U.S. patent application number 12/049055 was filed with the patent office on 2008-09-11 for analog external cavity laser.
Invention is credited to Vladimir Kupershmidt, Frans Kusnadi, John Major, Sabeur Siala.
Application Number | 20080219304 12/049055 |
Document ID | / |
Family ID | 35054384 |
Filed Date | 2008-09-11 |
United States Patent
Application |
20080219304 |
Kind Code |
A1 |
Kupershmidt; Vladimir ; et
al. |
September 11, 2008 |
ANALOG EXTERNAL CAVITY LASER
Abstract
The present invention relates to the analog external cavity
lasers (ECLs) including designs, materials, methods of
manufacturing and methods of use for such ECLs and packages for
such ECLs. Numerous criteria are presented that lead to improved
cost/performance for ECLs and for systems incorporating such
ECLs.
Inventors: |
Kupershmidt; Vladimir;
(Pleasanton, CA) ; Kusnadi; Frans; (San Jose,
CA) ; Major; John; (San Jose, CA) ; Siala;
Sabeur; (Sunnyvale, CA) |
Correspondence
Address: |
EMCORE CORPORATION
1600 EUBANK BLVD, S.E.
ALBUQUERQUE
NM
87123
US
|
Family ID: |
35054384 |
Appl. No.: |
12/049055 |
Filed: |
March 14, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11097745 |
Apr 1, 2005 |
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12049055 |
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60558927 |
Apr 2, 2004 |
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60562762 |
Apr 16, 2004 |
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60638679 |
Dec 23, 2004 |
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Current U.S.
Class: |
372/29.02 |
Current CPC
Class: |
H01S 5/0683 20130101;
H01S 3/1055 20130101; H01S 5/0427 20130101; H01S 3/1398 20130101;
H01S 5/4087 20130101; H01S 5/028 20130101; H01S 5/0687 20130101;
H01S 5/4062 20130101; H01S 2301/03 20130101; H01S 5/4012 20130101;
H01S 5/141 20130101; H01S 5/147 20130101; H01S 5/0078 20130101;
H01S 5/02438 20130101; H01S 5/02251 20210101; H01S 5/02325
20210101; H01S 5/0064 20130101; H01S 5/02216 20130101; H01S 5/06804
20130101; H01S 5/02415 20130101 |
Class at
Publication: |
372/29.02 |
International
Class: |
H01S 3/13 20060101
H01S003/13 |
Claims
1-18. (canceled)
19. An optical transmitter comprising: an external cavity laser for
generating an optical signal and transmitting the optical signal
over a dispersive fiber optic link; a first piezoelectric
transducer coupled to the external cavity laser; an electronic
circuit coupled to the piezoelectric transducer to change spectral
characteristics of the external cavity laser through changing
physical properties of the external cavity laser by applying a time
varying stress to the external cavity laser, thereby reducing an
effect of noise in a received signal arising from stimulated
Brillouin scattering (SBS) generated in the dispersive fiber optic
link.
20. The optical transmitter of claim 19 arranged so that the
optical signal is launched at 1550 nm.
21. The optical transmitter of claim 19 wherein the external cavity
laser comprises a semiconductor laser coupled to a fiber Bragg
grating (FBG), the optical transmitter arranged so that the time
varying stress is applied to the FBG.
22. The optical transmitter of claim 19 wherein the external cavity
laser comprises: a) a light source having a reflective back facet
and a transmissive front facet, said light source further
comprising a Fabry-Perot gain element with an active length between
approximately 300 micrometers and approximately 600 micrometers,
wherein said light source has a symmetrical far-field beam profile;
and b) a partially reflective feedback element forming a laser
cavity in cooperation with said reflective back facet.
23. The optical transmitter of claim 22 wherein the ratio of mode
spacing to the bandwidth of the feedback element is from
approximately 0.5 to approximately 1.3.
24. The optical transmitter of claim 19 wherein the first
piezoelectric transducer is attached to the external cavity
laser.
25. The optical transmitter of claim 19 wherein at least a portion
of the external cavity laser is mounted to a substrate and the
first piezoelectric transducer is attached to the substrate.
26. The optical transmitter of claim 19 comprising a second
piezoelectric transducer, wherein the first piezoelectric
transducer is attached to a first side of the external cavity laser
and the second piezoelectric transducer is attached to a second
side of the external cavity laser, wherein the first side and
second side are substantially opposite to each other.
27. The optical transmitter of claim 19 wherein the piezoelectric
transducer comprises a piezoelectric coating disposed on the
external cavity laser.
28. In an optical system having an optical transmission source
comprising a light source optically coupled with an in-line grating
to form a laser, a method of lessening effects of noise in a
received signal arising from stimulated Brillouin scattering (SBS)
generated in a dispersive fiber optic link optically coupled with
the laser, the method comprising: applying a time varying stress to
the in-line grating with a first piezoelectric transducer so as to
change spectral characteristics of the in-line grating.
29. The method of claim 28 wherein the time varying stress applied
to the in-line grating is a periodic stress.
30. The method of claim 28 wherein the spectral characteristics
include a refractive index of the grating.
31. The method of claim 28 wherein the in-line grating is a fiber
Bragg grating (FBG).
32. The method of claim 28 wherein the laser is a narrow band
laser.
33. The method of claim 28, wherein the time varying stress is
applied to the grating at a rate that is sufficient to
substantially lessen the effects of the SBS.
34. The method of claim 28 wherein the first piezoelectric
transducer is attached to the in-line grating.
35. The method of claim 28 wherein the optical system comprises a
second piezoelectric transducer, wherein the first piezoelectric
transducer is attached to a first side of the in-line grating and
the second piezoelectric transducer is attached to a second side of
the in-line grating, wherein the first side and second side are
substantially opposite to each other.
36. A system comprising: a dispersive fiber optic link; a laser
optically coupled with the dispersive fiber optic link, wherein the
laser includes a narrow band optical source and a fiber Bragg
grating (FBG) forming an output facet of the laser; and first
piezoelectric means for dithering the spectral response of the FBG
by applying a time varying stress to the FBG to reduce noise in a
received signal arising from stimulated Brillouin scattering (SBS)
generated in the dispersive optical fiber link.
37. The system of claim 36 wherein the narrow band optical source
has a symmetrical far-field beam profile.
