the free spectral range of the wedge etalon 26 corresponds, at a minimum, to the full wavelength range of the wavelength grid defined by grid etalon 24, and typically to the wavelengths for which the laser medium has appreciable gain.
  • the wedge etalon 26 has a finesse which suppresses channels adjacent to a particular selected channel.
  • the wedge etalon 26 is used to select between multiple communication channels by changing the optical thickness between faces 32, 34 of wedge etalon 26. This is achieved by translating or driving wedge etalon 26 along axis x, which is parallel to the direction of taper of wedge etalon 26 and perpendicular to optical path 22 and the optical axis of external cavity laser 10.
  • Each of the pass bands defined by the wedge etalon 26 supports a selectable channel, and as the wedge is advanced or translated into optical path 22, the beam traveling along optical path 22 passes through increasingly thicker portions of wedge etalon 26 which support constructive interference between opposing faces 32, 34 at longer wavelength channels.
  • wedge etalon 26 As wedge etalon 26 is withdrawn from optical path 22, the beam will experience increasingly thinner portions of wedge etalon 26 and expose pass bands to the optical path 22 which support correspondingly shorter wavelength channels.
  • the free spectral range of wedge etalon 26 corresponds to the complete wavelength range of grid etalon 24 as noted above, so that a single loss minimum within the communications band can be tuned across the wavelength grid.
  • the combined feedback to gain medium 12 from the grid etalon 24 and wedge etalon 26 support lasing at the center wavelength of a selected channel. Across the tuning range, the free spectral range of the wedge etalon 26 is broader than that of grid etalon 24.
  • Wedge etalon 26 is positionally tuned via a tuning assembly which comprises a drive element 36 structured and configured to adjustably position wedge etalon 26 according to selected channels.
  • Drive element 36 may comprise a stepper motor together with suitable hardware for precision translation of wedge etalon 26.
  • Drive element may alternatively comprise various types of actuators, including, but not limited to, DC servomotors, solenoids, voice coil actuators, piezoelectric actuators, ultrasonic drivers, shape memory devices, and like linear actuators.
  • Controller 38 may include a data processor and memory (not shown) wherein are stored lookup tables of positional information for wedge etalon 26 which correspond to selectable channel wavelengths. Controller 38 may be internal to driver element 36, or may be external and shared in other component positioning and servo functions of the invention as described below.
  • controller 38 When external cavity laser 10 is tuned to a different communication channel, controller 38 signals drive element 36 according to positional data in the look up table, and drive element 36 translates or drives wedge etalon 26 to the correct position wherein the optical thickness of the portion of the wedge etalon 26 positioned in optical path 22 provides constructive interference which supports the selected channel.
  • a linear encoder 40 may be used in association with wedge etalon 26 and drive element 36 to ensure correct positioning of wedge etalon 26 by driver 36.
  • Wedge etalon 26 may include opaque regions 42, 44 at its ends that are optically detectable and which serve to verify the position of wedge etalon 26 when it has been positionally tuned to its longest or shortest channel wavelength.
  • Opaque regions 26 may be coated on surface 32 and/or surface 34 and provide an encoder mechanism usable in the positional tuning of wedge etalon, effectively functioning, in conjunction with the laser beam, as an optical interrupt switch. Since the switch is integral to the etalon 26 and laser beam, however, it offers improved accuracy in position reading due to the absence of intervening structures between the etalon 26 and an external switch, and it does so with a reduction in the number of parts needed with a concurrent reduction in cost of the assembly.
  • controller 38 can anticipate when an opaque region 42, 44 will enter optical path 22. Appearance of an opaque region 42, 44 in optical path 22 at a point other than predicted will indicate an encoder error, and the controller 38 can make an appropriate correction based on the detected presence of an opaque region 42, 44 in optical path 22. Additional opaque regions (not shown) may be included elsewhere on wedge etalon 26.
  • FIG. 2A through FIG. 2C show external cavity pass bands PB1 (also called cavity modes), grid etalon pass bands PB2, and wedge etalon pass bands PB3. Relative gain is shown on the vertical axis and wavelength on the horizontal axis.
  • free spectral range of the wedge etalon 26 (FSRchannei sei) is greater than the free spectral range of the grid etalon 24 (FSRori d Gen ), which in turn is greater than the free spectral range of the external cavity (FSRc av i t y).
