U.S. patent application number 09/769082 was filed with the patent office on 2002-07-25 for laser communication system.
This patent application is currently assigned to fSONA COMMUNICATIONS CORPORATION. Invention is credited to Carlson, Robert T., Draganov, Vladimir, Holcomb, Terry Lee, Mecherle, George Stephen, Wang, Fang-Xin.
Application Number | 20020097468 09/769082 |
Document ID | / |
Family ID | 25084395 |
Filed Date | 2002-07-25 |
United States Patent
Application |
20020097468 |
Kind Code |
A1 |
Mecherle, George Stephen ;
et al. |
July 25, 2002 |
Laser communication system
Abstract
An optical wireless transceiver for communicating broadband
signals through free space includes an input, a regenerator, a
splitter and a plurality of lasers in transmitter modules. A very
fast (low f-number) optical receiver module includes a reflector,
preferably a Mangin mirror or parabolic reflector with field
corrector, aligned with an input aperture. A photodiode receives
the signal from the reflector for subsequent demodulation. A
background rejection filter is disposed between the reflector and
the photodiode at the focal point of the mirror. The transceiver
provides signal regeneration and switchable data rates. Connections
are made to optical or electrical digital inputs and outputs
bearing signals of various protocols. The plurality of lasers
includes adjustable-beamwidth collimating lenses. Monitoring
circuitry including a controller monitors the system. A stand-alone
backup RF transceiver operating in conjunction with the laser
transceiver provides enhanced availability. An efficient
high-current, high power laser driver capable of modulating a laser
between 100 and 1500 mA at data rates greater than 10 Mbits/sec is
provided. A highly efficient thermoelectric cooler operates to cool
the laser diode, or other objects requiring cooling.
Inventors: |
Mecherle, George Stephen;
(Hawthorne, CA) ; Holcomb, Terry Lee; (Torrance,
CA) ; Carlson, Robert T.; (Surrey, CA) ; Wang,
Fang-Xin; (New Westminister, CA) ; Draganov,
Vladimir; (Coquitlam, CA) |
Correspondence
Address: |
LYON & LYON LLP
633 WEST FIFTH STREET
SUITE 4700
LOS ANGELES
CA
90071
US
|
Assignee: |
fSONA COMMUNICATIONS
CORPORATION
|
Family ID: |
25084395 |
Appl. No.: |
09/769082 |
Filed: |
January 24, 2001 |
Current U.S.
Class: |
398/128 ;
398/118 |
Current CPC
Class: |
H04B 10/1123 20130101;
H04B 10/032 20130101 |
Class at
Publication: |
359/152 ;
359/172 |
International
Class: |
H04B 010/00 |
Claims
What is claimed is:
1. A communication link for providing two-way communication through
free space, the link including a first transceiver and a second
transceiver, wherein at least one transceiver comprises: an input
signal interface for receiving one or more digital signals; a
splitter in communication with the input to split the one or more
digital signals into a plurality of approximately equal laser data
signals; a plurality of lasers displaced from one another and
facing in parallel directions, each of the lasers being in
communication with the splitter; a plurality of laser drivers, each
laser driver being coupled to one of the lasers and to the
splitter, wherein the laser drivers receive the laser data signals
and provide amplified laser data signals to the lasers at high
power and high frequency.
2. The communication link of claim 1, wherein the input signal is
characterized by a data rate of at least 10 Mbits/second, and each
laser is supplied with a nominal current of at least 100 mA.
3. The communication link of claim 1, wherein the digital signals
comprise optically transmitted data signals.
4. The communication link of claim 1, wherein said at least one
transceiver further comprises a signal regenerator in communication
with the one or more digital input signals.
5. The communication link of claim 4, wherein the regenerator
includes a first clock and data recovery circuit, and the clock and
data recovery circuit may be switched between one of a plurality of
clock frequencies.
6. The communication link of claim 1, wherein each of the plurality
of lasers includes a laser diode coupled to the laser driver and
receiving the amplified laser data signals.
7. The communication link of claim 6, wherein each of the plurality
of lasers further includes a lens for receiving and collimating the
laser diode output into a beam having a beamwidth.
8. The communication link of claim 7, wherein the resulting
beamwidth may be adjusted.
9. The communication link of claim 8, wherein the beamwidth may be
adjusted between from 0.3 mrad to approximately 3.5 mrad.
