WO1998011636A1 - Moyens economiques pour moduler un laser - Google Patents

Moyens economiques pour moduler un laser Download PDF

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Publication number
WO1998011636A1
WO1998011636A1 PCT/US1997/017139 US9717139W WO9811636A1 WO 1998011636 A1 WO1998011636 A1 WO 1998011636A1 US 9717139 W US9717139 W US 9717139W WO 9811636 A1 WO9811636 A1 WO 9811636A1
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Prior art keywords
laser
eccentric
plug
optical fiber
combination
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PCT/US1997/017139
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English (en)
Inventor
Herman C. Lowenhar
Burton L. Hulland
Original Assignee
Kol Ohr Corporation
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Publication date
Application filed by Kol Ohr Corporation filed Critical Kol Ohr Corporation
Priority to AU44997/97A priority Critical patent/AU4499797A/en
Priority to US09/269,145 priority patent/US6868101B1/en
Publication of WO1998011636A1 publication Critical patent/WO1998011636A1/fr

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Classifications

    • 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
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02061—Grating external to the fibre and in contact with the fibre, e.g. evanescently coupled, gratings applied to the fibre end
    • 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
    • G02B6/42—Coupling light guides with opto-electronic elements
    • G02B6/4201—Packages, e.g. shape, construction, internal or external details
    • G02B6/4202—Packages, e.g. shape, construction, internal or external details for coupling an active element with fibres without intermediate optical elements, e.g. fibres with plane ends, fibres with shaped ends, bundles
    • 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
    • G02B6/42—Coupling light guides with opto-electronic elements
    • G02B6/4201—Packages, e.g. shape, construction, internal or external details
    • G02B6/4219—Mechanical fixtures for holding or positioning the elements relative to each other in the couplings; Alignment methods for the elements, e.g. measuring or observing methods especially used therefor
    • 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
    • G02B6/42—Coupling light guides with opto-electronic elements
    • G02B6/4201—Packages, e.g. shape, construction, internal or external details
    • G02B6/4219—Mechanical fixtures for holding or positioning the elements relative to each other in the couplings; Alignment methods for the elements, e.g. measuring or observing methods especially used therefor
    • G02B6/422—Active alignment, i.e. moving the elements in response to the detected degree of coupling or position of the elements
    • G02B6/4226—Positioning means for moving the elements into alignment, e.g. alignment screws, deformation of the mount
    • 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
    • G02B6/42—Coupling light guides with opto-electronic elements
    • G02B6/4201—Packages, e.g. shape, construction, internal or external details
    • G02B6/4219—Mechanical fixtures for holding or positioning the elements relative to each other in the couplings; Alignment methods for the elements, e.g. measuring or observing methods especially used therefor
    • G02B6/4228—Passive alignment, i.e. without a detection of the degree of coupling or the position of the elements
    • 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
    • H01S5/062—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes
    • H01S5/06209—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes in single-section lasers
    • H01S5/06213—Amplitude modulation
    • 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
    • H01S5/0687—Stabilising the frequency of the laser
    • 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
    • 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
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/105—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the mutual position or the reflecting properties of the reflectors of the cavity, e.g. by controlling the cavity length
    • H01S3/1055—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the mutual position or the reflecting properties of the reflectors of the cavity, e.g. by controlling the cavity length one of the reflectors being constituted by a diffraction grating
    • 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
    • H01S5/0225—Out-coupling of light
    • H01S5/02251—Out-coupling of light using optical fibres
    • 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
    • H01S5/146—External cavity lasers using a fiber as external cavity
    • 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
    • H01S5/146—External cavity lasers using a fiber as external cavity
    • H01S5/147—External cavity lasers using a fiber as external cavity having specially shaped fibre, e.g. lensed or tapered end portion