38. The system of claim 36 wherein the first piezoelectric means is
attached to the FBG.
39. The system of claim 36 wherein the FBG is mounted to a
substrate and the piezoelectric means is attached to the
substrate.
40. The system of claim 36 comprising a second piezoelectric means,
wherein the first piezoelectric means is attached to a first side
of the FBG and the second piezoelectric means is attached to a
second side of the FBG, wherein the first side and second side are
substantially opposite to each other.
41. The system of claim 36 wherein the piezoelectric means
comprises a piezoelectric coating disposed on the FBG.
42. The system of claim 36 wherein the laser comprises a feedback
element and the ratio of mode spacing to the bandwidth of the
feedback element is from approximately 0.5 to approximately 1.3.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from provisional patent
application Ser. No. 60/558,927 filed Apr. 2, 2004, and provisional
patent application Ser. No. 60/562,762, filed Apr. 16, 2004, and
provisional patent application Ser. No. 60/638,679 filed Dec. 23,
2004, pursuant to one or more of 35 U.S.C. .sctn. 119, .sctn. 120,
.sctn. 365. The entire contents of all cited provisional patent
applications are incorporated herein by reference for all
purposes.
BACKGROUND OF THE INVENTION
[0002] 1. Field of Invention
[0003] This invention relates generally to the field of laser light
sources and, more particularly, to external cavity laser light
sources.
[0004] 2. Description of the Prior Art
[0005] Directly modulated distributed feedback (DFB) laser diodes
are widely used in many applications, including the transmission of
multiple channels of analog modulated signals as typically used for
analog broadcast, digital simulcast and narrowcast (QAM-format) in
cable television (CATV) and telecom networks for video, data and
voice-over-IP distribution. Other applications of DFB laser diodes
(or simply "DFBs" for economy of language) include the transmission
of radio frequency (RF) signals over optical fibers
("RF-over-fiber") in which RF signals are transmitted over a single
strand of optical fiber, typically employing the widely-deployed
standard single-mode fiber (SMF) for use at a wavelength of
approximately 1310 nm (1310 nanometers). These applications are by
way of illustration and not limitation as other applications for
DFB laser diodes exist and are being continuously developed.
[0006] For CATV and RF-over-fiber applications, the desired system
performance has been largely achieved at the wavelength of 1310 nm
due in important part to the use of lasers and other components
that generate and maintain acceptably low levels of analog
distortion. Such analog distortion is typically represented by
various numerical parameters such as composite second order
distortion (CSO or Inter-Modulation Distortion 2nd Order--IMD2 or
2nd order Intercept Point-IP2) and composite triple beat distortion
(CTB or IMD3 or IP3). The desired low levels of analog distortion
are typically achieved and maintained through a combination of low
distortion DFB lasers and very low (effectively zero) dispersion at
1310 nm in the SMF that is employed. However, there are large
potential benefits to be realized by building CATV systems, video
and data transmission systems, and RF-over-fiber systems to operate
using the wavelength band around 1550 nm. The potential benefits
arise in part from the availability of optical amplification means
(such as erbium-doped-fiber amplifiers, EDFA) and the ability to
take advantage of wavelength division multiplexing (WDM)
technology. However, widespread deployment of 1550 nm networks for
CATV and wireless distribution has been hampered by several
factors, including the chirp produced by direct-modulated DFB
lasers. Chirp adversely interacts with the non-zero dispersion in
standard SMFs around the 1550 nm band and the gain slope in typical
EDFAs to severely limit system performance as discussed, for
example, by E. Bergmann et al., "Dispersion-Induced Composite
Second-Order Distortion at 1.5 .mu.m," IEEE Photonics Tech. Lett.,
Vol. 3, No. 1, pp. 59-61 (January 1991).
[0007] In addition, the deployment of new and more efficient
modulation schemes has been delayed due to as yet unmet
requirements for higher performing, lower cost optical
transmitters. For instance, quadrature-phase-shift-keying (QPSK)
modulation techniques can potentially double the system's bit rate
of transmission without an increase in the bandwidth required of
the electrical components of the system. Also, QPSK modulation
techniques can potentially provide a more compact spectrum at the
same system bit rate than is possible with binary modulation.
However, utilizing QPSK techniques typically requires low cost
laser sources having narrower linewidth than typical DFBs that are
currently commercially available (see, for example, S. Norimatsu,
et al. "An 8 Gb/s QPSK Optical Homodyne Detection Experiment Using
External-Cavity Laser Diodes", IEEE Photonics Tech. Lett., Vol. 4,
No. 7, pp. 765-767, July 1992.
[0008] Among the major barriers to the wider use of
direct-modulated DFBs around the 1550 nm band is the high chirp and
the relatively high intrinsic distortion of the solitary DFB laser.
Such effects arise chiefly because of three physical phenomena (i)
spatial hole burning effect, (ii) leakage current, and (iii)
intrinsic nonlinear response.
[0009] Another limitation of direct-modulated DFBs operating near
the 1550 nm band is the relatively large linewidth, of the order of
1 MHz. Such a linewidth limits the Relative Intensity Noise of the
laser (RIN) which in turn limits the transmission carrier-to-noise
ratio (CNR). Such effects lead to limited transmission performance
under high channel loadings and for longer transmission
distances.
[0010] Multiple approaches have been proposed to address these
issues, but they generally have technical and/or cost drawbacks.
These solutions include (a) The use of dispersion compensators,
which are usually expensive, complex, and cumbersome, typically
requiring customization of each fiber span. (b) The use of
externally-modulated continuous wave (CW) DFBs coupled to a
Mach-Zehnder modulator. This combination can exhibit practically
zero chirp in some circumstances, but the high cost makes this
solution over-engineered and too expensive for all but a few
specialized, typically low volume, applications. (c) The use of
electro-absorption modulated lasers (EMLs) which suffer from narrow
operating margins, the requirement for complex predistortion
circuitry to reduce the relatively large intrinsic harmonic
distortion, and the low carrier-to-noise ratio (CNR) (due to the
relatively low optical output power).
