`
`IN THE UNITED STATES PATENT AND TRADEMARK OFFICE
`
`In re Patent of:
`
`Lebens et al.
`
`U.S. Patent No.:
`
`6,095,661
`
`
`
`Issue Date:
`
`August 1, 2000
`
`Appl. Serial No.:
`
`09/044,559
`
`Filing Date:
`
`March 19, 1998
`
`Title:
`
`METHOD AND APPARATUS FOR AN L.E.D.
`
`FLASHLIGHT
`
`
`PETITION FOR INTER PARTES REVIEW OF UNITED STATES PATENT
`NO. 6,095,661 PURSUANT TO 35 U.S.C. §§ 311–319, 37 C.F.R. § 42
`
`
`Exhibit LG-1015
`
`U.S. Patent No. 5,373,387 (“Bosch’’)
`
`

`

`United States Patent
`
`[19]
`
`[11] Patent Number:
`
`5,373,387
`
`Bosch et al.
`Dec. 13, 1994
`[45] Date of Patent:
`
`USOOS373387A
`
`[54] METHOD FOR CONTROLLING THE
`AMPLITUDE OF AN OPTICAL SIGNAL
`
`[75]
`
`Inventors:
`
`Fridolin L. Bosch; Ton V. Nguyen,
`both of Bethlehem, Pa.
`
`[73] Assignee: AT&T Corp., Murray Hill, NJ.
`
`[21] Appl. No.: 136,503
`
`[22] Filed:
`
`Oct. 14, 1993
`
`Int. Cl.5 ........................................... .. H04B 10/04
`[51]
`[52] us. Cl. .................................. .. 359/187; 359/186;
`359/161; 372/31; 375/22
`[58] Field ofSearch .............. ..359/110,153,161—162,
`359/180—181, 184-187; 372/31, 38; 375/22
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`................ .. 372/31
`8/1987 Yoshimoto et a1.
`4,689,795
`4,713,841 12/1987 Porter et a1.
`.........
`.. 359/186
`
`5,153,765 10/1992 Grunziger ......................... .. 359/180
`OTHER PUBLICATIONS
`
`“GaAlAs Laser Transmitter For Lightwave Transmis-
`sion Systems”, The Bell System Technical Journal, vol.
`57, No. 6, Jul.—Aug. 1978, P. W. Schumate, Jr., P. S.
`Chen, and P. W. Dorman, pp. 1823-1836.
`“Laser Level Control For High Bit Rate Optical Fibre
`Systems”, 13th Circuits and Systems International Sym-
`
`posium Proceedings, Houston, Texas, Apr. 1980, D. W.
`Smith and T. G. Hodgkinson, pp. 926-930.
`
`Primary Examiner—Herbert Goldstein
`Assistant Examiner—Kinfe-Michael Negash
`Attorney, Agent, or Firm—Scott W. McLellan
`
`[57]
`
`ABSTRACT
`
`An amplitude control scheme for a high bit rate digital
`optical transmitter is disclosed. The data to be transmit-
`ted by the laser is pulse-width modulated by a low fre-
`quency signal. The pulse-width modulated rsignal
`is
`applied to the laser via a laser driver and to a mark
`density reference generator. The magnitude of the low
`frequency components from the mark density reference
`generator is a signal indicative of the desired amplitude
`of the laser light pulses. A back-face photodiode con-
`verts a portion of the laser light into an electrical signal,
`the magnitude of the low frequency portion thereof
`being a signal indicative of the actual amplitude of the
`laser light pulses. The actual amplitude of the laser light
`pulses is computed to the desired amplitude and the
`laser driver output amplitude may then adjusted to
`compensate for variations in the laser performance.
`
`9 Claims, 2 Drawing Sheets
`
`MONTTOR
`
`PHOTODIODE
`
`LASER
`
`HIGH
`SPEED
`DATA
`INPUT
`D
`
`
`_—1
`LASER
`r15
`{
`::, LIGHT
`
`PULSE WIDTH
`1 OUTPUT
`_ ._ .1
`MODULATOR
`L________
`
`Ibias
`
`MARK DENSITY
`REFERENCE
`
`
`
`GENERATOR
`LASER BIAS
`
`CONTROLLER
`
`
`T————T'—_____-”____—]
`
`FREQUENCY
`OSCILLATOR
`
`
`
`Exhibit LG—1015 Page 1
`
`Exhibit LG-1015 Page 1
`
`

