Title: Apparatus for frequency tuning and locking and method for operating same
Abstract: An apparatus comprising a first reference element having an output power that varies monotonically with input frequency over an operating frequency range and receiving at least a portion of an output beam of light from an optical source. A second reference element having an output power that is frequency dependent receives at least a portion of the output beam of light. A first optical detector measures the power of a first reference beam of light from the first reference element. A second optical detector measures the power of a second reference beam of light from the second reference element. Electronic circuitry is coupled to the first and second optical detectors for receiving first and second reference signals therefrom and producing a coarse error signal for permitting coarse adjustment and a fine error signal for permitting fine adjustment of the frequency of the output beam of light.
Patent Number: 6,898,221 Issued on 05/24/2005 to Berger, et al.
| Inventors:
|
Berger; Jill D. (Los Gatos, CA);
Dutta; Subrata K. (San Jose, CA);
Fennema; Alan A. (San Jose, CA);
Gorbounova; Olga A. (Fremont, CA);
Hrinya; Stephen J. (San Jose, CA);
Ilkov; Fedor A. (Sunnyvale, CA);
King; David A. (Menlo Park, CA);
Tavernier; Heather L. (Mountain View, CA);
Tselikov; Alexander A. (Fremont, CA)
|
| Assignee:
|
Iolon, Inc. (San Jose, CA)
|
| Appl. No.:
|
099414 |
| Filed:
|
March 15, 2002 |
| Current U.S. Class: |
372/32; 372/20; 372/29.011; 372/38.01 |
| Intern'l Class: |
H01S 003/13; H01S003/10 |
| Field of Search: |
372/20,32,98,290.11,380.1
|
References Cited [Referenced By]
U.S. Patent Documents
| 3970838 | Jul., 1976 | Goodwin et al.
| |
| 5331651 | Jul., 1994 | Becker et al.
| |
| 5825792 | Oct., 1998 | Villeneuve et al.
| |
| 5970076 | Oct., 1999 | Hamada.
| |
| 6005995 | Dec., 1999 | Chen et al.
| |
| 6043883 | Mar., 2000 | Leckel et al.
| |
| 6122301 | Sep., 2000 | Tei et al.
| |
| 6134253 | Oct., 2000 | Munks et al.
| |
| 6243403 | Jun., 2001 | Broutin et al.
| |
| 6272157 | Aug., 2001 | Broutin et al.
| |
| 6366592 | Apr., 2002 | Flanders.
| |
| 6400737 | Jun., 2002 | Broutin et al.
| |
| 6529534 | Mar., 2003 | Yasuda.
| |
| 2001/0003482 | Jun., 2001 | Zare et al.
| |
| 2002/0088920 | Jul., 2002 | Imajuku et al.
| |
| Foreign Patent Documents |
| WO 0108277 | Feb., 2001 | WO.
| |
| WO 0111738 | Feb., 2001 | WO.
| |
Other References
Yariv, Amnon, "Optical Electronics, Fourth Edition", Saunder College Publishing,
pp. 112-115.
Frenkel et al., "Angle-Tuned Etalon Filters for Optical Channel Selection In
High Density Wavelength Division Multiplexed Systems", Journal of Lightwave
Technology, vol. 7, No. 4, Apr. 1989, pp. 615-624.
|
Primary Examiner: Harvey; Minsun Oh
Assistant Examiner: Menefee; James
Attorney, Agent or Firm: Dorsey & Whitney LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
The application claims priority to U.S. provisional patent application Ser. No.
60/276,570 filed Mar. 15, 2001, U.S. provisional patent application Ser. No. 60/347,934
filed Oct. 26, 2001 and U.S. provisional patent application Ser. No. 60/355,147
filed Feb. 8, 2002, the entire contents of which are incorporated herein by this reference.
Claims
1. An apparatus for frequency tuning and locking for use with an optical source
tunable over an optical frequency range and producing an output beam of light having
a frequency comprising a first reference element having an output power that varies
monotonically with input frequency over the operating frequency range and being
adapted to receive at least a portion of the output beam of light, the first reference
element producing a first reference beam of light, a second reference element having
an output power that is frequency dependent and being adapted to receive at least
a portion of the output beam of light, the second reference element producing a
second reference beam of light, a first optical detector for measuring the power
of the first reference beam of light and producing a first reference signal, a
second optical detector for measuring the power of the second reference beam of
light and producing a second reference signal and electronic circuitry coupled
to the first and second optical detectors for receiving the first and second reference
signals and producing a coarse error signal for permitting coarse adjustment of
the frequency of the output beam of light and a fine error signal for permitting
fine adjustment of the frequency of the output beam of light.
2. The apparatus of claim 1 wherein the first reference element has an output
power that is substantially linear with frequency.
3. The apparatus of claim 2 wherein the linearity of the output power has a root
mean square correlation to a straight line better than 0.998.
4. The apparatus of claim 1 wherein the first reference element is a tuning element.
5. The apparatus of claim 4 wherein tuning element is selected from the group
consisting of a multi-layer dielectric thin film reflector, a dispersive phase
retarder plus at least one polarizer, a slowly varying light absorbing material
and a dispersive Faraday retarder plus at least one polarizer.