[0011] The high cost of the optical transmitter solutions mentioned
above has proven to be an important factor leading to the
development of a number of techniques that enhance the performance
of direct-modulated DFBs. These approaches generally include one or
a combination of the following techniques: (i) Electronic
predistortion techniques to correct for degradation in second order
distortion (for example, see U.S. Pat. Nos. 5,436,749; 4,992,754;
5,227,736) and to correct for third order distortion (for example,
U.S. Pat. No. 5,172,068). The main drawbacks of such techniques
include the added expense and the need for customization in
manufacturing to accommodate different levels of distortion
correction. (ii) Optical injection locking techniques, whereby
light from a master laser is injected into a slave laser whose
output is then locked to the master laser. However, this method has
had very limited commercial success because of its high cost and
complexity (for example, see H. Sung et al., "Dependence of
Semiconductor Laser Intermodulation Distortions on Fiber Length and
its Reduction by Optical Injection Locking, "Conference
Proceedings, Paper WE2 10, p. 186-200). (iii) Optical linearization
techniques whereby a DFB is optically enhanced using an external
optical element (for example, see U.S. Pat. No. 6,538,789).
[0012] Thus, a need exists in the art for an improved distributed
Bragg reflector laser diode having low analog distortion and/or
that can be made available at relatively low cost.
SUMMARY OF THE INVENTION
[0013] The present invention relates to the design, packaging, and
manufacturing of direct-modulated analog external cavity lasers
(ECLs) having improved cost-performance ratio for transmission of
analog and semi-analog (such as quadrature amplitude modulated
(QAM)) signals, particularly for broadcast, digital simulcast and
narrowcast (QAM) applications.
[0014] Furthermore, some embodiments of the present invention
relate to the design of direct-modulated analog ECLs with
simultaneously (i) controlling chirp (from extremely low to high in
magnitude); (ii) low intermodulation distortions (second- and third
orders); and low Relative Intensity Noise (RIN) which provide high
performance and low cost devices, methods and/or systems for
transmission of analog signals in the 1550 wavelength range.
[0015] The present invention further relates to and includes
packaging design and packaging criteria (particularly 14-pin
butterfly packaging common in the industry) and with the external
laser cavity implemented using a fiber Bragg grating element
terminated with an integrating high coupling lens and light source
(Fabry-Perot (FP) chip), all mounted on the same solid substrate.
Such an approach provides long-term package stability that is
particularly advantageous in analog transmission systems.
[0016] The present invention further relates to and includes
designs and methods for increasing the coupling efficiency within
the cavity between the FP chip and the grating element while
reducing unwanted reflectivity within the cavity of the analog
ECL.
[0017] The present invention further relates to methods for
reducing the amount of reflected light coupling back into cavity of
the analog ECL. Suppressing reflected optical energy from coupling
back into the cavity is paramount to the performance of the analog
ECL in transmission systems. Methods for achieving high level of
suppression of reflective light include the incorporation of an
in-line optical isolator in the pigtail of the analog ECL and/or
the appropriate design of the reflectivity of the reflective
surface of the external cavity.
[0018] The present invention relates further to the enhancement and
use of the so-called distortion dip in analog ECLs by properly
designing the reflective external reflective element and by
appropriate temperature control of the ECL.
[0019] The present invention relates further to the design and
implementation of both intra-cavity and extra-cavity methods for
suppressing Stimulated Brillouin Scattering (SBS) which, if not
adequately reduced, can severely limit the amount of optical power
that can be launched into the fiber.
[0020] The present invention relates further to the design and
implementation of techniques for tuning and stabilizing the
emission wavelength of the analog ECL's output power to be within
industry standards for dense wavelength division multiplexed (DWDM)
systems, without sacrificing desirable distortion, chirp, and RIN
properties of the analog ECL.
[0021] The present invention relates further to methods for
reducing the so-called frequency tilt, which is manifested by
higher level of before-link and after-link distortion affecting the
higher frequency channels launched into the fiber.
[0022] The present invention relates further to methods for
designing analog ECLs so as to enable the successful incorporation
of known predistortion and Electronic Dispersion Compensation (EDC)
technologies to further improve the distortion and reach
performance of transmitters utilizing these analog ECLs.
[0023] The present invention relates further to methods for
fabricating analog ECLs using Bragg gratings created inside the
cores of various optical fibers, including standard non-dispersion
shifted single mode fiber (such as Corning SMF-28),
dispersion-shifted single mode fiber, polarization-maintaining
single mode fiber, and graded and step-index multimode fiber.
[0024] These and other advantages apparent to those skilled in the
art are achieved in accordance with various embodiments of the
present invention as described in detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The drawings herein are not to scale and the depictions of
relative sizes and scale of components within a drawing and between
drawings are schematic and also not to scale.
[0026] To facilitate understanding, identical reference numerals
have been used, where possible, to designate identical elements
that are common to the figures.
[0027] The techniques of the present invention can readily be
understood by considering the following detailed description in
conjunction with the following drawings, in which;
[0028] FIG. 1 depicts a typical ECL configuration in block diagram
form.
[0029] FIG. 2 is a schematic depiction of an ECL constructed with:
(a) a fiber Bragg grating, and (b) a Bragg grating written in a
waveguide on a PLC.
[0030] FIG. 3 is a schematic depiction of an external cavity
laser.
[0031] FIG. 4 is a graphical depiction of ECL emission wavelength
vs TEC temperature denoting mode hop regions.
[0032] FIG. 5 is a graphical depiction of the spectral response of
an ECL near a mode hop region, also depicting a jump in peak
wavelength as the temperature is changed from 20.2 deg. C. to 19.8
deg. C.
[0033] FIG. 6 is a graphical depiction of slope efficiency vs bias
current, that is the first derivative of the L-I curve of the
ECL.
[0034] FIG. 7 is a schematic depiction of an ECL having the chip
and grating assembled on the same substrate to ensure improved
stability. Two attachment points can maintain the FBG section under
a controlled stress (compression or tension) in order to achieve
improved performance under direct analog modulation.
[0035] FIG. 8 is a graphical depiction of a distortion profile of
one ECL embodiment including a distortion dip.
[0036] FIG. 9 is a graphical depiction of second order distortion
(e.g. IMD2) and chirp as functions of temperature range between
mode hops for one embodiment of FBG design for an analog ECL. The
figure depicts the ability to select a desired chirp level by the
appropriate selection of the operating temperature, that is the TEC
temperature. Experimental results are depicted.