10. The communication link of claim 8, wherein the beamwidth may be
adjusted by repositioning the lens.
11. The communication link of claim 7, wherein the lens collimates
the laser diode output into a beam having a beamwidth of less than
3.5 mrad.
12. The communication link of claim 6, wherein the laser driver
includes a modulation signal amplifier coupled with the splitter
and a DC bias circuit coupled between the modulation signal
amplifier and the laser diode.
13. The communication link of claim 12, wherein the laser driver
further includes a sampling photodiode, and the modulation signal
amplifier is coupled with the sampling photodiode and responsive to
the output of the sampling photodiode.
14. The communication link of claim 13, wherein the laser diode
emits a laser beam, and the sampling photodiode monitors the power
of the laser beam.
15. The communication link of claim 12, wherein said at least one
transceiver further comprises a thermoelectric cooler in thermal
communication with the laser diode.
16. The communication link of claim 6, wherein the laser driver
operates at a current of approximately between 100 milliAmperes and
1500 milliAmperes.
17. The communication link of claim 6, wherein the laser diode
generates an average power of at least 80 milliwatts.
18. The communication link of claim 17, wherein said at least one
transceiver has four laser diodes.
19. The communication link of claim 1, wherein said at least one
transceiver further comprises a visual sighting scope aligned with
the lasers.
20. The communication link of claim 1, wherein said at least one
transceiver further comprises a charge coupled detector.
21. The communication link of claim 1, wherein the one or more
digital signals comprises packet-based communication signals in
accordance with at least one data transmission protocol.
22. The communication link of claim 1, wherein the at least one
data transmission protocol complies with a protocol selected from
the group consisting of TCP/IP, IPX, Fast Ethernet, SONET, and
ATM.
23. The communication link of claim 1, wherein the transceiver is
capable of operating on different physical layers.
24. The communication link of claim 1, wherein the transceiver is
capable of operating on at least one layer selected from the group
consisting of STS-3, STS-12, OC-3, and OC-12.
25. The communication link of claim 1 wherein said at least one
transceiver further comprises: an aperture; a reflector in line
with the aperture; a photodiode at the focal point of the
reflector; and an output from the photodiode.
26. The communication link of claim 25, wherein the reflector has
an f-number of about 0.07
27. The communication link of claim 25, wherein the reflector is a
Mangin mirror.
28. The communication link of claim 25, wherein the reflector is a
parabolic reflector coupled with at least one corrector lens.
29. The communication link of claim 25, wherein the reflector is a
mirror having a general conic or aspheric optical surface and
coupled with at least one corrector lens.
30. The communication link of claim 25, wherein the link is capable
of transmitting and receiving broadband signals through free space
across a distance of at least eight kilometers in favorable weather
conditions.
31. The communication link of claim 25, wherein the link is capable
of transmitting and receiving broadband signals through free space
across a distance of at least approximately two kilometers in foggy
conditions according to a London, England fog environment with 99%
availability.
32. The communication link of claim 25, wherein said at least one
transceiver further comprises: a preamplifier coupled with the
photodiode; a regenerator coupled with the preamplifier; and an
output signal interface coupled with the regenerator.
33. The communication link of claim 32, wherein the second clock
and data recovery circuit may be switched between one of a
plurality of clock frequencies.
34. The communication link of claim 25, wherein said at least one
transceiver further comprises a background rejection filter near
the focal point of the reflector.
35. The communication link of claim 25, wherein the background
rejection filter is flat in shape.
36. The communication link of claim 25, wherein the background
rejection filter is hemispherical in shape.
37. The communication link of claim 25, wherein the background
rejection filter is a bandpass filter.
38. The communication link of claim 36, wherein the hemispherical
filter is an optical interference filter and has a nominal center
wavelength of approximately 1550 nanometers.
39. The communication link of claim 38, wherein the hemispherical
interference filter has a narrow bandwidth of approximately 100
nanometers.
40. The communication link of claim 34, wherein said background
rejection filter is a long wave pass filter having a threshold
passage wavelength, said at least one transceiver further comprises
a detector having a predictable responsivity roll-off at a
wavelength above the threshold passage wavelength of the long wave
pass filter.
41. The communication link of claim 25, further comprising a
controller.
42. The communication link of claim 25, further comprising a radio
frequency backup transceiver.
43. The communication link of claim 1, wherein said at least one
transceiver includes monitoring circuitry for monitoring signal
strength or transceiver status.