6. The apparatus of claim 5 wherein the tuning element is a dielectric reflector.
7. The apparatus of claim 1 wherein the first reference element has an output
power that varies monotonically with input frequency over a frequency range of
at least 0.4 THz.
8. The apparatus of claim 1 wherein the second reference element is a fine reference
element having a multi-peaked transmission spectrum.
9. The apparatus of claim 8 wherein the second reference element has a periodic
transmission spectrum.
10. The apparatus of claim 9 wherein the second reference element is selected
from the group consisting of an interferometer and a metal film dielectric reflector.
11. The apparatus of claim 10 wherein the interferometer is an etalon.
12. The apparatus of claim 11 wherein the etalon has a finesse ranging from two
to four.
13. The apparatus of claim 12 wherein the etalon has a finesse of approximately two.
14. The apparatus of claim 11 wherein the electronic circuitry provides a capture
range approximating the free spectral range of the etalon.
15. The apparatus of claim 11 wherein the etalon has first and second spaced-apart
reflectors and a medium of fused silica disposed between the first and second reflectors.
16. The apparatus of claim 11 wherein the etalon has a plurality of spaced-apart
transmission peaks with increasing and decreasing portions, the electronic circuitry
including a sign changing element for permitting locking on the increasing and
decreasing portions of each transmission peak.
17. The apparatus of claim 8 wherein the second reference element has a nonperiodic
transmission spectrum.
18. The apparatus of claim 8 wherein the second reference element is selected
from the group consisting of a liquid absorption cell, a gas absorption cell, a
resonant absorption cell and a solid reference material.
19. The apparatus of claim 1 further comprising an optical power divider for
directing a first portion of the output beam of light to the first reference element
and a second portion of the output beam of light to the second reference element.
20. The apparatus of claim 1 further comprising an additional optical detector
for measuring the power of at least a portion of the output beam of light and producing
a power reference signal, the electronic circuitry being electrically coupled to
the additional optical detector and including circuitry for processing the second
reference signal with the power reference signal to produce the fine error signal.
21. The apparatus of claim 1 wherein the first reference element produces an
additional beam of light, an additional optical detector for measuring the power
of the additional beam of light and producing an additional signal and summation
circuitry coupled to the first optical detector and the additional optical detector
for adding the first reference signal and the additional signal to produce a power
reference signal, the electronic circuitry being electrically coupled to the summation
circuitry and including circuitry for processing the first reference signal with
the power reference signal to produce the coarse error signal and the fine error signal.
22. The apparatus of claim 21 wherein the additional optical detector produces
a locking beam of light, the locking beam of light being the portion of the output
beam of light received by the second reference element.
23. The apparatus of claim 1 further comprising an additional optical detector
for measuring the power of at least a portion of the output beam of light and producing
a power reference signal, the electronic circuitry being electrically coupled to
the additional optical detector and including circuitry for processing the first
reference signal with the power reference signal to produce the coarse error signal.
24. The apparatus of claim 23 wherein the second reference element is a fine
reference element having a multi-peaked transmission spectrum.
25. The apparatus of claim 23 wherein the first reference element has an output
power that is substantially linear with frequency.
26. A method for tuning and locking the output beam of an optical source tunable
over an optical frequency range with a first reference element having an output
power that varies monotonically with input frequency over the optical frequency
range and with a second reference element having an output power that varies with
frequency and is provided with a plurality of peaks over the optical frequency
range comprising the steps of selecting a frequency of the output beam from the
optical source, impinging a first portion of the output beam on the first reference
element to produce a first reference beam which permits the optical source to tune
to a frequency near the selected frequency and impinging a second portion on the
output beam on the second reference element to produce a second reference beam
which permits the optical source to lock on the selected frequency.
27. The method of claim 26 wherein the selecting step includes the steps of selecting
a frequency located on an increasing portion of one of the plurality of peaks of
the second reference element and selecting a frequency located on a decreasing
portion of one of the plurality of peaks of the second reference element.
28. The method of claim 26 wherein the step of impinging a second portion of
the output beam on the second reference element includes the step of measuring
the power of the second reference beam to provide a reference signal, further comprising
the steps of measuring the output power of a portion of the output beam to provide
a power reference signal, adjusting the power reference signal as a function of
the selected frequency to produce a set point value and subtracting the set point
value from the reference signal to produce an error signal.
29. The method of claim 28 further comprising the step of changing the sign of
the error signal.
30. The method of claim 26 wherein the step of impinging a second portion of
the output beam on the second reference clement includes the step of measuring
the power of the second reference beam to provide a reference signal, further comprising
the step of adjusting the reference signal by a predetermined constant stored in
a computer memory and corresponding to the selected frequency.
31. The method of claim 26 wherein the first portion of the output beam includes
the second portion of the output beam.
32. An apparatus for locking the frequency of an output beam of light from an
optical source tunable to a plurality of frequencies over an optical frequency
range comprising a coarse reference element having an output power that varies
monotonically with input frequency over the optical frequency range and being adapted
to receive at least a portion of the output beam of light, the coarse reference
element producing a first reference beam of light, a fine reference element adapted
to receive at least a portion of the output beam of light, the fine reference element
producing a second reference beam of light, a first optical detector for measuring
the power of the first reference beam of light and producing a first reference
signal, a second optical detector for measuring the power of the second reference
beam of light and producing a second reference signal, a computer memory having
a plurality of predetermined constants corresponding respectively to the plurality
of frequencies of the optical source and electronic circuitry for adjusting the
second reference signal by the predetermined constant corresponding to the selected
frequency to produce a fine reference signal.