[0037] FIG. 10 is a graphical depiction of second order distortion
(e.g., IMD2) (left vertical axis) and chirp (right vertical axis)
as functions of the temperature range between mode hops for an
embodiment of the FBG design (the FBG with knee profile) for an
analog ECL for the example in which the one-sided butterfly profile
(distortion dip) is aligned with maximum chirp. The figure depicts
results of numerical simulation.
[0038] FIG. 11 is a graphical depiction of FBG reflectivity profile
as a function of wavelength span. The "knee" spectral profile
facilitates the alignment of the OSBDP (distortion dip) with the
high chirp region as depicted in FIG. 10.
[0039] FIG. 12 are schematic depictions of ECL multi-wavelength
arrays including: (a) multiple distinct optical output beams; and,
(b) multiple output beams multiplexed into a single strand of fiber
or onto one waveguide on a PLC.
[0040] FIG. 13 is a schematic depiction of SBS suppression applied
to analog ECL using an External Phase Modulator as a Bragg grating
written inside a polarization-maintaining fiber (PMF) in the
embodiment depicted in this figure.
[0041] FIG. 14 is a schematic depiction of an analog ECL with an
in-line optical isolator in the pigtail.
[0042] FIG. 15 depicts CSO tilt of analog ECL and its improvement
using wider bandwidth FBG. CSO tilt is defined here as the
difference in CSO at a medium frequency of 314.5 MHz and at a high
frequency of 547.5 MHz.
[0043] FIG. 16 depicts CSO tilt of analog ECL at high frequency
(547.5 MHz) and its improvement using chirped FBG.
[0044] FIG. 17 is a schematic depiction of an analog ECL with an
integrated optical isolator and optical filter spliced into the
pigtail of the analog ECL. The optical filter reduces spontaneous
emission.
[0045] FIG. 18 is a graphical depiction of attenuation and
dispersion vs. wavelength of standard single-mode fiber and
dispersion shifted fiber.
DETAILED DESCRIPTION OF THE INVENTION
[0046] After considering the following description, those skilled
in the art will clearly realize that the teachings of the invention
can be readily utilized for the design, fabrication, packaging
and/or use of external cavity lasers (ECLs), particularly analog
ECLs
[0047] The technologies described herein relate to analog external
cavity lasers including techniques for designing, packaging and
improving the performance of ECLs for use in analog and CATV fiber
optic communications systems. This field of application is by way
of example and not limitation since systems, techniques, processes,
devices and materials described herein can find applications in
other fields as well. The analog ECLs described herein are
direct-modulated laser sources providing significant advantages
compared with other devices and systems for analog optical
transmission. These advantages include higher performance in terms
of distortion and chirp (for example), at lower costs and with
improved design margins.
[0048] A typical ECL includes a laser diode chip 200 having one
highly reflective face, typically coated with high reflectivity
(HR) film 202, and the opposite face transmissive to the light
produced by the laser diode but typically coated with an
anti-reflective (AR) coating 201. In combination with an optical
feedback element 103 an optical transfer function, F(.lamda.) is
formed as depicted in FIG. 1. The optical transfer function,
F(.lamda.), is usually a grating, sometimes referred to as a Bragg
grating, and behaves effectively as both a mirror and a filter,
reflecting some light incident thereon, 104, back to the laser
diode chip 200 while transmitting another portion of the light 105
to produce the output of the laser. The laser light produced by the
ECL bounces back and forth between the highly reflective (HR) side
of the laser diode 202 (that is, the side opposite the AR-coating)
and the grating 103, forming thereby the laser cavity.
[0049] Pursuant to some embodiments, the present invention includes
an external cavity laser (ECL) in which the grating is created
inside an optical fiber, (and thus often called a fiber Bragg
grating (FBG)), depicted as 203 illustrated in FIG. 2(a). In other
embodiments, the grating can be created inside planar waveguides on
planar lightwave circuits (PLC) as illustrated as 203 in FIG. 2(b).
In yet other ECL embodiments, the optical transfer function can
comprise a bulk grating, an acousto-optic filter, an etalon-type
thick or thin film filter, built in glass, semiconductor or other
material. In other embodiments, the grating can be created in
semiconductor material (such as indium phosphide, InP). Additional
ECL embodiments include the use of a tunable grating wherein the
optical transfer function can be tuned using such techniques as
spatial movement of the grating (e.g., using
micro-electro-mechanical systems (MEMS)) or the application of an
RF signal to tune an acousto-optic based optical transfer function,
or the application of stress to tune a thin- or thick-film based
tunable optical filter, or the use of a tunable grating written in
or on a semiconductor chip, among other techniques. Such
embodiments may, but need not, include a beam-shaping element
between the light-generating chip and the external reflective
surface.
[0050] The analog ECLs described herein are typically described in
connection with an external cavity employing a fiber Bragg grating
(FBG) as reflective element. However, this is by way of
illustration and not limitation since the techniques described
herein can readily be utilized in connection with external cavities
making use of other reflective elements, as apparent to those with
ordinary skills in the art.
[0051] Laser source modules are typically specified for use in
practical communication systems (or other applications) in which
the laser module can be subject to wide ambient temperature ranges
(e.g., -20.degree. C. to +75.degree. C.). Since the operating
wavelength is typically a sensitive function of temperature, it is
known practice to provide temperature sensing and temperature
controlling devices in the module package to maintain a narrower
range of operating temperatures (say on the order of 15-35.degree.
C.) within the module package environment. A widely used cooling
device is a thermoelectric cooler (TEC).
[0052] Embodiments of the present invention include analog external
cavity lasers advantageously designed and built according to
several criteria. The development and employment of such criteria
comprise aspects of the present invention, The resulting analog
ECLs achieve several advantages in comparison with typical prior
art devices.
[0053] The packaging of the ECL is an important component of the
overall ECL system. Some embodiments of the present invention
include design criteria for the ECL packaging. By way of
illustration and not limitation, the embodiments described herein
relate to an FBG-based analog ECL including a lens integrated at
the tip of the fiber containing the FBG.
[0054] Analog ECLs pursuant to some embodiments of the present
invention include a combination of favorable choices of Fabry-Perot
(FP) gain element (or laser diode chip, 200), external cavity
design and proper gap adjustment between the optical component
containing the grating and the laser diode chip, wherein the gap
adjustment affects the optical coupling. Diffraction losses and
advantageously detuned loading conditions are also considered in
ECL designs pursuant to some embodiments of the present invention.