  • the band pass peaks PB1 of the external cavity periodically align with the center wavelengths of pass bands PB2 defined by the wavelength grid of grid etalon 24.
  • the wavelength grid extends over sixty four channels spaced apart by one half nanometer (nm) or 62GHz, with the shortest wavelength channel at 1532 nm, and the longest wavelength channel at 1563.5 nm.
  • the width for a grid etalon pass band 56 at half maximum is shown in FIG. 2B, and the width for a wedge etalon pass band 58 at half maximum is shown in FIG. 2C.
  • the positioning of grid etalon 24 and wedge etalon 26 within the external cavity improves side mode suppression.
  • the tuning of the band pass PB3 of wedge etalon 26 between a channel centered at 1549.5 nm and an adjacent channel at 1550 nm is illustrated graphically in FIG. 3A-3C, wherein the selection of a channel generated by grid etalon 24 and the attenuation of adjacent channels or modes is shown.
  • the external cavity pass bands PB1 shown in FIG. 2A-2C are omitted from FIG. 3A-3C for clarity.
  • the grid etalon 24 selects periodic longitudinal modes of the external cavity corresponding to the grid channel spacing while rejecting neighboring modes.
  • the wedge etalon 26 selects a particular channel in the wavelength grid and rejects all other channels.
  • the selected channel or lasing mode is stationary at one particular channel for filter offsets in the range of approximately plus or minus one half channel spacing. For larger channel offsets the lasing mode jumps to the next adjacent channel.
  • FIG. 4 shows a wedge etalon 26 fabricated as a thin film device.
  • the device is based on the interference of light.
  • the device includes of stacks of alternating thin dielectric films. Such films are described by optical thickness in fractions of a wavelength of light
  • the dielectrics are classed into low-index (L) layers, typically silica, and high-index (H) layers, typically metal oxides of metals such as tantalum, titanium, or niobium, for example. Index refers to the optical index of refraction.
  • a spacer layer 46 which comprises a single half wave ( ⁇ /2) layer (half wave optical thickness, HWOT), or an odd integral multiple of one half-wavelength optical thickness.
  • the spacer layer may be made of a low index (L) material, such as silica, or a high index (H) material. Neither material gives significant optical performance over the other, but the use of a high index material reduces the dependence of pass band wavelength shift of the device relative to the angle of incidence of the laser beam on the device. Thus, for applications where the etalon is tilted for purposes of tuning, use of a high index material reduces the degree of tilting required to obtain the same wavelength shift comparably with use of a low index material.
  • the temperature dependence of the center wavelength of the pass band can vary depending on whether a high index material or low index material is employed for the spacer element 46.
  • the spacer layer 46 may be any odd integral multiple of one half- wavelength optical thickness, e.g., l ⁇ , 3/2, 5/2, 7/2 ...., etc.
  • the choice of a higher order optical thickness changes the far out-of-pass band characteristics of the etalon.
  • a higher order layer may be chosen to obtain particular out-of-pass band characteristics at the expense of a thicker etalon.
  • an etalon according to the present invention is not to be limited to one spacer layer surrounded by two V* wave layer stacks, more than one spacer layer may be employed, where the spacer layers are each separated from one another by a V ⁇ wave stack as described.
  • the spacer layers typically, but not necessarily have the same thickness. That is, the thicknesses may vary by an odd integral multiple of one half wavelength.
  • the interference filter / etalon 26 must have particular properties in order to perform its function.
  • the transmission pass band of the device must have sufficiently high transmission at the center of the pass band in order to permit an adequate external cavity reflectivity, when the pass band for the desired wavelength is centered on the beam in the laser cavity.
  • the transmission pass band must have a sufficiently low transmission for adjacent modes of the etalon 26, which selects wavelengths, so that radiation at these wavelengths is adequately suppressed. These are conflicting requirements.
  • the transmission of an ideal, thin-film, Fabry-Perot, interference filter, at the center of its pass band, and with no wedge, is 100 percent.
  • the transmission at the center of the pass band of a wedged filter is less than 100 percent.
  • Transmission depends on the full width at half maximum of the basic filter design and the ratio of the beam size to the tuning rate of the device. The reduction of transmission below 100 percent is due to the filter being tuned away from the wavelength of the laser beam, as determined by the wavelength generating etalon, over most of the laser beam, due to the wedge of the filter. Only the line at the center of the beam and perpendicular to the direction of wedge has 100 percent transmission for a perfect filter.