33. The apparatus of claim 32 wherein the fine reference element is an etalon.
34. The apparatus of claim 32 wherein the fine reference element has a transmission
spectrum that is periodic and includes a plurality of spaced-apart transmission
peaks having respective increasing and decreasing portions, the electronic circuitry
including an element to change the sign of the fine reference signal to reflect
a selected frequency corresponding to the decreasing portion of one of the plurality
of transmission peaks.
35. The apparatus of claim 32 wherein the electronic circuitry includes an adjustable
gain amplifier.
36. The apparatus of claim 32 wherein the electronic circuitry includes a processor.
37. An apparatus for frequency tuning and locking for use with an optical source
tunable over an optical frequency range and producing an output beam of light having
a frequency comprising a first reference element having an output that varies with
input frequency over the operating frequency range and being adapted to receive
at least a portion of the output beam of light, the first reference element producing
a first reference beam of light, a second reference element having an output power
that is frequency dependent and being adapted to receive at least a portion of
the output beam of light, the second reference element producing a second reference
beam of light, a first optical detector for measuring the power of the first reference
beam of light and producing a first reference signal, a second optical detector
for measuring the power of the second reference beam and producing a second reference
signal and electronic circuitry coupled to the first and second optical detectors
for receiving the first and second reference signals and processing the first reference
signal to produce a coarse error signal so as to permit coarse adjustment of the
frequency of the output beam of light and processing the second reference signal
to produce a fine error signal so as to permit fine adjustment of the frequency
of the output beam of light.
38. The apparatus of claim 37 wherein the second reference element is a fine
reference element having a multi-peaked transmission spectrum.
39. The apparatus of claim 37 wherein the first reference element has an output
power that is substantially linear with frequency.
40. The apparatus of claim 37 wherein the optical frequency range comprises eight
to 1000 tunable frequencies.
41. An apparatus for frequency tuning and locking for use with an optical source
tunable over an optical frequency range in the 50 GHz ITU grid and producing an
output beam of light having a frequency comprising a first reference element having
an output that varies monotonically with input frequency over the operating frequency
range and being adapted to receive at least a portion of the output beam of light,
the first reference element producing a first reference beam of light, a second
reference element having an output power that is frequency dependent and being
adapted to receive at least a portion of the output beam of light, the second reference
element producing a second reference beam of light, a first optical detector for
measuring the power of the first reference beam of light and producing a first
reference signal, a second optical detector for measuring the power of the second
reference beam and producing a second reference signal and electronic circuitry
coupled to the first and second optical detectors for receiving the first and second
reference signals and processing the first reference signal to produce a coarse
error signal so as to permit coarse adjustment of the frequency of the output beam
of light and processing the second reference signal to produce a fine error signal
so as to permit fine adjustment of the frequency of the output beam of light.
42. The apparatus of claim 41 wherein the second reference element is a fine
reference element having a multi-peaked transmission spectrum.
43. The apparatus of claim 41 wherein the first reference element has an output
power that is substantially linear with frequency.
44. The apparatus of claim 41 wherein the optical frequency range comprises eight
to 1000 tunable frequencies.
Description
FIELD OF THE INVENTION
The present invention relates generally optical sources and more particularly
to apparatus for locking the frequency of a tunable optical source.
BACKGROUND
Tunable optical sources are commonly used for applications in the ultraviolet,
visible and infrared regions of the optical spectrum. The output frequency or wavelength
of such a source may be tuned in a continuous or stepwise fashion. In one exemplary
application, single-frequency, extended cavity semiconductor lasers (ECL's) with
a tuning range of approximately 40 nanometers around a center wavelength of 1540
nanometers have been developed for telecommunications applications. In many applications,
it is often desirable to lock the output frequency of a tunable optical source
to a predetermined frequency. In a telecommunications system, for example, the
output wavelength may be locked to a single frequency in a predetermined International
Telecommunications Union (ITU) frequency grid.
Frequency or wavelength lockers have heretofore been provided. The use
of an etalon in a wavelength locker is described in U.S. Pat. No. 6,005,995 of
Chen et al. Properties of Fabry-Perot etalons are discussed in "Angle-tuned etalon
filters for optical channel selection in high density wavelength division multiplexed
systems," by A. Frenkel and C. Lin, Journal of Lightwave Technology, Vol. 7, pp.
615-624 (1989). See also §4.1 of Optical Electronics, A. Yariv, 4
th Ed.,
Saunders College Publishing, Philadelphia, 1991.