Definitions

  • This invention relates to wideband modulators appropriate to extended-cavity lasers and to wavelength division multiplex networks using such lasers.
  • one of the least costly wideband modulating means is an external lithium niobate waveguide modulator.
  • an input signal is split in two and transmitted along parallel paths in lithium niobate waveguides.
  • phase shifts are introduced which cause destructive interference when the two paths are later re-merged.
  • Another type of wideband modulator is based on electro-absorption within the laser. If used with ordinary laser sources, these types of modulators may add on the order of one-fifth or less of the cost of the laser to system costs. However, if used with the previously described low cost lasers, the same modulators would add on the order of triple the laser cost to system costs.
  • a truly low cost modulator is needed if such low cost lasers are to be utilized advantageously.
  • Application Serial No. 08/234,955 relates to a class of devices known as fiber-extended-cavity lasers.
  • a modified laser diode is typically used as the active element. While an ordinary laser diode has two reflective ends that serve to define the resonant optical cavity that is required for lacing to take place, laser diodes used in fiber-extended- cavities have one of those ends anti-reflectively coated so they can no longer Iase, and that end is coupled to an optical waveguide, typically a side-polished fiber, that is, a portion of an ordinary optical fiber from one side of which most of the cladding has been removed.
  • an optical waveguide typically a side-polished fiber, that is, a portion of an ordinary optical fiber from one side of which most of the cladding has been removed.
  • the grating By positioning a feedback grating at the polished portion of the fiber, the grating will be coupled to the evanescent wave that is accessible there.
  • the grating serves as the second reflective end needed to complete a resonant cavity.
  • Patent 5,315,436 also discloses two other methods of continuously tuning a semiconductor laser that use gratings with parallel lines.
  • the feedback grating is loosely, grazingly coupled to the evanescent wave.
  • Such grazing coupling ensures that the grating lines along the entire grating participate substantially equally in the feedback process, as is required for optimum sidemode suppression.
  • the gratings used in each of these three tuning methods are relatively long.
  • An OWT is a tracking spectrometer that incorporates a tunable laser, such as those just described, and that automatically tunes that laser to output a wavelength that lies at a desired offset from a reference wavelength fed to the OWT.
  • the offset is established by the design of the OWT.
  • the tunable laser it incorporates may be modulated by the modulating means disclosed herein.
  • OWTs serve important functions in fiber-optic network architectures, illustratively the generation of hierarchically ordered downstream and upstream wavelength arrays, they are critical to the stability of the tunable lasers themselves. They resolve a number of stability issues stemming from thermal, mechanical and coupling problems.
  • a feature present in all three of the tunable laser structures mentioned above is their long cavity length, typically on the order of an inch.
  • Typical laser diodes have an active region that is between 100-500 microns long, so the cavity is between 50 and 250 times longer than the cavity of an ordinary Fabry-Perot laser. This presents an opportunity to directly modulate the laser while reducing the wavelength pulling that occurs with such modulation by approximately two orders of magnitude.
  • the feedback grating used for tuning also effects the sidemode suppression that is critical to the prevention of cross-channel interference in dense wavelength division multiplexing.
  • the grating must be at least as long as the cavity.
  • the maximum modulation rate of the novel modulation devices and method disclosed herein is inversely proportional to cavity length. Since the means by which the laser diode is coupled to the fiber of the optical cavity critically impacts both the length of the feedback grating (and thus its cost, since long gratings are costly) and the maximum achievable modulation rate, those coupling means will be dealt with at length.
  • the present invention discloses a novel means for low cost wideband modulation of an extended-cavity laser. These entail the use of each pulse in an input modulating pulse train as the trigger for the generation of a brief pulse, herein called a "notch" pulse, that is, a pulse that briefly diminishes (or notches) the normal laser drive current. Each "notch" pulse is used to drive the semiconductor laser embedded in an extended cavity with a current that flows opposite to its normal drive current. This serves to diminish the inverted charge-carrier population essential to lacing action, so the laser output falls sharply in a form of amplitude modulation.