`

`US. Patent
`
`Dec. 13, 1994
`
`Sheet 1 of 2
`
`5,373,387
`
`MONIIOR
`PHOTODIODE
`LASER
`
`—‘1
`_>} LASER
`_.. LIGHT
`| OUTPUT
`— — J
`Ibios
`
`LASER BIAS
`CONTROLLER
`
`'7
`I
`:
`I
`I
`
`L
`
`40
`
`m
`
`FIG.
`
`1
`
`r____CQ
`30
`
`FREQUENCY
`OSCILLATOR
`
`HIGH
`SPEED
`DATA
`INPUT
`D
`
`I-
`:
`
`| i
`
`|
`:
`I
`I
`
`I
`
`
`
`PULSE WIDTH
`MODULATOR
`_
`MARK DENSITY
`REFERENCE
`
`
`GENERATOR
`
`
`
`ANPLNUOE
`CONIROILER
`
`
`I
`I
`I
`I
`I
`I
`L ____________________ _ _ __I
`
`
`
`T0 36,22
`(MARK
`DENSITY
`REFERENCE)
`
`Q
`
`ERROR SIGNAL FROM 42
`
`Exhibit LG—1015 Page 2
`
`Exhibit LG-1015 Page 2
`
`

`

`US. Patent
`
`Dec. 13, 1994
`
`Sheet 2 of 2
`
`5,373,387
`
`FIG. 4
`
` PULSE WIDTH
`DEPTH-PS
`
`
` AMPLITUDECONTROLSIGNALLACS)
`
`MODULAHON
`
`
`
`
`
`0
`
`LASER LIGHT PULSE AMPIJTUDE
`
`Exhibit LG—1015 Page 3
`
`Exhibit LG-1015 Page 3
`
`