Unfortunately, etalons and other reference elements with multipeaked
transmission spectra used in frequency or wavelength lockers typically require
an additional measurement system to distinguish between transmission peaks. Systems
that provide this additional measurement functionality have been provided. See,
for example, International Publication Number WO 01/08277 assigned to New Focus,
Inc. which discloses a channel selector and an etalon. An alternative approach
is disclosed in International Publication Number WO 01/11738 assigned to Coretek,
Inc. wherein a first etalon is used as a wavelength locker and a filter and second
etalon are used to determine a single known wavelength that can be used to calibrate
the transmission profile of the first etalon. A disadvantage of this approach is
that the system must tune back to the single reference wavelength in order to recalibrate
if a particular instability in the source causes discontinuities in the source
wavelength control. U.S. Pat. No. 5,970,076 of Hamada discloses identifying peaks
in an etalon transmission using the combination of a fiber grating, optical coupler
and an etalon with a large free spectral range. Disadvantages of this approach
include a comparatively limited range of operating frequencies and a large degree
of optomechanical complexity.
Frequency or wavelength lockers using reference elements with monotonic
transmission functions have been disclosed. See, for example, U.S. Pat. No. 6,043,883
of Leckel et al., which discloses a wavemeter device that uses the dispersive phase
retardation of a polarized input beam to generate a monotonic transmission function.
A limitation of this device is that the derived error signal has a shallow slope
that is not optimal for frequency locking to a grid of narrowly spaced frequencies
because environmental and electronic noise act to limit the ability to distinguish
between and lock to individual grid frequencies.
U.S. Pat. No. 6,134,253 of Munks et al. discloses an apparatus wherein the input
beam from the device to be frequency or wavelength locked is split into two secondary
beams that each incident on a filter. The apparatus disadvantageously relies on
actively tuning one or both of the filters. A wavelength locker is disclosed in
U.S. Pat. No. 6,122,301 of Tei et al. that relies on the measurement of the light
power reflected and transmitted by a dielectric reflector with a transmission function
that varies monotonically with wavelength. Unfortunately, the error signal that
can be derived from this optical apparatus has a shallow slope that is not optimal
for wavelength locking.
In general, it is an object of the present invention to provide an apparatus
for
frequency tuning and locking for use with an optical tunable source.
Another object of the invention is to provide an apparatus for frequency
tuning and locking that separately optimizes the locking and tuning signals.
Another object of the invention is to provide an apparatus for frequency
tuning and locking of the above character which has a relatively high immunity
to external environmental forces such as mechanical shock and vibration.
Another object of the invention is to provide an apparatus for frequency
tuning and locking of the above character which is suitable for use in an optical
telecommunications system.
Another object of the invention is to provide an apparatus for frequency
tuning and locking of the above character that is of minimal size.
Another object of the invention is to provide an apparatus for frequency
tuning and locking of the above character that has a minimal component count.
Another object of the invention is to provide an apparatus for frequency
tuning and locking of the above character that may be manufactured at a reduced cost.
SUMMARY OF THE INVENTION
An apparatus for frequency tuning and locking for use with an optical source
tunable
over an optical frequency range and producing an output beam of light having a
frequency is provided. The apparatus comprises a first reference element having
an output power that varies monotonically with input frequency over the operating
frequency range and is adapted to receive at least a portion of the output beam
of light. The first reference element produces a first reference beam of light.
A second reference element having an output power that is frequency dependent is
adapted to receive at least a portion of the output beam of light and produce a
second reference beam of light is provided. A first optical detector measures the
power of the first reference beam of light and produces a first reference signal.
A second optical detector measures the power of the second reference beam of light
and produces a second reference signal. Electronic circuitry is coupled to the
first and second optical detectors for receiving the first and second reference
signals and producing a coarse error signal for permitting coarse adjustment of
the frequency of the output beam of light and a fine error signal for permitting
fine adjustment of the frequency of the output beam of light.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are somewhat schematic and are incorporated
in and form a part of this specification, illustrate several embodiments of the
invention and, together with the description, serve to explain the principles of
the invention.
FIG. 1 is a block diagram of a lightwave communications system incorporating
a stabilized laser having the apparatus for frequency tuning and locking of the
present invention.
FIG. 2 is a block diagram of the apparatus for frequency tuning and locking
of the present invention utilized in the stabilized laser of the lightwave communications
system of FIG. 1.
FIG. 3 is a graph showing the ratio of the intensity of the transmitted beam
over the intensity of the incident beam as a function of frequency for the tuning
reference element in the apparatus for frequency tuning and locking of FIG. 2.
FIG. 4 is a graph showing the ratio of the intensity of the transmitted beam
over the intensity of the incident beam as a function of frequency for the locking
reference element in the apparatus for frequency tuning and locking of FIG. 2.
FIG. 5 is one embodiment of the locking reference element utilized in the apparatus
for frequency tuning and locking of FIG. 2.
FIG. 6 is a block diagram of the calculation device of the apparatus for frequency
tuning and locking of FIG. 2.
FIG. 7 is a block diagram of the wavelength control unit of the lightwave communications
system of FIG. 1 incorporating the frequency tuning and locking device of the present invention.
FIG. 8 is a block diagram of an embodiment of a combined calculation device
and wavelength control unit for use in the lightwave communications system of FIG. 1.
FIG. 9 is a flow chart of the operation of the apparatus for frequency tuning
and locking of FIG. 2.
FIG. 10 is another embodiment of the apparatus for frequency tuning and locking
of the present invention.
FIG. 11 is a further embodiment of the apparatus for frequency tuning and locking
of the present invention.