  • the "notch" pulse is of brief duration so that upon its termination the laser promptly begins to restore its output power level by amplifying the residual signal level in successive passes through the optical cavity. At a receiver location the leading edge of that laser output signal waveform can then be used to trigger a pulse generator to replicate the input modulating pulse train.
  • a means for suppressing even that small residual wavelength pulling is also disclosed, and applied to both extended-cavity lasers and ordinary lasers.
  • the invention further discloses a simple, miniaturized means of micropositioning, based on the differential rotation of nested conical plugs, that can serve to couple the semiconductor laser to the cavity and that is inexpensive so that it can remain as a permanent part of the structure.
  • Fig. la shows a laser diode coupled to a side-polished fiber, which is itself coupled to an asymmetrically divergent grating.
  • Fig. lb shows in cross-sectional view the position of the side-polished fiber in relation to the grating carrier.
  • Fig. 2 shows a guide block containing a side-polished fiber, and a moveable carrier with a grating that is controllably coupled to the side-polished fiber.
  • Fig. 3 shows a portion of the guide block of Fig. 2, to which laser mounting and coupling elements have been added.
  • Figs. 4a and 4b show respectively the outer and inner nestable conical plugs of a micro-positioner for an optical fiber.
  • Fig. 4c shows the conical plugs of Figs. 4a and 4b nested.
  • Fig. 5 shows a functional block diagram of an Offset Wavelength Tracker with a modulator for its tunable laser.
  • Fig. 6 shows a functional block diagram of the modulation method.
  • Figs. 7a, 7b, 7c, 7d and 7e illustrate the relative timing of various signals connected with the modulator.
  • Figs. 8a, 8b, 8c, 8d and 8e diagram some of the various ways in which a laser diode can be modulated by the means disclosed herein.
  • Fig. 9a shows a cross-sectional view of an optical fiber whose center fiber is concentric with its surrounding cladding.
  • Fig. 9b shows a cross-sectional view of an optical fiber whose "center” fiber is eccentric with respect to its surrounding cladding.
  • Figs. 10a and 10b illustrate two of the scanning patterns available with the micro-positioners described herein.
  • Fig. 11 is Fig. 10 of U.S. Patent 5,315,436, which shows the use of eccentric optical fiber for compensation of manufacturing tolerances.
  • Fig. la illustrates an extended-cavity laser with which the present invention is applicable.
  • Cavity length is illustratively considered to be approximately the distance from the rear, or left-hand, end of diode laser 9 to the center of grating 5. This is much longer than the length of the active region, the latter being substantially the length of diode laser 9.
  • the dimensions of diode laser 9 relative to the grating 5 are greatly exaggerated in these figures, so that, in reality, the cavity's active region is only a quite small fraction of cavity length.
  • Fig. 2 shows a guide block 16 in which the side-polished optic cable is embedded. The block is to be moved relative to the grating 5 in a grazing manner to achieve continuous tuning of the laser, as described in the aforementioned patent.
  • Fig. 3 shows a portion of a guide block 116 and laser diode mount, corresponding to the narrow, central portion of guide block 16 of Fig. 2, with added elements for mounting a laser and coupling to it. This portion, the part bounded by section lines A and B, is shown as 116 of Fig. 3. This partial view shows details of the micro-positioning coupling method that otherwise would be concealed. Comparison of Fig. 3 with Fig. 2 shows that forward of line C-C. there have been added:
  • a horizontal extension section 120 extending only from the upper surface of 116 so as to leave a gap 130 below.
  • Nested cones 123 and 124 have integral spur gears, accessible from the side or the bottom, by means of which they may be independently rotated.
  • a horizontal section 125 extending from 121 and comprising a shelf 126 on which a laser chip 127 may be mounted.
  • the proximate face 131 of laser 127 typically a cleaved face, serves as one reflective end of a fiber-extended laser cavity.
  • the laser's distal face, anti- reflectively coated, is coupled to the end 129 of center fiber 128 of optical fiber 104.
  • End 129 of optical fiber 104 is preferably formed into a microlens that is anti-reflectively coated to enhance coupling to the laser chip's emitting spot. It will also be closer to the chip than diagrammatical ly shown in this view, which exaggerates the spacing in order to show the tip of the conical micro-positioner.
  • Optical fiber 104 has had most of the cladding removed (ground away) from one side to form along its length a side-polished region where coupling may be effected between a grating and the evanescent wave.
  • a divergent grating so disposed as to effect coupling to the evanescent wave, can be moved to tune the extended-cavity laser.
  • guide block 16 of Fig. 2 can be made to contain a plurality of side-polished fibers 4, illustratively parallel to and uniformly spaced from one another.
  • a single feedback grating serves to tune a plurality of laser diodes, each of which would be coupled to a respective side-polished fiber, and each of which would be separately modulated.