`

`1
`
`5,373,387
`
`2
`
`METHOD FOR CONTROLLING THE AMPLITUDE
`OF AN OPTICAL SIGNAL
`
`BACKGROUND OF THE INVENTION
`
`CROSS-REFERENCE TO RELATED
`APPLICATION
`
`This application is related to a co-pending patent
`application titled “Amplitude Detection Scheme For
`Optical Transmitter Control”, by F. L. Bosch and T. V.
`Nguyen, Ser. No. 08/136,358,
`filed simultaneously
`with, and assigned to the same assignee, as this applica-
`tion.
`
`1. Field of the Invention an optical transmitter and,
`more particularly, to a method of detection useful in
`providing control of laser modulation over the lifetime
`of the optical transmitter.
`2. Description of the Prior Art
`A continuing concern in the field of laser-based opti-
`cal transmitters is the change in laser characteristics
`with temperature and aging. The change in laser char-
`acteristics manifests itself as a combination of a shift in
`the laser threshold, (the current at which the laser be-
`gins lasing), and the L-I slope (the light output L vs.
`current I characteristic). As a result, special operating
`strategies for controlling the laser bias and modulation
`currents have been developed. In most cases, a photodi-
`ode is mounted in the same package as the laser and a
`portion of the light exiting the rear face of the laser is
`captured by the photodiode and used to monitor the
`laser performance. In particular, a feedback loop com-
`paring the photodiode current to a reference signal
`maintains the average light output at a desired level by
`automatically adjusting the bias current. The reference
`signal is referred to here as a “mark density” reference
`signal, representing the average density over time of
`pulses which the laser converts into light. See, for ex-
`ample, an article entitled “GaAlAs Laser Transmitters
`for Lightwave Transmission Systems”, appearing in the
`Bell System Technical Journal, Vol. 57, No. 6, July-
`August 1978, beginning at page 1823 and included
`herein by reference.
`During initial use, and as long as only the laser thresh-
`old exhibits changes as a function of aging, the above
`photodiode monitor arrangement is suitable. However,
`when the laser L~I slope begins to change as a function
`of age, the light amplitude (and with it, the critical
`extinction ratio of ON-light to OFF-light) will change.
`Thus, a need remains for a means of controlling both the
`average light output and the light amplitude.
`One arrangement proposed to provide this need is
`described in an article entitled “Laser Level Control for
`High Bit Rate Optical Fibre Systems”. by D. W. Smith
`et a1. and presented at the 13th Circuits and Systems
`International Symposium, Houston, Tex., April 1980
`(appearing at pages 926—30 of the Proceedings). In this
`case, a low-frequency ON-state slope sensing arrange-
`ment, utilized in a high bit rate transmitter, permits
`indirect amplitude control. However, the arrangement
`as proposed works only with lasers having an excep-
`tionally linear L-I slope, irrespective of temperature or
`aging. Typical lasers, however, have non-linear L-I
`slope which may render this approach unusable.
`Thus, a need remains for a more robust arrangement
`for monitoring both average light and light amplitude in
`an optical transmitter that is usable with non-ideal la-
`sers.
`
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`SUMMARY OF THE INVENTION
`
`The need remaining in the prior art is addressed by
`the present invention which relates to an amplitude
`detection scheme for an optical tramsmitter and, more
`particularly, to a scheme useful in providing control of
`the laser modulation over the lifetime of the optical
`transmitter.
`
`In accordance with an exemplary embodiment of the
`present invention, a digital laser transmitter is provided
`having a laser responsive to a laser driver and a back-
`face photodetector for converting at least part of the
`light from the laser into a monitor signal. The amplitude
`of a the digital laser light output signal is monitored and
`controlled by the method of: pulse width modulating a
`high bit rate digital data input signal with a low fre-
`quency modulation signal to form a pulse width modu-
`lated data signal which is coupled to the laser driver;
`extracting the low frequency AC component of the
`monitor signal to form a laser amplitude control signal;
`and comparing said laser amplitude reference signal to a
`reference signal and providing as an output an ampli-
`tude error signal indicative of any difference in value
`between the laser control and reference signals. The
`laser driver includes a control input for varying the
`drive to the laser and the control input is responsive to
`the amplitude error signal.
`In a preferred embodiment of the present invention,
`the amplitude detection arrangement is used in conjunc-
`tion with an average light output control, as provided
`by a conventional monitor photodiode. Therefore, a
`dual-loop control arrangement may be provided which
`includes correction or both average light and light am—
`plitude.
`Other and further embodiments and advantages of
`the present invention will become apparent during the
`course of the following discussion and by reference to
`the accompanying drawings.
`
`BRIEF DESCRIPTION OF THE DRAWING
`
`Referring now to the drawings,
`FIG. 1 illustrates an exemplary laser transmitter uti-
`lizing the amplitude detection and control arrangement
`of the present invention;
`FIG. 2 is a simplified schematic diagram of the laser
`driver of FIG. 1;
`FIG. 3 is a simplified schematic diagram of the mark
`density reference generator of FIG. 1; and
`FIG. 4 is a plot (not to scale) of a light amplitude
`control signal measured against the laser light pulse
`amplitude for different modulation depths in one exem-
`plary embodiment of the invention.
`
`DETAILED DESCRIPTION
`
`An exemplary digital lightwave transmitter 10 utiliz-
`ing both average light and low frequency light ampli-
`tude control is illustrated in FIG. 1. In general, an in-
`coming digital data signal (electrical) D, operating at a
`predetermined bit rate is coupled into the low fre-
`quency light amplitude control means 12. Low fre-
`quency means 12 will be discussed in more detail below.
`Data signal D subsequently passes through a laser
`driver 14 and is applied as the modulation input to a
`laser device 16. A bias current Ibias is also applied as an
`input to laser device 16. The light output signal from
`laser device 16 may then be coupled into an optical fiber
`(not shown) to propagate along the signal path as a
`digital optical data signal.
`
`Exhibit LG—1015 Page 4
`
`Exhibit LG-1015 Page 4
`
`