In general, advantageous levels of CSO distortion may be determined
by the FP chip performance or by external cavity design having
particular amplitude and phase profiles. Therefore, design criteria
developed and used pursuant to some embodiments of the present
invention typically include certain constraints on the FP chip.
[0055] Design and packaging criteria have been developed pursuant
to some embodiments of the present invention based on both
experimentation and on a computer model (FIG. 3), in 300 to address
packaging sensitivity and coupling of the FP chip to the component
containing the grating 301. Typical criteria include the
following:
[0056] The Fabry-Perot gain element advantageously provides a
constraint on the active length, (La in FIG. 3) between
approximately 300 .mu.m and approximately 600 .mu.m
(.mu.m=micrometer=10.sup.-6 meter)
[0057] The lower limit on the length of the FP chip, La, is
determined chiefly by the linearity of the plot of the optical
output power of the ECL versus the laser's injection current
(usually referred to as the L-I curve) and the adiabatic chirp
response of the Fabry-Perot chip in the frequency range of interest
(e.g., in the range up to approximately 10-3,000 MHz for CATV and
RF-over-fiber applications. This relationship tends to be followed
since adiabatic chirp response is typically a function of 1/La. The
upper limit of La is chiefly dictated by the temperature span
between mode hops which is directly related to FP chip length.
[0058] Temperature span between mode hops depends upon many
factors, including the length of the FP chip and, less importantly,
the relative position of the grating element with respect to the
gain chip. As the ECL TEC temperature is changed, mode hops are
manifested by an increase in the ECL emission wavelength occurring
suddenly after a linear region, in an approximately step-increase
fashion as depicted in FIG. 4. The mode hop region exhibits a
hysteresis effect, whereby different traces for wavelength versus
TEC temperature are obtained depending on whether the temperature
is swept in the upward or downward direction. For example, an
FBG-based ECL built with an InGaAsP/InP FP chip with an active
length of about 500 .mu.m exhibits a temperature interval of about
8.degree. C. between mode hops as shown in FIG. 4. Operating the
ECL at or close to the mode hop region is typically disadvantageous
because of potential instability of the output of the analog ECL.
It is prudent to include a "safety margin" of the temperature
operating point away from positions of mode hops of about 1.degree.
C. or more. Thus, the available space/margin for ECL analog
packaging with a 500 .mu.m chip length is thus reduced to
5-6.degree. C., and for a longer chip, to an even smaller margin.
It is therefore an aspect of some embodiments of the present
invention to select the length of the gain element chip and of the
overall cavity so as to allow an operating temperature window of at
least a few deg. C. between mode hops.
[0059] The high reflectivity (back facet) coating on the
Fabry-Perot gain element chip typically has a reflectivity above
about 75-95%, and an AR coating (front facet) with reflectivity
less than about 0.05%. Some embodiments of the present invention
have the waveguide positioned at an angle (typically about 6-10
deg) from the front facet of the gain element chip thereby relaxing
the AR coating specifications for the front facet.
[0060] Constraints on the AR coating are typically dictated by the
desired level of distortion and available temperature margin, which
depend on the magnitude of the AR coating. The temperature
separation between mode hops for a 500 .mu.m chip is only about
8.degree. C. and the temperature width of the region of mode hops
is very small, approximately 0.15-0.2 deg. C. for an AR less than
about 0.05%. If the AR coating is not of sufficiently high quality,
for example >0.07%, then the temperature width of the mode hop
region increases to about 0.5-0.7 deg. C., caused in part by the
increased competition between lasing modes within the hysteresis
region. As the ECL temperature is scanned from a stable region
through a mode hop region, the modal behavior of the ECL typically
changes from single mode with unacceptably low side-mode
suppression ratio (SMSR) to multi spectral modes as illustrated in
FIG. 5. When the AR coating is less than about 0.05% reflectivity,
single mode operation with SMSR larger than about 35 dB could be
reached within .+-.0.2 degree from the boundaries of the mode
hop.
[0061] External cavity design typically requires a high-performance
FP, particularly low leakage current, high slope efficiency, small
alpha (.alpha.)-factor (less than about 4) and magnitude of
adiabatic chirp. One of the indications of a high-efficiency FP
chip with low leakage current is a very small change in the
differential resistance measured at high bias current compared with
its threshold value. Additional requirements on the FP chip arise
from the fact that analog ECLs typically require high optical
coupling efficiency between the chip and the reflective element of
the external cavity. Therefore, it is advantageous to use a type of
buried-heterostructure (BH) with a symmetrical far-field beam
profile (for example, having beam ellipticity less than about 1.2).
Such far-field beam profiles allow the use of fiber-integrated
lenses in an FBG-based analog ECL with high coupling efficiency
(for example, around 56-70%).
[0062] Parameters such as those enumerated herein typically allow
practical packaging of ECLs with advantageous amounts of available
power.
[0063] Another aspect of some embodiments of the present invention
relates to achieving low distortion analog ECLs by means of a
cavity design that has a linewidth enhancement factor
(.alpha.-factor) at the wavelength corresponding to the gain peak
of the FP chip less than about 4.
[0064] In addition to the limited magnitude of the .alpha.-factor
related to the FP chirp at the gain peak, the distortion
performance of analog ECLs pursuant to some embodiments of the
present invention benefits from so-called blue detuning. Blue
detuning is the amount of offset between the ECLs lasing wavelength
from the lower wavelength side (blue side) of the gain peak of the
FP chip. Blue detuning leads to a reduction of the .alpha.-factor
with wavelength, which is advantageous for achieving lower
distortion in analog ECLs. Typical amount of blue detuning is
within a range of about 20-50 nm from the gain peak. To suppress
competition from lasing at the peak of FP gain, it is typically
necessary to use a high-performance AR coating, with typical AR
coating less than about 0.05%
[0065] To reduce levels of distortion, the design of the external
cavity should avoid coupling the optical signal from the coated
(HR/AR) FP chip to a high-Q resonator. That is, keeping the
reflectivity of the grating (R) below about 25% typically leads to
favorable cavity design.