  • the effect of tuning such an array of equidistantly spaced, parallel cavities would be to shift their wavelengths, as a group, up or down in the lasers' operating band.
  • Fig. 4 shows details of micro-positioning plugs 123 and 124, including typical values of the offset inner wall dimensions of hollow plug 123 and of the offset center bore of solid plug 124 that 123 is designed to accept.
  • a laser chip is used, rather than the typical laser diode in a can with an output window. Commercially available couplers are typically designed to couple to the latter.
  • Coupling to an unhoused laser diode chip is advantageous since it permits access to both ends, whereas access is restricted to only the emitting face when the chip is viewed through the window of a can-mount.
  • the reflectivity of each of the cleaved ends of a typical laser diode is on the order of 30 percent, so either end can be used for output signal.
  • the laser In the design of the OWT disclosed in U.S. Patent 5,315,436, the laser must be coupled to its tuning structure, and must also provide an ou ⁇ ut both to the network and as reference wavelength to the next-in-line OWTs.
  • Use of both ends of the laser chip can effect both functions without the relatively high cost of a separate coupler.
  • use of the laser's back wall as an ou ⁇ ut signal port permits the insertion there, external to the resonant cavity, of the shutter that is required to prevent cross-channel interference while the OWT is tuning itself to its hierarchically required wavelength.
  • gap 130 is preferably made only long enough to admit nested conical plugs 123 and 124 into conical seat 122.
  • Gap 129 must be kept as short as possible in order to minimize that part of cavity length that is not coupled to the grating, thus keep cavity length as short as possible.
  • Patent 5,315,436 any modulation- induced change in effective cavity length only takes place within the active element, since the remainder of the cavity, being passive, remains unchanged.
  • the effect is to diminish the wavelength shift by substantially the ratio of the length of the laser diode's cavity to the length of the entire fiber-extended cavity, i.e., in the exemplary cavities under consideration, by approximately 100:1.
  • a shift of one nanometer is thus reduced to 0.01 nanometer, a negligible change in even a dense Wavelength Division Multiplex network, where adjacent channels may be only one nanometer apart.
  • the modulation method disclosed herein takes advantage of this fact.
  • this small wavelength shift may be objectionable. It is possible to reduce even that small shift essentially to zero, by building into the cavity a segment of an optically active material and driving that segment with a countervailing signal, that changes the index of refraction of that segment in a direction opposite to and by an amount proportional to the change in the index of refraction of the laser diode being modulated, thus keeping the effective cavity length constant despite the refractive index changes.
  • That segment may be one section of a two-section semiconductor laser, whose other section comprises the gain medium.
  • high laser gain is desirable, in order to speed post-modulation laser ou ⁇ ut power level restoration and thus maximum modulation rate.
  • Typical active channel lengths vary between 100 and 500 microns, and for a given type of laser, gain is proportional to channel length.
  • the need for relatively high gain, as expressed in a design choice in which a long active channel is chosen rather than a short active channel, will however have minor effects on wavelength pulling because, practically speaking, the ratio of cavity length to active channel length remains high whatever the channel length.
  • a common problem in fabricating assemblies containing lasers is how to couple an optical fiber to that point on the laser where emission takes place.
  • the emitting spot is typically only 3 or 4 microns wide, while the center fiber of a typical optical fiber line is 9 microns in diameter. Aligning the two so they are efficiently coupled requires precision coupling means.
  • the shortest such means is a tubular coupler containing a positionable focussing lens and a three-axis alignment mechanism controlled by three respective, fine-pitch set screws.
  • a tubular coupler were used in a fiber-extended-cavity laser, it would extend the cavity by its own length plus the length of its ou ⁇ ut pigtail.
  • Miniaturized micro-positioning devices are provided in accordance with the invention for easily and inexpensively coupling to such small emitting spots. These devices can also serve as micro-positioning means for the correction of manufacturing tolerances, illustratively in the detector head of an Offset Wavelength Tracker, such as shown in Fig. 10b of U.S. Patent 5,315,436, reproduced here in Fig. 11.
  • manufacturing tolerances in the spacing Y between the centers of the two pairs of photodetector elements and the spacing between the two signal input ports can be compensated by adjusting the spacing X between the two center fibers at those input ports. While other means of accomplishing these two functions are possible, the simplicity and low cost of the preferred embodiment recommends its use.