`

`3
`As seen in FIG. 1, a portion of the optical output
`signal from laser device 16 also exits the rear face of the
`device and illuminates a monitor photodetector 20, here
`a photodiode. The optical signal is converted into an
`electrical current within monitor photodetector 20,
`denoted as IMON. As is well-known in the art, IMONmay
`be applied as a first input to a control means 22 for
`monitoring the average light output signal from laser
`device 16. As described above. IMONmay be compared
`to the mark density level of the electrical pulses to the
`laser driver 14, a substantially DC signal, to provide any
`necessary adjustments to the level of 131,43 to maintain
`the desired average light output.
`It is the control means 12, as will be described in
`greater detail hereinafter, that provides the additional
`ability to monitor and control the amplitude of the laser
`output signal.
`As illustrated in the exemplary embodiment of FIG.
`1, an exemplary high bit rate digital data stream D is
`being supplied as an input to control means 12. In accor-
`dance with the teachings of the present invention, that
`stream D is applied as a first input to a pulse width
`modulator 30. Data stream D is then pulse width modu-
`lated by a low frequency signal MOD from a low fre-
`quency oscillator 32 to form a pulse-width modulated
`data output signal DpWM. The signal DpWMdrives laser
`driver 14 which, in turn, drives the laser diode 16. As a
`consequence, the laser light pulses from diode 16 are
`also pulse-width modulated. For purposes of this discus-
`sion, the term “low frequency” refers to a signal having
`a frequency significantly lower than the frequency of
`the digital data that it modulates in pulse width modula-
`tor 30.
`An illustrative laser driver 14 is shown in FIG. 2. The
`
`driver 14 allows the laser light pulses to vary in ampli-
`tude depending on the current supplied by the current
`source 50, as will be discussed below.
`The pulse-width modulated data signal DPWM addi-
`tionally drives a mark density reference generator 15
`which generates a signal indicative of the density of the
`pulses applied to the laser 16. An exemplary generator
`15 is shown in FIG. 3 and is similar to the laser driver 14
`of FIG. 2 but with fixed bias (tail) current. Returning to
`FIG. 1, the signal from mark density reference genera-
`tor 15 has two components, a DC component (MDSDc)
`representing the average level of the pulses and an AC
`component (MDSAC) representing, for purposes here,
`the effect of the pulse-width modulation on the data
`signal D. The DC component MDSDc is coupled to the
`laser bias control 22, discussed above. The AC compo-
`nent MDSAcis amplified by a low frequency AC-cou-
`pled amplifier 36 and demodulated to form an amplitude
`reference signal (ARS), as discussed below in more
`detail, since the signal to the mark density reference
`generator 15 has not yet passed through the laser 16.
`The ARS signal represents the desired amplitude of the
`laser light pulse.
`As stated above, the laser light pulses from laser 16
`will have the low frequency pulse width modulation
`impressed thereon. Monitor photodetector (photodi-
`ode) 20 will, therefore, re-convert this low frequency
`signal into an electrical equivalent which may then be
`evaluated to determine any change in the light ampli-
`tude of the laser output signal. In particular, the output
`signal from monitor photodetector 20 will contain both
`DC and AC components, the DC component (IMON -
`DC) is related to the bias current, as described above.
`Therefore, the AC component (IMON - AC) may be
`
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`5,373,387
`
`4
`amplified in a manner similar to the AC component of
`the mark density reference signal MDSAC, utilizing a
`second low frequency AC-coupled amplifier 38
`(wherein amplifier 38 should be essentially matched in
`performance to first amplifier 36). The signal IMON- AC
`is subsequently demodulated to generate an amplitude
`control signal, ACS. The signal ACS is indicative of the
`amplitude of the actual laser output signal. In accor-
`dance with present invention, this amplitude signal may
`be controlled by comparing the actual output signal
`ACS to the generated reference signal ARS within a
`light amplitude controller 42 to produce an error signal
`E. The output error signal E from controller 42 will
`provide an indication of any difference between these
`two signals and serves as a control signal to laser driver
`14 to either increase or decrease the amplitude of the
`driver signal applied to the laser. Referring temporarily
`back to FIG. 2, the error signal E from controller 42
`varies a current source 50 to adjust the amplitude of the
`current through the laser 16 and, hence the amplitude of
`the laser light output pulse therefrom.
`Therefore, by appling pulse width modulation to the
`laser modulating current, and detecting the low fre-
`quency component in a monitor photodiode, a relative
`measure of the light amplitude may be obtained, and
`any changes in this amplitude be corrected.
`The signals ARS and ACS are essentially indicative
`of the amplitude of the low frequency signals from
`corresponding amplifiers 36, 38. As such, the detectors
`may be conventional envelope detectors. However, it
`has been found that a conventional envelope detector is
`unsuitable due to the low signal levels and high noise
`environment within a laser transmitter package, i.e., the
`low frequency signals have a low signal-to—noise ratio.
`To improve the signal—to-noise ratio of the demodula-
`tion of the low frequency signals, it is preferred that
`synchronous demodulation be used to detect the modu-
`lating signal in the Dmm; and IMON- AC signals. Thus,
`synchronous demodulators 34, 40, here shown as multi-
`pliers, utilize the low frequency modulation signal
`MOD as the demodulation reference signal.
`It is noted that the low frequency oscillator 32 may
`produce a square wave, a sinusoidal wave, triangular
`wave, etc. or may be a random or pseudorandom signal.
`The frequency of the oscillator 32 should be lower than
`the data D bit rate, preferably two orders of magnitude
`or more lower.
`
`To confirm the above exemplary embodiment, por-
`tions of a digital laser transmitter have been fabricated
`and tested using a GaAs photodiode back—face monitor
`with a 1.3 pm multimode laser. The low frequency
`oscillator used was a 5 KHz square wave oscillator,
`pulse-width modulating a 1.1 Gb/s digital data signal at
`different modulation depths to produce data pulse
`width variations of 8, 25 and 80 ps. As shown in FIG. 4,
`the amplitude control signal (ACS, FIG. 1) was mea-
`sured and was found to be linear with light amplitude
`from the laser 16. Further. the entire transmitter has
`been simulated using a data rate of 10 Mbits/sec and a
`low frequency modulation rate of 100 KHz. The laser
`was simulated to shift its threshold current and L-I
`slope by approximately 17 and 22 percent, respectively,
`while operating “closed loop” under the control of the
`light amplitude control means 12 (FIG. 1). The simula-
`tions verify that the means 12 does control the light
`amplitude from the laser 16 with the different L-I slopes
`and threshold shifts.
`
`Exhibit LG—1015 Page 5
`
`Exhibit LG-1015 Page 5
`
`