[0066] Concurrently, if grating reflectivity becomes very small,
for example R -5%, then the cavity typically becomes unstable as
mode competition increases and packaging of such a cavity becomes
extremely difficult. Thus, a favorable range for R is approximately
5%<R<25%.
[0067] It is advantageous to increase and strive to maximize the
coupling efficiency (C) of the light from the coated (HR/AR) FP
chip into the grating element, such as a single mode fiber for the
case of FBG-based ECL, or a planar waveguide for the case of
PLC-based ECL, or any combination of FP chip, coupling optics and
external resonator which constitute an external cavity. Maximizing
coupling efficiency can be approached by improving the optical beam
quality exiting the FP chip and by proper design of the coupling
optics as depicted in FIG. 1.
[0068] An approximate lower limit on the minimum coupled power is
typically dictated by two factors: Constraints on the available
power for the analog laser (typically >7 dBm) and the criteria
described herein for the coupling efficiency C, which leads to the
effective ECL "front facet" reflectivity
R.sub.eff=C.sup.2*R.sub.max. If C.about.0.7 (for typical practical
implementations), and R=15%, then R.sub.eff=7.4% which indicates a
weak cavity and high FP mode competition. These considerations
again lead us to derive certain conditions on AR coating
reflectivity minimization. The upper constraint on coupling
efficiency (Cmax) is dictated by the practicality of the coupling
optics and sensitivity to the alignment.
[0069] Integrated lens design is an important embodiment that
provides mechanical and detuned loading stability in the ECL
package.
[0070] Design of the external cavity (external FBG or grating on
PLC substrate) also advantageously follows certain criteria,
pursuant to some embodiments of the present invention. Among these
is the criterion that, in a single mode operation, SMSR >35
dB.
[0071] Other criteria include that the ratio of mode spacing
.DELTA..lamda.sp (cold cavity when the ECL bias current=Ibias
.about.Ith, near the lasing threshold) to the bandwidth (BW) of
Bragg grating, is advantageously in the range
0.5<.DELTA..lamda.sp/BW<1.3.
[0072] Such criteria as described herein tend to avoid mode
competition by producing a high level of discrimination between a
dominant mode and the secondary modes (that is, generate single
mode operation with SMSR >35 dB). At the same time, such
criteria can be used as a practical optimization parameter during
the assembly of the analog ECL cavity.
[0073] Relationships between the passive length of the ECL cavity
(Lp in FIG. 3) and the so-called effective length, Leff, of the ECL
grating, permit the chirp of the external cavity to be effectively
tailored depending on the application. Leff for a so-called weak
grating (R less than about 15%) is a function of the grating
bandwidth and the reflectivity profile.
[0074] For both an external FBG or a Bragg grating on a
PLC-platform, the side-lobe suppression of the grating (SLS)
advantageously exceeds about 15 dB in order to provide SMSR of the
ECL >35 dB within 1.degree. C. temperature interval from mode
hop temperature Th.
[0075] One example of FBG design pursuant to some embodiments of
the present invention is an apodized grating as a practical
implementation. Criteria related to the chirp requirements still
allow the designer of analog ECLs a choice of certain relationships
between passive length and ECL effective length.
[0076] Another aspect of the present invention relates to the
exploitation and tailoring of the detuned loading in ECL in order
to reduce the second and/or third order distortion under direct
analog modulation while simultaneously achieving a desired chirp
level. In ECL designs pursuant to some embodiments of the present
invention, the detuned loading parameter
X.sub.DL=(.lamda.-.lamda.p)/BW (where .lamda.p is peak wavelength
of the grating profile and .lamda. is the lasing wavelength of the
ECL) plays an important role in the mechanism of ECL distortion
(both 2nd order, CSO, and 3.sup.rd order, CTB). In the assembly of
the external cavity of ECLs, such parameters are typically
sensitive functions of the gap between the AR-coated facet of the
FP chip and the effective reflectivity plane of the grating.
[0077] Modeling and analysis of the distortion mechanism and
practical experiments show that for essentially any type of
reflectivity profile of the optical element F(.lamda.), a unique
signature for the dependence of slope efficiency (which is the
first order derivative of the L-I curve, shown in FIG. 6) on bias
current is obtained, which provides minimum distortion across the
temperature range between mode hop positions. Such a signature is
attributed to the fact that the threshold current of the ECL and
the effective reflectivity of the grating depend on a detuned
loading effect, which varies with bias current and temperature. A
certain detuned loading condition is advantageously employed in
order to achieve reduced distortion and this detuned loading
condition gives a certain signature shape of the slope efficiency
curve. Reduced and substantially minimum distortion is typically
achieved when the slope of the efficiency curve peaks around the
threshold current then decreases as the bias current is increased,
to reach a minimum substantially at the target optical output
power, and then increases again. One aspect of some embodiments of
the present invention relates to the use of such signatures in the
design, construction and assembly of analog ECLs.
[0078] This signature of the slope efficiency curve is typically
sensitive to the detuned loading. Thus, to ensure the mechanical
stability of the ECL across environmental changes, the chip and the
grating of an FBG-based ECL are advantageously assembled on the
same substrate, as illustrated in FIG. 7.
[0079] One aspect of some embodiments of the present invention
relates to actively measuring and monitoring the second order
distortion (CSO or IMD2 or IP2) as part of the assembly process of
the analog ECL while the external cavity is being aligned. When a
desired level of distortion is achieved, the optical feedback
element (F(.lamda.)) is fixed in place. For example, in case of
FBG-based ECL, the FBG is aligned then fixed in place using the
attachment points as illustrated in FIG. 7. This technique
typically results in improved manufacturing yields and improved
uniformity from one ECL device to another. In addition, monitoring
the distortion and/or chirp during assembly enables low-cost
customization of the device if needed.
[0080] With respect to the second order distortion, analog ECL has
a unique feature consisting of a region of ECL operating
temperature where 2nd and 3rd order distortion is small at all
carrier frequencies. This feature is called "one-sided butterfly
distortion profile" (OSBDP) or "distortion dip." This distortion
dip typically occurs within a narrow operating temperature range of
.DELTA.T approximately 1-2 deg. C., usually occurring close to the
mode hop position, as shown in FIG. 8. Analog ECLs pursuant to some
embodiments of the present invention typically have CSO that is
comparable or better than that offered by DFB CSO performance at
any operating temperature, but at significantly lower chirp than
DFBs. This makes analog ECL lasers very attractive for broadband
and narrowband analog applications (CATV, QAM, RF-over-fiber, among
others) and may result in dramatically increased transmission
distances and expanded applications such as fiber-to-home/premises
(FTTH/P).