  • the basic filter design can be calculated as described by Macleod, H. A. in Thin Film Optical Filters 2 nd Edition, McGraw-Hill, 1989, on pages 244-269, the entirety of which is incorporate herein by reference thereto, or by using any of a number of thin-film design programs, such as TFCalc. Thin Film Design Software for Windows, available from Software Spectra, Inc., Portland, Oregon, for example.
  • the transmission of the device can then be calculated by integrating the product of the transmission band pass value at any point on the filter and the normalized irradiance of the laser beam at that point, over the extent of the laser beam.
  • the method of determining the optimum filter starts with determining the performance requirements and constraints.
  • Requirements include the mimmum allowable transmission, at the position of maximum transmission for a laser beam, and the required suppression of the adjacent modes from the wavelength selector etalon.
  • Constraints include the wavelengths that the device must be able to tune, the size of the laser beam, and the length of scan possible for the device.
  • the scan rate or tuning ratio is the wavelength range divided by the length of scan.
  • the linear scan gives the minimum scan rate.
  • Real filters are more or less nonlinear, and the scan rates will be greater than and less than the linear rate at some locations along the filter. Real filters have losses in transmission due to variation in film thickness from the design values due to normal manufacturing errors, from scattering, particularly at film interfaces, and from absorption of light within the layers. These losses can be a small fraction for a well-made device, only a few percent, but it is typical to. allow 10 percent.
  • the design process begins by setting values based on allowances for the laser.
  • a typical maximum scan distance is 18 mm.
  • the wavelength range is also determined by the application.
  • a laser for the telecommunications C-Band covers 1528 nm to 1565 nm, or a span of 37 nm.
  • the size of the laser beam can be varied, by using different focal length lenses to shape the beam, but the choice is constrained by the wavelength of light defined by our application and by the natural spreading of a Gaussian laser beam due to diffraction. See Hecht, Eugene, et al., Optics 3 rd Edition, Addison- Wesley 1997, for a discussion of Gaussian laser beams, such discussion being incorporate herein by reference thereto.
  • a practical beam size at the device is 0.38 mm across the 1/e 2 points, where e is the base of natural logarithms.
  • the designer can choose a small range around this value by using a lens with an appropriate focal length.
  • the desired external cavity reflectivity leads to a minimum peak transmittance of 78 percent for the device.
  • the design reflectivity should therefore be about 88 percent or greater to account for absorption, scatter and manufacturing errors.
  • the full width at half maximum of the pass band of the filter needs to be 1.5 nm or less in order to suppress the adjacent wavelengths passed by the etalon, which defines the allowed wavelengths, by 55 dB or more. This was determined empirically by constructing and testing physical models, for which the separation of the adjacent wavelengths was 0.4 nm. We determined these values from a constructed model, because we were unable to determine the required full width at half maximum with a mathematical model.
  • the shape of the pass band needs to be that of a single-cavity or a multiple, aligned-cavities, Fabry-Perot filter, for which the shape from the peak to the half -power points is approximately Gaussian or Lorentzian.
  • the theoretical transmission for a filter with a scan rate of 3.0 nm/mm would be 88 percent, determined by the integration process, already described, for a pass band with a 1.5 nm full width at half maximum.
  • a pass band much less than 1.5 nm would be undesirable, because it would have a peak transmission less than the required 78 percent, with an allowance of 10 percent for absorption, scatter, and manufacturing errors.
  • the full width at half maximum is therefore required to be greater than 1.0 nm and less than 1.5 nm.
  • the scan rate is required to be less than or equal to 3.0 nm/mm. We could meet these requirements with a linear scan that is 14 mm or greater. We could operate with a laser beam, which is 0.40 mm or less in diameter across the 1/e 2 points.
  • Half wave layer 46 and quarter wave stacks 48, 50 are tapered in thickness in order to define a "wedge" shape.
  • the actual "wedge” may be the half wave layer 46, with quarter wave stacks 48, 50 providing reflective or partially reflective surfaces therefore.