`

`5
`Having described the preferred embodiment of this
`invention, it will now be apparent to one of skill in the
`art that other embodiments incorporating its concept
`may be used. Therefore, this invention should not be
`limited to the disclosed embodiment, but rather should
`be limited only by the spirit and scope of the appended
`claims.
`We claim:
`
`1. In a digital laser transmitter having a laser respon-
`sive to a laser driver and a back-face photodetector for
`converting at least part of the light from the laser into a
`monitor signal, a method for monitoring and controlling
`the amplitude of a digital laser light output signal,
`CHARACTERIZED BY THE STEPS OF:
`
`pulse width modulating a high bit rate digital data
`input signal with a low frequency modulation sig-
`nal to form a pulse width modulated data signal
`which is coupled to the laser driver;
`extracting the low frequency AC component of the
`monitor signal to form a laser amplitude control
`signal; and
`comparing said laser amplitude control signal to a
`reference signal and providing as an output an
`amplitude error signal indicative of any difference
`in value between the laser amplitude control and
`reference signals;
`wherein the laser driver includes a control input for
`varying the drive to the laser and the control input
`is responsive to the amplitude error signal.
`2. The method as defined in claim 1 further character-
`
`ized by the step of:
`extracting the low frequency AC component of the
`pulse width modulated data signal to form the
`reference signal.
`3. The method as defined in claim 2 further character-
`
`ized by the steps of:
`comparing the DC component of the reference signal
`to the DC component of the monitor signal and
`providing an amplitude control signal, indicative of
`the difference therebetween, as a bias control signal
`to the laser.
`
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`5,373,387
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`6
`4. The method as defined in claim 3, the extracting
`step forming the laser amplitude control signal being
`characterized by:
`filtering the monitor signal to pass the low frequency
`AC component thereof;
`demodulating the low frequency AC component;
`wherein the demodulated low frequency AC compo-
`nent is the amplitude control signal.
`5. The method as defined in claim 4, the extracting
`step forming the reference signal is characterized by:
`measuring the mark density of the pulse width modu-
`lated data signal to form a mark density signal;
`filtering the mark density signal to pass the low fre-
`quency AC component thereof;
`demodulating the low frequency AC component;
`wherein the demodulated low frequency AC compo-
`nent is the reference signal.
`6. The method as defined in claim 5 wherein the low
`frequency modulation signal is a square wave.
`7. The method as defined in claim 5 wherein the low
`frequency modulation signal is a sine wave.
`8. In a digital laser transmitter having a laser respon-
`sive to a laser driver and a back-face photodetector for
`converting at least part of the light from the laser into a
`monitor signal, a method for measuring the amplitude of
`a digital laser light output signal CHARACTERIZED
`BY THE STEPS OF:
`
`pulse width modulating a high bit rate digital data
`input signal with a low frequency modulation sig-
`nal to form a pulse width modulated data signal
`which is coupled to the laser driver;
`extracting the low frequency AC component of the
`monitor signal to form a laser amplitude control
`signal;
`wherein the laser amplitude control signal corre-
`sponds to the amplitude of the laser light output
`signal.
`9. The method as defined in claim 8, the extracting
`step forming the laser amplitude control signal being
`characterized by:
`filtering the monitor signal to pass the low frequency
`AC component thereof;
`demodulating the low frequency AC component;
`wherein the demodulated low frequency AC compo-
`nent is the laser amplitude control signal.
`*
`*
`*
`*
`*
`
`Exhibit LG—1015 Page 6
`
`Exhibit LG-1015 Page 6
`
`