  • Fig. 8a shows in cross-section an optical fiber that contains a concentric center fiber
  • Fig 8b an optical fiber that contains an eccentric center fiber.
  • This principle of rotating an eccentrically placed fiber can be modified to scan an area, as follows. Let the eccentric fiber be inserted into an eccentric plug (see Fig. 4), which can also be rotated. If the plug and the fiber are then rotated at rates that differ substantially, die fiber tip will be scanned in a pattern resembling a connected series of partially overlapping circles. If the two rotation rates are closely similar, the fiber's tip will be scanned in a spiral pattern.
  • an area centered at the true center of the eccentric plug can be scanned. If that true center is positioned in the vicinity of the emitting spot of a laser, and that area is scanned by rotating the fiber and the plug, then the center fiber will eventually be positioned over the emitting spot, and thus coupled to the laser.
  • the coupling efficiency can be maximized.
  • Fig. 4 shows a typical design for such plugs. Note that if the inner wall of the hollow plug 124 of Fig. 4a is offset from the center line of its plug by one-thousandth of an inch, and if the center bore of the solid plug 123 of Fig. 4b is offset from the center line of its plug by one-thousandth of an inch, then a circular area with a diameter of four-thousandths of an inch can be scanned. Offsets can be chosen that are appropriate to effect any desired scan. Note, too, the integrally molded spur gear on each of the plugs 123, 124, that adapts them for automated scanning by a drive mechanism.
  • Marks, not shown, at the gear end of each plug indicate the position of maximum offset and the point opposite it on the circumference, to aid in initial setup. If both plugs were rotated at the same rate, the tip of an optical fiber inserted into the plug shown in Fig. 4b would be scanned in a stationary circle. If the plugs are rotated at rates that differ substantially the tip of that fiber will be scanned in a pattern resembling a connected series of partially overlapping circles that overlap and surround the center point of the scanned area, as illustrated in Fig. 9a. Finally, if the plugs are rotated at rates that are slightly different, the tip of that fiber will be scanned in a spiral pattern as illustrated in Fig. 9b.
  • each pulse in an incoming pulse train will be used to generate a modulating "notch" pulse shorter than the incoming pulse.
  • This "notch" pulse is used to directly modulate the laser diode, by applying to the laser diode a brief countercurrent to its normal drive current. This causes a sharp drop in laser ou ⁇ ut power. Subsequent recovery of the laser ou ⁇ ut to its initial power level takes place between modulating pulses, as successive round-trip traversals of the laser cavity by the light emitted by what remains of the emitting population builds laser ou ⁇ ut back to full power level.
  • the received pulse train can serve as triggers for a pulse generator that can regenerate the original input pulse train.
  • a segment of optically active material, transmissive of the propagating wavelength and inserted in the optical cavity could be driven by a countervailing waveform synchronized with the modulating waveform to alter that segment's index of refraction, and therefore its effective optical length in the opposite direction, to maintain substantially constant effective cavity length despite the modulation's effect on the laser diode's index of refraction.
  • FIG. 7 illustrates the relative timing of the pulse trains.
  • the input pulse train of drive current to the laser diode is shown in 7a.
  • a pulse generator Upon sensing the leading edge of each pulse in that train, a pulse generator generates a corresponding, brief pulse illustratively pulses like those of 7b or 7c, that are used to briefly diminish or "notch" the laser diode drive current to diminish the charge carrier population inversion in the active region.
  • the resultant falloff and subsequent recovery in laser ou ⁇ ut power level is illustrated in 7d.
  • the depth of modulation is controlled to provide adequate margin against amplification to saturation by the subsequent system amplifiers, while permitting recovery to substantially full power level with a small number of traversals of the optical cavity.
  • a pulse generator that can be triggered by either the leading edge of the fall in received laser power or by the leading edge of the subsequent rise in received laser power can be used to replicate the original input modulation pulse train, which is then available for ordinary processing. The net effect is an increment to the time delay in the receipt of the input pulse train, as would occur if the transmission path were lengthened.
  • notch pulses of different form than those shown in Figs. 7b and 7c may be used.
  • Fig. 8 illustrates five different circuits for modulating a laser diode by this means.
  • the "notch" pulses generated by generator/driver generated by generator/driver