DESCRIPTION OF THE INVENTION
Locked or stabilized lasers can be used in a variety of applications where
it is desirable to tune to a plurality of selected frequencies. One preferred application
is a light wave communications or telecommunications system of the type shown in
FIG.
1. One or more optical transmitters
22 for inputting respective
modulated optical signals into the system are provided in the system. For simplicity,
only one of such optical transmitters
22 is shown in FIG.
1 and includes
a locked or stabilized laser
23. Any suitable tunable optical source
24
is included within stabilized laser
23 for producing an output beam of light
of a selected frequency and wavelength. Optical source
24 has at least one
output beam of light, such as beam
26 in FIG. 1 and, in this regard, can
have multiple output beams with identical spectral features. Where multiple beams
are provided, the majority of the output power is carried by the primary output
beam, such as beam
26, and significantly less power by the other secondary
beams. In one preferred embodiment, the optical source
24 is an extended
cavity semiconductor laser of the type disclosed in U.S. patent application Ser.
No. 09/728,212 filed Nov. 29, 2000, the entire content of which is incorporated
herein by this reference. Optical source
24 may be tunable across a band
of wavelengths within the erbium fiber gain bandwidth, for example, the Conventional
or C-band covering a range of approximately 35 nanometers from 1530 nanometers
to 1565 nanometers. The corresponding frequency span is approximately 4.4 THz with
a lower frequency bound of approximately 191 THz. The source
24 may, for
example, be locked to one of a grid of frequencies with a fixed separation of 50
GHz and a reference frequency of 193.1 THz, commonly referred to as a 50 GHz ITU grid.
Input beam
26 is incident on a optical power divider
27 of any
suitable type so as to be split into a reference beam
31 and an output beam
32. In one embodiment, power divider
27 is a dielectric or polarization
beam splitter having a reflectivity ranging from approximately 1% to 20% and preferably
approximately 5%. Reference beam
31 is directed into a suitable wavelength
locker such as the apparatus for frequency tuning and locking
33 of the
present invention. A command interface signal
36 is supplied to apparatus
or wavelength locker
33 for selecting the frequency of input beam
26
produced by tunable optical source
24. Wavelength or frequency locker
33
is electrically coupled to a wavelength control unit
41 and, as more filly
described below, provides a coarse or tuning error signal
42 and a fine
or locking error signal
43 to control unit
41. Command interface
signal
36 is additionally supplied to wavelength control unit
41,
which is electrically coupled to optical source
24 so as to provide a wavelength
control signal
46 to the optical source.
Optical transmitter
22 further includes an optical modulator
47
of any suitable type which receives output beam
32 of stabilized laser
23.
An input electronic information signal is provided to modulator
47 for permitting
output beam
32 to be encoded in a conventional manner. Optical transmitter
22 can optionally further include a tunable or other optical filter for
shaping the encoded beam received from modulator
47 and an amplifier for
amplifying such beam. Such optional filter and amplifier are shown together for
simplicity in FIG.
1 and identified by reference number
48.
The output beam from single-frequency optical transmitter
22, identified
as λ
1 in FIG. 1, is thereafter combined with one or more additional
modulated beams, each of which is tuned to a different operating frequency and
wavelength. Such additional modulated beams are identified as λ
2 through
λ
m in FIG.
1 and form a single transmitting station. A
conventional multiplexer
51 is provided for combining such modulated beams
into a single multiplexed beam
52. An optional amplifier
53, such
as an erbium fiber or Raman amplifier, can be provided for amplifying multiplexed
beam
52 before such beam is coupled into a fiber optic transport system
54. The transport system may include other optical amplifiers, tunable filters,
optical switches, add/drop multiplexers, power equalizers and other system components
for amplifying, routing and shaping the spectrum of the multiplexed light wave
signal. Other transmitting stations can input one or more modulated optical beams
into fiber optic transport system
54. Exemplary modulated beams λ
m+1
through λ
p from another transmitting station are shown as
being inputted into the system
54 in FIG.
1.
Multiplexed beam
52 is delivered by transport system
54
to an optical receiver
61 which includes any suitable frequency selector
such as an optical filter that may be tunable or a demultiplexer. Receiver
61
shown in FIG. 1 has a tunable optical filter
62 for each of the modulated
λ
1 through λ
p beams. An optical power divider
63 separates the multiplexed beam into p beams and directs each such beam
to a respective optical filter
62, only one of which is shown in FIG.
1.
The passband of each tunable optical filter
62 serves to select the frequency
signal generated by the optical transmitter
22 corresponding to such frequency.
Each resulting modulated beams is then directed to a suitable combination of components
to extract the electronic signal therefrom. For example, the output beam for filter
62 is directed to a detector
64 which measures the power of the beam
and converts the information carried thereby into an output electronic information
signal. If a demultiplexer (not shown) is used for separating the signals from
multiplexed beam
52, the several channels produced thereby are similarly
directed to different detectors that convert the optical information signals to
respective electronic output signals.
Stabilized laser
23 and particularly wavelength locker
33
are shown in greater detail in FIG.
2. As described therein, the wavelength
locker
33 includes a first or tuning reference element
71, second
or locking reference element
72 and electronic circuitry which includes
calculator or calculation device
73. At least a portion of input beam
26
from optical source
24 is received by or impinges upon tuning reference
element or tuning element
71 and at least a portion of such light beam is
received by or impinges upon locking reference element or locking element
72.