  • a portion of the substrate 52 may have only antireflective coatings on two opposing sides and none of the layers 48, 46 and 50, forming a non-filtered portion (not shown) This non- filtered portion can allow the laser beam to pass unfiltered and thereby provide calibration signals or signals which may be used to perform diagnostic tests.
  • the relative thickness of layers 46, 48, 50 as shown in FIG. 4 are exaggerated for clarity, and the degree of taper is greatly exaggerated.
  • Channel selection for external cavity laser 10 can be carried out within a tuning range of about 1530 nm to 1565 nm using the thin film wedge etalon of FIG. 4.
  • wedge etalon 26 is about 18 millimeters long, and the thickness of half wave layer 46 is about 510 nm at the narrow end, and about 518 nm thick at the wide end of wedge etalon 26, so that half wave layer 46 is tapered in thickness by eight nanometers over a length of 18 millimeters.
  • wedge etalon 26 will need to define 100 different communication channels, and half wave layer 46 will provide 100 different transmission zones (not shown) corresponding to the 100 selectable channels.
  • half wave layer 46 will provide 100 different transmission zones (not shown) corresponding to the 100 selectable channels.
  • the individual zones will be separated by a distance of 180 microns.
  • the coefficient of thermal expansion of the materials of the half wave layer 46, quarter wave layers 48, 50 and substrate 52 are selected to minimize dimensional fluctuation within a standard operating temperature range.
  • the etalon 26 is preferably mounted on a mount 60 by which the etalon is driven.
  • Mount 60 is preferably made of metal such as AISI 416 stainless steel.
  • any material that exhibits mechanical and thermal properties sufficient for this application may be employed, such as Covar (an iron-nickel alloy with a coefficient of thermal expansion to match common borosilicate glasses) for example.
  • Polymers which are sufficiently dimensionally stable and exhibit mechanical and thermal properties sufficient for this application may also be used, and may also reduce the cost of the design.
  • Coefficients of thermal expansion in the components of the present invention must be very small or compensate for one another. Dimensional stability is a key factor in the proper functioning of the present invention. The wavelength calibration of the laser will change over a period of years due to dimensional instability.
  • Heat treatment and annealing of the mount are preferred in order to minimize any contribution to dimensional instability by the mount 60.
  • a key consideration in mounting the etalon is the minimization of stress imposed on the etalon 26.
  • a single point attachment is made substantially on the center of the bottom 26b, of the etalon 26.
  • metal mount 60 is provided with a central pad 62 and a pair of similar pads 64 and 66, adjacent the central pad 62. All three pads are preferably formed of the same material, which may be the same material as the mount and may be integral with the mount.
  • Each pad 62,64,66 is provided with a raised shoulder 62a,64a,66a which facilitates the positioning of the etalon 26 during mounted as described in more detail below.
  • the main surface 62b of pad 62 is recessed to a level slightly below the levels of the main surfaces 64b and 66b which are equal in height.
  • the recessed surface 62b is provided to accommodate the volume of an adhesive which is used to secure pad 62 to a central portion of the bottom end 62b of etalon 62.
  • a typical adhesive is a filled epoxy with about 80 -95% (by weight) silica fill, typically about 90%.
  • a silica fill as described minimizes shrinkage of the epoxy as it cures.
  • the shrinkage from application to full curing is only about .25% of the original volume of the adhesive when applied. It should be noted here that the present invention is not limited to the use of epoxy with 90% silica fill, as other extremely low shrink adhesives, as well as other low stress attachment means could be used o accomplish the fixation.
  • the mounting of the etalon 26 on mount 60 is accomplished by first applying an adhesive to the main surface 62b of pad 62.
  • a volume of at least about 0.5 mg is required for a complete joint.
  • a volume of about 0.5 to 1.2 mg is typically used, often about 1.0 mg, to account for manufacturing errors that may exist in the parts to be joined.
  • the etalon is next positioned by centering the bottom end 26b over the central pad 62. Shoulders 62a,64a,66a facilitate the proper positioning of etalon 26 as a side of the etalon (adjacent an edge of the bottom surface 26b) abuts the shoulders when properly placed.
  • the volume of adhesive applied to surface 62a levels the height of this surface with surfaces 64a and 66a.
  • the bottom surface 26b is supported by the three pads and connected through a "single point" attachment, with very little stress applied along any portion of the bottom surface 26 of the etalon.