  • radio-frequency transformer 210 is coupled to driver signal lines 220 and 221 through radio-frequency transformer 210.
  • 212 represents a D.C. bias line.
  • This method can be used to briefly push the laser drive current toward zero with a countercurrent pulse or, with a large enough signal, even beyond zero. The latter might be used to invert the sense of the drive current, to speed the fall in laser ou ⁇ ut level by sweeping some of the excited-state charge carriers out of the active region, yet without entirely quenching laser ou ⁇ ut by excessive depletion of the population inversion.
  • the modulating "notch” pulses drive field-effect transistor (FET) 213.
  • FET field-effect transistor
  • a second FET 214 is combined with FET 213 in a so-called totem-pole configuration.
  • Generator/driver 201 differs from 200, in that it ou ⁇ uts an additional signal on signal line 222. That signal is typically pulsed on in synchronism with the modulating "notch" pulse, to discharge any stray capacitances.
  • FET 214 has been given its own bias line 216. This permits the laser diode to be driven with a reverse-bias signal.
  • generator/driver 202 differs from 201, in that it outputs on signal line 223 the countervailing signal previously mentioned as a way of reducing the wavelength shift due to modulation substantially to zero.
  • a two-section laser diode 216 is employed, the second section of which is differently doped than the primary gain section. That second section has the desired optical transmissivity and is accessible through additional bonding pad 215.
  • the composite change in index of refraction can be reduced substantially to zero.
  • the counter- vailing signal is not shown in the waveforms of Fig. 7.
  • that segment of material can be a section of the diode that is differently doped than the main section, and is driven by a separate signal lead, through its own bonding pad.
  • a laser would be constructed as a Distributed Feedback (DFB) laser, whose wavelength is established by a feedback grating, illustratively a grating extending along the entire active region of the laser, and which grating, thus being as long as the optical cavity, is also effective in sidemode suppression.
  • DFB Distributed Feedback
  • Fig. 5 shows a modulator of a design that accords with the above principles driving the laser diode of an OWT.
  • the tip of a side-polished fiber positioned by micro-positioner 312 both couples laser diode ou ⁇ ut to the fiber for interaction with the feedback grating 314 and couples back into the laser diode feedback at the wavelength represented by the grating's interline periodicity where it intersects the side-polished fiber. That bi-directional flow of signal is represented by the double-headed arrow.
  • the side-polished fiber exiting the micro-positioner 312 is loosely coupled to the feedback grating 314, and then fed into the wavelength comparator input port.
  • the wavelength comparator 340 the wavelength ou ⁇ ut by the tunable laser is compared to the wavelength of the reference signal 350. Any difference between the two wavelengths that is greater than or less than the designed offset wavelength produces a corrective error signal for servo B 355 that drives the feedback grating actuator 318, which moves the grating, as indicated by the two directional arrows, tuning the ou ⁇ ut wavelength until zero error results.
  • the loop containing servo A 360 drives its own actuator 362 to keep the reference wavelength centered on a split photodetector, to serve as a fiduciary mark.
  • a separate optical fiber 370 here shown below the laser diode 310, has its tip coupled to the cleaved, uncoated rear wall of the laser diode from which the main ou ⁇ ut signal is taken. Note that the arrow from the laser to the micro-positioner 372 has only one head, since there is essentially no energy reflected back to the laser. Note the use of a micro-positioner 352 at the reference wavelength input port of the wavelength comparator 340. This serves to move the optical fiber tip to compensate for manufacturing tolerances that would otherwise result in wavelength offset errors.
  • Fig. 6 shows the modulation method in functional block diagram form.
  • the input modulation pulse train drives both a "notch" pulse generator and, optionally, a countervailing signal generator, each of which feeds a driver.
  • the modulation driver "notches” the laser diode drive current by any desired means, including any one of the circuits shown in Figs. 8.
  • the optional countervailing signal is here shown driving one section of a two-section laser diode, though it could as readily be used to drive a separate segment of a suitable optically active material.
  • the modulated laser ou ⁇ ut then traverses a transmission path to a receiver. There, the leading edge of each received pulse can serve as a trigger for a pulse train regenerator, that can replicate the original input modulation pulse train. While not an inherent part of the modulator, this latter function is included here for clarity.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • Electromagnetism (AREA)
  • Semiconductor Lasers (AREA)