More specifically, a first portion of reference beam
31 is directed to the
tuning reference element by means of an optical power divider of any suitable type
such as described above with respect to beam splitter
27. Such optical power
divider is preferably a second beam splitter
74 which splits the reference
beam
31 into a tuning beam
76 directed towards tuning reference element
71 and a locking beam
77 directed towards locking reference element
72.
Tuning or coarse reference element
71 has an output power that varies
monotonically with input frequency over the operating frequency range of the wavelength
locker
33. A suitable transmission curve for tuning reference element
71
is shown in FIG.
3. As illustrated therein, the output transmission of the
tuning reference element has a unique power for each input frequency of tuning
beam
76. Over the frequencies of interest, the transmission variation is
preferably large and may range from 20% to 95% and preferably approximately 75%.
The first derivative of the transmission function over the frequencies of interest
is preferably substantially constant. More specifically, the route mean squared
(RMS) or R-squared value of the transmission of the element
71, that is
the measure of the linearity of the such transmission function, is preferentially
greater than 0.998 across the operating frequency range of wavelength locker
33.
Representative monotonic elements suitable for use as a tuning reference
element
71 include a multi-layer dielectric thin film filter or reflector,
a dispersive phase retarder plus at least one polarizer, a slowly varying light
absorbing material such as germanium over the range of 1.7 microns to 1.95 microns
and a combination of a dispersive Faraday retarder plus at least one polarizer.
In one preferred embodiment, tuning element
71 is a dielectric reflector.
Tuning reference element
71 produces a first reference beam of light
in the form of filtered tuning beam
81 which is received by a first optical
detector
82 electrically coupled to calculation device
73. The first
optical detector
82 measures the power of filtered tuning beam
81
and produces a first reference signal or tuning detector signal
83 which
is processed by calculation device
73 in the manner set forth below to provide
a coarse error signal in the form of tuning error signal
42 for coarse adjustment
of the frequency of input beam
26.
A second portion of reference beam
31 in the form of at least a portion
of locking beam
77 is received by locking or fine reference element
72.
Locking element
72 has an output power that is frequency dependent and preferably
has a multi-peaked transmission spectrum that can be periodic or nonperiodic. The
locking element more preferably has a periodically-varying transmission spectrum
provided with a plurality of transmission maxima in form of peaks
84 as
shown in FIG.
4. Each of such peaks
84 has an increasing portion
85 and a decreasing portion
86. Locking element
72 can be
of any suitable type. Elements
72 which may have nonperiodic spectra include
liquid and gas absorption cells, resonant absorption cells and solid absorbing
or reference materials. Elements
72 with periodic spectra include interferometers
and metal film dielectric filters or reflectors. Suitable interferometers include
Mach Zender interferometers in bulk optic and guided wave form, fiber Bragg interferometers,
air-gapped etalons and Fabry-Perot etalons as shown in FIG.
5. The locking
element
72 is preferably an etalon that is formed by first and second spaced-apart
reflectors
91 and a medium
92 disposed between the reflectors. Medium
92 is formed from any suitable material such as fused silica, for a solid
etalon, or air, for a air-gapped etalon. Locking element or etalon
72 preferably
has a finesse ranging from two to four and preferably approximately two.
The etalon
72 has a capture range
93, shown as 2Δ in FIG.
4, preferably approximating the free spectral range of the etalon and a locking
range
94 which may be approximately equal to approximating one half the
capture range
93. Such capture range is the frequency span over which the
control system of laser
23 can uniquely identify the target frequency.
Locking element
72 produces a second reference beam of light in the
form of filtered locking beam
96 which is received by a second optical detector
97 electrically coupled to calculation device
73. Second optical
detector
97 measures the power of filtered locking beam
96 and produces
a second reference signal in the form of locking detector signal
98 which
is processed by the calculation device
73 in the manner described below.
The portion of locking beam
77 incident on tuning element
72 is the
incident beam in FIG.
5 and the filtered locking beam
96 is the transmitted
beam in FIG.
5.
Wavelength locker
33 further includes an additional optical detector
106 for measuring the power of input beam
26 (see FIG.
2).
In one embodiment, the additional or third optical detector
106 receives
a portion of locking beam
77 by means of an optical power divider of any
suitable type such as described above. Such optical power divider is preferably
a beam splitter
107 which splits the locking beam
77 into a power
measurement beam
108 which impinges upon third optical detector
106
and a modified locking beam
109 which impinges on locking element
72.
A third optical detector
106 measures the power of measurement beam
108
and produces a power reference signal or power detector signal
111. The
third optical detector
106 is electrically coupled to calculation device
73 and the power detector signal
111 is processed by calculation
device
73 in the manner described below.
One preferred embodiment of calculation device
73 includes a suitable
processor such as digital signal processor
121 having internal program memory
122 (see FIG.
6). External memory such as non-volatile setpoint memory
123 is included in calculation device
73 and coupled to processor
121 by any suitable means such as a bus. A command interface input or signal
36 corresponding to a selected frequency and wavelength of stabilized later
23 is inputted into processor
121 by any suitable means such as a
bus. Analog tuning detector signal
83 is amplified by an amplifier
127
and then converted to a digital signal by any suitable analog to digital converter
128. The A/D converter
128 is coupled to processor
121 by
any suitable means such as a bus for delivering such digital signal to the processor.