  • the central pad 62 is typically about 3 mm wide and about 3.5mm long (although these dimensions can be varied, as would be readily apparent to those of ordinary skill in the art) and therefor provides a very small area of attachment.
  • a portion of the etalon 26 is shown in phantom in Fig. 5 to aid in visualizing the orientation of attachment.
  • the index of refraction and the physical thickness of each layer of the etalon will vary with temperature.
  • the effective index of refraction of the overall etalon 26 varies with temperature.
  • the result of such variations is a variation with temperature of the wavelength that is emitted from the etalon 26, for any given position.
  • This phenomenon is known as thermal wavelength drift (TWD) and it results from at least two temperature dependent factors.
  • TWD thermal wavelength drift
  • One factor is the temperature induced variation in the effective index of refraction of each individual layer 46, 48, 50
  • a second factor is variation in the thickness of each individual layer 46, 48, 50, since these layers may expand and become relatively thinner (such as with increasing temperature for example) or shrink or compress and become relatively thicker.
  • a change in thickness changes the optical wavelength of that component.
  • any layer therefore varies with temperature. Although efforts have been made to minimize this effect, through selection of complementary materials for the various layers and the like, the effect has not been eliminated. Lasers which can be tuned for entire telecommunications bands, and which operate over typical required temperatures, require additional adjustment of the tunable element's position to compensate for TWD due to changes in operating temperature.
  • Measurements are generally taken over the whole temperature range over which the assembly will be expected to operate, e.g., from about -5°C to about 70°C in increments ranging from about 1 to 20°C and more typically about 5 to 10°C, with each different temperature being plotted in association with the location of the center of the pass band.
  • This information is stored in non- volatile memory 39 in controller 38 as a look up table to be used for compensating for thermal wavelength drift.
  • laser light is passed through the etalon 26 at one end thereof, and a resultant wavelength of the pass band is measured and plotted against the linear position of the etalon 26.
  • the etalon is incrementally translated to what is expected to be the next channel selection and the steps of passing the laser light, measuring the wavelength of the pass band and recording the wavelength as a function of the linear position are repeated. These steps are repeated for every useable channel over the length of the etalon 26 and wavelength/linear position information is recorded in non- volatile memory 39 of controller 38 as a look up table.
  • a graphical representation of the wavelength/linear position data stored in the lookup table is shown in Fig.
  • Values for wavelength correction typically are about 3 pm/°C over the temperature range described, giving a total range of about -90 pm (-0.09 nm) to about +135 pm (+.135 nm) for the corresponding temperature range of about -5°C to about 70°C. Tuning compensation to 50 pm or less is desirable
  • external cavity laser 10 is hermetically sealed (not shown) and a temperature sensor 35, typically a thermocouple (although a thermistor may alternatively be used, for example) is provided within the sealed environment to provide feedback to the controller 38 as to the temperature of the components therein, and thus the temperature that the etalon 26 is experiencing.
  • the controller Upon selection of a particular channel or wavelength by an operator of the laser, the controller references the look up table in memory 39 which stores the position data of the etalon relative to wavelengths. This reference provides the proper position of the etalon 26 under baseline conditions, i.e., without regard to the temperature, and might be referred to as a gross positioning data point.
  • the controller 38 references the lookup table in nonvolatile memory 39 which stores the temperature coefficients of tuning from which the controller can calculate an adjustment in the positioning of the etalon 26 to compensate for thermal wavelength drift as described above.
  • One implementation of the etalon 26 according to the present invention at the central wavelength of 1545 nm, consists of the 32 layers listed in the Table below.
  • the physical thickness of the layers varies by about ⁇ 1.5 percent to provide a variation in the central wavelength of the band pass of about 3 percent. This variation, ideally, is a linear change on either side of the center of the filter. The change occurs over the scan distance of the filter.
  • the layer thickness (Thickness) is in nm.
  • Layer number 1 is contacting a silica substrate with an index of refraction (Index) of 1.44.
  • Layer 32 is exposed to air.