Abstract

La présente invention concerne un procédé de modulation et un appareil pour moduler un laser à cavité étendue (211). Pour mettre en oeuvre ledit procédé et employer ledit appareil (200-203), on utilise chaque impulsion dans un train d'impulsions de modulation d'entrée, en tant que déclencheur servant à produire une impulsion brève ou dite 'encoche', c'est-à-dire une impulsion qui réduit brièvement le courant d'attaque normal. Chaque impulsion 'encoche' est utilisée pour commander le laser à semi-conducteur (211) intégré dans une cavité étendue avec un courant qui s'écoule dans le sens inverse du courant d'attaque normal. Ceci sert à réduire la population de porteurs de charge inversée qui est essentielle pour l'effet laser, de sorte que la sortie laser chute brutalement sous forme d'une modulation d'amplitude. L'invention concerne également des moyens simples, miniaturisés (372, 352, 312) de micropositionnement, sur la base de la rotation différentielle de fiches coniques, qui peuvent servir à coupler le laser à semi-conducteur à la cavité.
PCT/US1997/017139 1996-09-13 1997-09-12 Moyens economiques pour moduler un laser WO1998011636A1 (fr)

Priority Applications (2)

Application Number Priority Date Filing Date Title
AU44997/97A AU4499797A (en) 1996-09-13 1997-09-12 Low cost means of modulating a laser
US09/269,145 US6868101B1 (en) 1996-09-13 1997-09-12 Low cost means of modulating a laser

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
US2625596P 1996-09-13 1996-09-13
US60/026,255 1996-09-13
US2973396P 1996-10-23 1996-10-23
US60/029,733 1996-10-23

Publications (1)

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WO1998011636A1 true WO1998011636A1 (fr) 1998-03-19

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AU (1) AU4499797A (fr)
WO (1) WO1998011636A1 (fr)

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4875217A (en) * 1987-05-22 1989-10-17 Siemens Aktiengesellschaft Laser diode array with lateral coupling
US4932034A (en) * 1988-03-11 1990-06-05 Kokusai Denshin Denwa Kabushiki Kaisha Distributed feedback semiconductor laser device and current injection
US5315436A (en) * 1990-06-06 1994-05-24 Lowenhar Herman Leonard Fiber optics system
US5337392A (en) * 1992-07-29 1994-08-09 Alcatel Cit Method of coupling an optical fiber to an optical component on the same substrate

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4875217A (en) * 1987-05-22 1989-10-17 Siemens Aktiengesellschaft Laser diode array with lateral coupling
US4932034A (en) * 1988-03-11 1990-06-05 Kokusai Denshin Denwa Kabushiki Kaisha Distributed feedback semiconductor laser device and current injection
US5315436A (en) * 1990-06-06 1994-05-24 Lowenhar Herman Leonard Fiber optics system
US5337392A (en) * 1992-07-29 1994-08-09 Alcatel Cit Method of coupling an optical fiber to an optical component on the same substrate

Also Published As

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AU4499797A (en) 1998-04-02

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