Tuning error signal
42 is generated by processor
121 in the manner
describe below and outputted by the processor by any suitable means such as a bus.
Analog power detector signal
111 is similarly modified for delivery to digital
signal processor
121. Specifically, such signal
111 is passed through
an amplifier
132 and then converted to a digital signal by analog to digital
convertor
133 coupled to processor
121 by any suitable means such
as a bus. Amplifiers
127 and
132 change the current modulated signals
83 and
111 produced respectively by optical detectors
82 and
106 to voltage modulated signals which are more suitable for processing
in digital signal processor
121.
Calculation device
73 further includes a locking error generator
141 which receives and processes power detector signal
111 and locking
detector signal
98 to generate a signal for delivery to processor
121.
Analog locking error generator
141 includes an amplifier
143 for
changing the current modulated locking detector signal
98 to a voltage modulated
signal before delivery of such signal to one input of summation circuitry in the
form of summer
144. A programmable gain amplifier such as locking frequency
adjust amplifier
147 is coupled to the other input of summer
144.
The input of amplifier
147 is coupled to the output of power detector amplifier
132 for receiving power detector signal
111. Adjust amplifier
147
is electrically coupled to digital signal processor
121 by any suitable
means such as a bus so that the gain of frequency adjust amplifier
147 can
be controlled by a gain signal
148 produced by processor
121. The
amplifier
147 has a negative gain so as to additionally serve as a sign-changing
element. The output of locking frequency adjust amplifier
147 is a negative
number which is added to the amplified value of locking detector signal
98
outputted from amplifier
143 by summer
144 and delivered to a second
programmable gain amplifier in the form of error magnitude adjust amplifier
151.
Digital signal processor
121 is electrically coupled to error magnitude
adjust amplifier
151 by any suitable means such as a bus so that the programmable
gain of the error magnitude adjust amplifier can be controlled by a second gain
signal
152 produced by processor
121.
The output of locking error magnitude adjust amplifier
151 is coupled
to digital signal processor
121 which outputs an analog locking error signal
153. A suitable converter such as analog to digital converter
156
receives locking error signal
153 and converts it to a digitized locking
error signal
158 for processing by processor
121. A/D converter
156
is electrically coupled to the digital signal processor
121 by any suitable
means such as a bus for delivering signal
158 to the processor. Locking
error signal
43 is outputted from processor
121 for permitting fine
adjustment of the frequency of input beam
26.
Tuning error signal
42 and locking error signal
43 generated
by wavelength locker
33 and the command interface signal
36 are received
by wavelength control unit
41 and processed therein to produce wavelength
control signal
46 (see FIGS.
1 and
7). Wavelength control
unit
41 includes a suitable processor such as digital signal processor
171
having internal memory such as program memory
172. External memory is coupled
to processor
171 and, as shown in FIG. 7, includes non-volatile servo memory
173 coupled to the processor
171 by any suitable means such as a
bus. Wavelength control unit
41, and specifically digital signal processor
171 thereof, is electrically coupled to wavelength locker
33 by any
suitable means such as a first bus for receiving the digital tuning error signal
42 and a second bus for receiving the digital locking error signal
43.
Tuning error signal
42 and locking error signal
43 are processed
by digital signal processor
171 in the manner described below and outputted
to a suitable digital to analog converter
177 coupled to processor
171
by any suitable means such as a bus. Wavelength control signal
46 is outputted
by converter
177 to optical source
24.
It should be appreciated that wavelength control unit
41 and calculation
device
73 can have a variety of configurations and be within the scope of
the present invention. For example, digital signal processor
121 of calculation
device
73 can be combined with digital signal processor
171 of wavelength
control unit
41 into a signal processor. In addition, wavelength control
unit
41 and calculation device
73 can be combined in a single controller
for inclusion inside wavelength locker
33 or outside the locker but within
stabilized laser
23.
In one alternative embodiment of the present invention, wavelength control unit
41 and calculation device
73 are combined into a signal digital wavelength
controller
186 for inclusion in stabilized laser
23 (see FIG.
8).
Controller
186 includes a signal processor of any suitable type and preferably
a digital signal processor
187 having an internal control program
188
and memory (not shown). External non-volatile memory is included in controller
186 in the form of setpoint and servo memory
191 coupled to processor
187 by any suitable means such as a bus. Command interface signal
36
is inputted into digital signal processor
187 by any suitable means such
as a bus. Tuning detector signal
83, locking detector signal
98 and
power detector signal
111, each in an analog form, are amplified by respective
amplifiers
127,
143 and
132 and then inputted into respective
analog to digital converters
192,
193 and
194, each of which
is electrically coupled to processor
187 by a respective bus for delivering
the digitized detector signals to the processor
187. Controller
186
includes digital to analog converter
177 for converting the output digital
signal of processor
187 into the analog wavelength control signal
46.