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  • Physics & Mathematics (AREA)
  • Nonlinear Science (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • Electromagnetism (AREA)
  • Semiconductor Lasers (AREA)
  • Mechanical Light Control Or Optical Switches (AREA)
  • Optical Filters (AREA)

Abstract

L'invention concerne un laser à cavité externe (10) utilisant un filtre d'interférence à couche mince biseauté (26) en tant qu'élément d'accord. Elle concerne aussi des filtres d'interférence à couche mince tronconique à couches diélectriques destinés à l'accord de lasers à cavité externe. Elle concerne encore des procédés d'accordage de laser à cavité externe (10) consistant à réaliser un positionnement de réglage d'un filtre d'interférence à couche mince biseauté (26). Elle concerne enfin un procédé d'accordage d'un laser à cavité externe (10) afin de tenir compte du décalage thermique de longueur d'onde, un procédé de montage d'un étalon d'une manière sensiblement libre de contrainte, ainsi qu'une monture associée.
PCT/US2002/008566 2001-03-21 2002-03-19 Filtre d'interference a faisceau en couche mince variable et procede d'utilisation pour accordage de laser WO2003077386A1 (fr)

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Applications Claiming Priority (4)

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US09/814,464 US6816516B2 (en) 2001-03-21 2001-03-21 Error signal generation system
US09/814,464 2001-03-21
US09/900,412 2001-07-06
US09/900,412 US6717965B2 (en) 2001-07-06 2001-07-06 Graded thin film wedge interference filter and method of use for laser tuning

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Cited By (2)

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Publication number Priority date Publication date Assignee Title
EP1531529A3 (fr) * 2003-11-13 2005-06-01 Mitutoyo Corporation Laser à cavité externe avec élément d'accord rotatif
EP1587187A1 (fr) * 2004-04-13 2005-10-19 Fujitsu Limited Laser de petite taille accordable en longueur d'onde

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TW552645B (en) 2001-08-03 2003-09-11 Semiconductor Energy Lab Laser irradiating device, laser irradiating method and manufacturing method of semiconductor device
CN101730960B (zh) * 2007-07-05 2012-07-11 皇家飞利浦电子股份有限公司 外腔表面发射激光设备
GB0823084D0 (en) * 2008-12-18 2009-01-28 Renishaw Plc Laser Apparatus
CN104348076A (zh) * 2013-08-02 2015-02-11 福州高意通讯有限公司 一种可调谐滤波结构及激光器
CN103872563A (zh) * 2014-03-24 2014-06-18 苏州旭创科技有限公司 可调谐光学标准具及具有其的外腔激光器
CN105632944B (zh) * 2014-11-03 2018-08-14 上海微电子装备（集团）股份有限公司 一种多光束准同步激光封装装置及方法

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US6108355A (en) * 1998-10-16 2000-08-22 New Focus, Inc. Continuously-tunable external cavity laser

Patent Citations (1)

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Publication number Priority date Publication date Assignee Title
US6108355A (en) * 1998-10-16 2000-08-22 New Focus, Inc. Continuously-tunable external cavity laser

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1531529A3 (fr) * 2003-11-13 2005-06-01 Mitutoyo Corporation Laser à cavité externe avec élément d'accord rotatif
US7130320B2 (en) 2003-11-13 2006-10-31 Mitutoyo Corporation External cavity laser with rotary tuning element
EP1587187A1 (fr) * 2004-04-13 2005-10-19 Fujitsu Limited Laser de petite taille accordable en longueur d'onde
US7158547B2 (en) 2004-04-13 2007-01-02 Fujitsu Limited Wavelength tunable laser of small size

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