[0081] Another aspect of some embodiments of the present invention
relates to tailoring and tuning the chirp in such a way as to
provide an improved, substantially optimum chirp level at the
desired distortion performance, where the chirp is high enough to
suppress SBS but not too large so as to limit the maximum reach of
the transmitted analog signal. Having an ECL with low level of
distortion does not automatically guarantee adequate signal
performance over the analog fiber link, because low chirp interacts
with nonlinear effects in typical optical fibers to make SBS and
so-called double Raleigh scattering (interferometric noise) the
limiting factors for link performance. Furthermore, existing
methods of SBS suppression using RF dithering, which have been
developed for DFBs and externally modulated sources, may be
inadequate for suppressing SBS in ECL-based transmitters. One
approach to addressing the SBS issue with ECL-based transmitters is
to increase the chirp level while maintaining low distortion. One
embodiment suitable for achieving relatively high chirp analog ECLs
while maintaining low distortion and low RIN is to shorten the
cavity length by reducing the passive section Lp in FIG. 3.
[0082] FIG. 9 (experimental results) and FIG. 10 (simulated
computer model results) show two different designs of an analog ECL
in which a certain level of chirp can be selected while maintaining
low distortion (both 2.sup.nd and 3.sup.rd order) by operating the
ECL at the properly chosen temperature. This invention also relates
to exploiting and designing analog ECLs with (i) tunable chirp and,
(ii) increased levels of chirp. Tunable chirp allows the user to
"dial-in" the chirp level more appropriate for his application. To
achieve relatively high chirp (>35 MHz/mA) in analog ECLs while
maintaining low distortion, the grating element of the ECL
(including but not limited to FBG, waveguide grating in PLC-based
ECL, bulk grating, acousto-optic based grating) is designed
according to one of the following guidelines: (i) to be a chirped
grating; (ii) a grating with a spectral profile referred to as a
"knee" spectral profile as depicted in FIG. 11; (iii) a grating
with a spectral profile in which chirp is introduced by external
means such as gradients created by stress.
[0083] Electronic dithering techniques can also be used with ECLs
instead of, or in addition to, relatively high levels of chirp. One
technique is to apply to the analog ECL input triangular-shaped
pulses or a set of tones at relatively high frequency whereby the
total chirp is dominated mainly by transient components.
[0084] Another aspect of some embodiments of the present invention
relates to FBG-based analog ECLs whereby the FBG is assembled and
fixed in place under a mechanical stress (either under tension or
compression) in order to improve the performance of the ECL for
direct analog modulation, and/or to tune the emission wavelength to
be on the grid specified by the standardization bodies for
wavelength division multiplexing (WDM) systems. Mechanical stress
applied to the FBG is used to tailor the distortion, the chirp,
and/or the emission wavelength of the analog ECL. As an example, in
FIG. 7 the FBG section is maintained under controlled stress by
using two attachment joints (such as those resulting from
soldering, laser welding, or epoxy).
[0085] Another aspect of some embodiments of the present invention
relates to design and implementation of analog ECLs so as to have
their emission wavelengths centered on, or locked to, or adjacent
to, the industry-standardized ITU wavelength grid so that said ECLs
can be used in wavelength division multiplexed (WDM) systems
whereby multiple independent optical beams at different
wavelengths, each carrying different information are launched into
one strand of optical fiber. The grating element of typical analog
ECLs as described herein can be designed to have a peak wavelength
such that, after proper adjustment of ECL TEC temperature for
improved chirp and distortion, the emission wavelength of the ECL
remains within the ITU wavelength grid tolerances.
[0086] Another aspect of the invention relates to design and
implementation of an array of analog ECLs using monolithic and/or
hybrid integration. Each ECL element in the array emits at a
predetermined wavelength and can be independently modulated. The
elements in the array may, but need not, share the same temperature
controller. A typical embodiment of such an analog ECL array is
illustrated in FIG. 12. Examples for implementing such an ECL array
include the use of an FBG array or grating in a waveguide array on
a PLC. The multi-wavelength output from the array is passed through
an optical multiplexer in FIG. 2(b).
[0087] Another aspect of some embodiments of the present invention
relates to FBG-based analog ECLs whereby the FBG is created inside
a polarization-maintaining fiber (PMF). Such an embodiment results
in direct analog modulated ECLs having lower noise than those built
with standard single mode fiber. In addition, ECLs built with PMF
can be used as high quality low-noise CW lasers sources in an
externally modulated analog transmitter. In addition to controlled
polarization, ECLs built with PMF provides simultaneously narrow
linewidth, low relative intensity noise (RIN), and high optical
output power, which can then be coupled into the external
modulation means.
[0088] Another aspect of some embodiments of the present invention
relates to direct analog modulated ECLs fabricated using a Bragg
grating inside a PMF fiber pigtail which is interfaced with a phase
modulator (such as a lithium niobate modulator), as widely used in
prior art to suppress SBS in analog systems. This is depicted in
FIG. 13.
[0089] The present invention further relates to and includes
packaging design and packaging criteria (particularly 14-pin
butterfly packaging common in the industry) and with the external
laser cavity implemented using a fiber Bragg grating element
terminated with an integrating high coupling lens and light source
(Fabry-Perot (FP) chip), all mounted on the same solid substrate as
illustrated in FIG. 7. Such an approach provides long-term package
stability that is particularly advantageous in analog transmission
systems.
[0090] The present invention further relates to and includes
designs and methods for increasing the coupling efficiency within
the cavity between the FP chip and the grating element while
reducing unwanted reflectivity within the cavity of the analog ECL.
One embodiment for FBG-based analog ECLs is the use a microlens
fabricated at the tip of the fiber containing the grating.
Embodiments of such fiber lens include a tapered hemispherically
shaped lens and a hyperbolic shaped lens.
[0091] The present invention further relates to methods for
reducing the amount of reflected light coupling back into cavity of
the analog ECL. Suppressing reflected optical energy from coupling
back into the cavity is paramount to the performance of the analog
ECL in transmission systems. Such methods for achieving high level
of suppression of reflected light include the incorporation of an
in-line optical isolator in the pigtail as depicted in FIG. 14.