The operation and use of optical transmitter
22 and more particularly
stabilized laser
23 having wavelength control unit
41 and calculation
device
73 will now be described in the exemplary application of telecommunication
system
21. Optical transmitter
22 is used in the system
21
to produce a modulated optical beam at any one of the plurality of ITU grid point
frequencies or channels. Transmitter
22 can tune to any plurality of frequencies,
preferably at least eight frequencies and more preferably a plurality of frequencies
ranging from eight to 1000 in number. The frequency range corresponding to eight
frequencies having a fixed separation of 50 GHz is 0.4 THz. Exemplary placement
of such grid points on the transmission spectrum of locking reference element
72
are shown in FIG. 4, where a grid point
202 is provided on each increasing
portion
85 and decreasing portion
86 of each peak
84. The
spacing between each grid point is thus half the free spectral range of locking
reference element
72. Capture range
93 is approximately twice such
frequency spacing and thus approximately equal to such free spectral range.
A flow chart of the operation of stabilized laser
23 is shown in FIG.
9.
In step
203 therein, computer memory
123 of calculation device
73
and computer memory
173 of wavelength control unit
41 are precalibrated
with constants for each frequency or channel of operation of the stabilized laser
23. Calibration of the laser
23 is commenced by connecting the digital
signal processor
121 of the calculation device
73 and the digital
signal processor
171 or wavelength control unit
41 to an external
computer which communicates with processors
121 and
171 by means
of the bus utilized for providing command interface signal
36 to the processors.
The wavelength of input beam
26 is controlled by such external computer
during calibration. An external frequency measuring device, such as an optical
spectrum analyzer is connected to the external computer to provide a feedback signal
to such computer.
Estimated values for the setpoint constant or constants of wavelength control
locker
33, such as those that allow calculation of a gain signal
148
to locking frequency adjust amplifier
147, are supplied to processor
121
in step
203. Such a value of gain signal
148, for example, may be
chosen to correspond to initial frequency setpoint approximations that lie on a
horizontal line across the frequency spectrum of FIG.
4. Such frequency
setpoint approximations may lie on both the increasing portion
85 and decreasing
portion
86 of each transmission peak
84. The frequency of input beam
26 from optical source
24 is then tuned across the desired range
of operating wavelengths and the zero points for locking error signal
153
for each frequency determined. Such zero points correspond to the initial setpoint
approximations on the transmission curve of FIG.
4.
The setpoint constants for calculation device
73 corresponding to each
lock point
202 relating to a desired channel frequency of operation are
determined next. With the foregoing estimated values of gain signals
148
and
152 remaining unchanged, source
24 is tuned to a lock point
202
corresponding to a first desired frequency of operation. Gain signal
148
to locking frequency adjust amplifier
147 is then changed until the error
analog signal
153 is zero at the desired channel frequency. The data needed
by processor
121 to generate the value of gain signal
148 and the
gain signal
152 at the desired channel frequency are stored in setpoint
memory
123 of the calculation device
73 in any suitable manner such
as a look up table. Data needed to calculate the tuning signal setpoint and the
polarity of the locking error signal
43 are also stored in setpoint memory
123 at this time. The polarity of the locking error signal is chosen to
facilitate stable frequency locking of the optical source
23. Servo data
required for the optimized conversion of locking error signal
43 to wavelength
control signal
46 by the digital signal processor
171 are determined
and stored in the servo memory
173 by the external computer.
The setpoint and servo data of calculation device
73 and wavelength control
unit
41 for each other desired grid point frequency of operation are similarly
determined and stored in respective setpoint memory
123 and servo memory
173. The calibration sequence of step
203 for each such other frequency
is commenced with the initial estimated values of gain signals
148 and
152
being determined by the processor
121 from data supplied by the external
computer in the same manner as described above.
If operation of stabilized laser
23 is desired at any off-grid channel
frequency, the laser is similarly calibrated at such channel or channels and the
corresponding setpoint and servo data stored in the computer memory of the laser
23 for each such off-grid channel. As can be seen, the relative location
of grid pints
202 of each peak, the number of grid points
202 on
each peak and the frequency spacing between grid points
202 are arbitrary
and can be chosen and changed as desired.
The data stored in the setpoint memory of calculation device
73 and the
servo memory of wavelength control unit
41 provide optimized values for
the computation of the tuning and locking error signals
42 and
43
and wavelength control signal
46. Each grid point and off-grid frequency
of operation is calibrated separately to provide optimized tuning and locking performance
of the stabilized laser
23 in the presence of vibration, temperature excursions
and other perturbations.
After calibration step
203 has been completed and the external calibration
computer disconnected from processors
121 and
171, laser
23
can be installed for operation in an optical transmitter
22 for use in a
telecommunications system
21. Operation of the laser
23 is commenced
by turning on optical source
24, as shown in step
204 in FIG.
9.
In step
206, a command input signal
36 is supplied to laser
23
indicating the frequency or channel to which the laser is to be tuned. Upon receipt
of such signal
36, processor
121 retrieves from setpoint memory
123
the data required for the generation of the tuning signal setpoint, gain signals
148 and
152 and the locking polarity signal corresponding to the
command interface signal
36. Processor
171 retrieves from servo memory
173 the optimized servo constants for the conversion of locking error signal
43 to wavelength control signal
46. The servo constants for the conversion
of tuning error signal
42 to wavelength control signal
46 are typically
constant across the operating range of stabilized laser
23.
U