[0092] The present invention relates further to the enhancement and
use of the distortion dip in analog ECLs by properly designing the
reflective external reflective element and the temperature control
of the ECL.
[0093] The present invention relates further to the design and
implementation of intracavity methods for suppressing Stimulated
Brillouin Scattering (SBS) which, if not suppressed adequately, can
severely limit the amount of optical power that can be launched
into the fiber. Intracavity SBS suppression in analog ECLs is based
on modulating the effective optical path length of light inside the
cavity. Some embodiments of intracavity SBS suppression in analog
ECLs include (i) applying a modulated electrical signal to a
piezoelectric transducer or an electrostatic element placed
adjacent or onto the reflective element (such as FBG); (ii)
modulating the position of the reflective elements of the external
cavity (iii) superimposing a dithering electrical signal on the
gain element chip of the analog ECL.
[0094] The present analog ECL relates further to methods for
reducing the so-called frequency tilt, which is manifested by
higher level of before- and after-link distortion affecting the
higher frequency channels launched into the fiber. A method for
improving both the overall distortion and frequency tilt of analog
ECL is to reduce the dispersive nature of the grating element used
as the reflective element in the external cavity. Embodiments of
this invention for grating-based ECLs include the use of one or a
combination of one or more of the following techniques: (i) Design
the grating with a relatively large bandwidth (for example, a FBG
with optical profile bandwidth in the range of about 220-450 pm
where pm=picometer=10.sup.-12 meter) at full width at half maximum
(FWHM)), as seen from the experimental data presented in FIG. 15;
(ii) Use non-apodized grating as the reflective element of the
external cavity; (iii) Use chirped FBG with appropriate gradient
direction so as to cancel dispersion effects in the grating, as
shown by the experimental data presented in FIG. 16.
[0095] The present invention relates further to methods for
improving the design of analog ECLs so as to enable the successful
incorporation of predistortion and Electronic Dispersion
Compensation (EDC) technologies widely described in the prior art
to further improve the distortion and reach performance of
transmitters utilizing these analog ECLs.
[0096] The present invention relates further to methods for
building analog ECLs using Bragg gratings created inside the core
of various optical fibers including standard non-dispersion shifted
single mode fibers (such as Corning SMF-28), dispersion-shifted
single mode fibers, polarization-maintaining single mode fibers,
and graded and step-index multimode fibers.
[0097] The present invention further relates to reducing the amount
of spontaneous emission noise from the output beam emitted from the
analog ECL. Suppressing the spontaneous emission improves the
performance of the transmission especially when cascaded optical
amplifiers are present in the link. One embodiment involves the
splicing an in-line optical filter (such as a thin film filter)
into the analog ECL pigtail. Another embodiment is to integrate the
optical filter in the isolator used in the pigtail of the analog
ECL as depicted in FIG. 17.
[0098] Another aspect of some embodiments of the present invention
relates to direct analog modulated ECLs (including but not limited
to ECLs based on FBG, grating in PLC waveguide, bulk grating, and
acousto-optic grating) whereby the emission wavelength is in the
1310 nm wavelength range. Direct analog-modulated 1310 nm ECLs
provide benefits in both cases for the transmission medium:
dispersion-shifted fiber and standard non-dispersion shifted fiber.
In the case of dispersion-shifted fiber, the low chirp
characteristic of ECL improves reach capability and lowers link
distortion by reducing the interaction of laser chirp with the
non-zero dispersion in the fiber. This is similar to the benefit
obtained when 1550 nm ECL are used in standard SMF.
[0099] At the 1310 nm wavelength range in standard (non-dispersion
shifted) single mode fiber, the fiber dispersion is practically
zero as shown in FIG. 18. Thus chirp in the laser is not only
acceptable, but also desirable in order to reduce the nonlinear
effects introduced by the fiber at high levels of launch power.
Direct analog modulated ECLs provide enhanced intrinsic distortion
performance, but dithering methods (including but not limited to
electrical, and mechanical techniques) are typically necessary to
counteract the effect of low chirp (stimulated Brillouin
scattering--SBS and Rayleigh back-scattering) on analog
transmission when high optical power is launched into SMF.
[0100] Another method for reducing the nonlinear effects in fibers
at 1310 nm is to design the direct analog modulated ECL to emit at
a wavelength shifted away from the zero or near-zero dispersion
point of the SMF (see FIG. 18). In other words, the emission
wavelength of the ECL is intentionally designed and implemented to
be in the range of 1320-1350 nm, or after the water peak (that is,
>1390 nm).
[0101] Another aspect of some embodiments of the present invention
relates to optical transmitters incorporating direct analog
modulated ECLs, whereby such transmitters are tunable for
distortion and/or chirp in the field, and the tuning can be
implemented remotely from a central office (CO) or a network
operating center (NOC) through network management software. In
other words, the analog transmitter's distortion and/or chirp
levels and/or optical output power can be tuned or adjusted for an
improved or substantially optimized carrier-to-noise ratio (CNR)
(and other system parameters) adjusted for specific system links
during system equipment installation, or network provisioning, or
maintenance. This substantially eliminates the need to procure new
sets of transmitters and the associated delays and cost. One
embodiment of the tunable analog transmitter includes programming a
look-up table into the transmitter memory during the manufacturing
of the transmitter. This look-up table contains a "contour map" for
desired performance parameters (such as distortion, chirp,
wavelength shift, output power vs. TEC temperature). The user can
then use software commands to optimize link performance by
accessing and selecting from the programmed look-up table.
[0102] Another aspect of some embodiments of the present invention
relates to wavelength-tunable direct analog modulated ECLs.
Embodiments of wavelength-tunable analog ECLs include, but are not
limited to, (i) tuning of the FBG (such as using a piezoelectric
actuator), for ECLs based on FBG; (ii) grating created and tuned by
an RF signal applied to an acousto-optic material, whereby the
emission wavelength is tuned and selected by controlling the RF
signal, optionally in combination with a movable reflective mirror;
(iii) use of an etalon-based tunable filter.
[0103] Although various embodiments which incorporate the teachings
of the present invention have been shown and described in detail
herein, those skilled in the art can readily devise many other
varied embodiments that still incorporate these teachings.
* * * * *