44. The communication link of claim 43, wherein the backup
transceiver is activated upon detecting impairment of the laser
transceiver, and the backup transceiver is deactivated upon
detecting non-impairment of the laser transceiver.
45. The communication link of claim 41, wherein said at least one
laser transceiver operates with the backup transceiver in overflow
mode.
46. The communication link of claim 1, wherein said at least one
laser transceiver is intended for outdoor use and further comprises
a protective enclosure.
47. The communication link of claim 46, wherein the enclosure
includes a housing.
48. The communication link of claim 47, wherein the housing
includes at least one heat sink.
49. The communication link of claim 48, wherein at least one heat
sink is integral to the housing.
50. The communication link of claim 48, wherein said at least one
laser transceiver further comprises a thermoelectric cooler.
51. The communication link of claim 48, wherein said at least one
laser transceiver further comprises a thermoelectric cooler in
thermal communication with laser diode and with the housing.
52. The communication link of claim 46, wherein said at least one
laser transceiver further comprises an environmental control system
for maintaining a desired temperature and humidity within the
enclosure.
53. The communication link of claim 47, wherein said at least one
laser transceiver further comprises a primary aperture, and the
enclosure includes a stray light baffle across the aperture.
54. The communication link of claim 53, wherein the stray light
baffle is an aluminum honeycomb baffle.
55. The communication link of claim 1, wherein said at least one
transceiver further includes a multiplexer to combine multiple
signal inputs and a de-multiplexer to segregate multiple signal
outputs.
56. A transceiver of one or more digital signals comprising: an
input signal interface for receiving the one or more broadband
digital signals; a regenerator coupled with the input signal
interface; a splitter coupled with the regenerator to split the one
or more digital signals into one or more laser data signals; a high
power, high frequency laser driver coupled with the splitter to
condition the laser data signals; and a plurality of lasers coupled
with the laser driver to receive the laser data signals, the lasers
being laterally displaced from one another and facing in parallel
directions.
57. The transceiver of claim 56, wherein the regenerator includes a
first clock and data recovery circuit, and the first clock and data
recovery circuit may be switched between one of a plurality of
clock frequencies.
58. The transceiver of claim 56, wherein each of the plurality of
lasers includes a laser diode coupled to the laser driver and
receiving the conditioned laser data signals.
59. The transceiver of claim 58, wherein each of the plurality of
lasers further includes a lens receiving the laser output and
collimating the output into a beam having a beamwidth of 3.5 mrad
or less.
60. The transceiver of claim 58, wherein the laser driver includes
a modulation signal amplifier coupled with the splitter and a DC
bias circuit coupled between the modulation signal amplifier and
the laser diode.
61. The transceiver of claim 60, wherein the laser driver further
includes a sampling photodiode, and the modulation signal amplifier
is coupled with the sampling photodiode and responsive to the
output of the sampling photodiode.
62. The transceiver of claim 61, wherein the laser diode emits a
laser beam, and the sampling photodiode monitors the power of the
laser beam.
63. The transceiver of claim 60, wherein the laser driver further
includes a thermoelectric cooler adjacent to the laser diode.
64. The transceiver of claim 58, wherein the laser driver operates
at a current of approximately between 100 milliAmperes and 1500
milliAmperes.
65. The transceiver of claim 58, wherein the laser diode generates
an average power of at least 80 milliwatts.
66. The transceiver of claim 65, there being four laser diodes.
67. The transceiver of claim 56, further comprising a visual
sighting scope aligned with the lasers.
68. The transceiver of claim 56, further comprising a charge
coupled detector.
69. The transceiver of claim 56, wherein the one or more digital
signals comprises packet-based communication signals in accordance
with at least one data transmission protocol.
70. The transceiver of claim 69, wherein the at least one data
transmission protocol complies with a protocol selected from the
group consisting of TCP/IP, IPX, Fast Ethernet, SONET, and ATM.
71. The transceiver of claim 69, wherein the transceiver is capable
of operating on different physical layers.
72. The transceiver of claim 69, wherein the transceiver is capable
of operating on at least one layer claim at least one layer
selected from the group consisting of STS-3, STS-12, OC-3, and
OC-12.
73. The transceiver of claim 56, further comprising: an aperture; a
reflector in line with the aperture; a photodiode at the focal
point of the reflector; a preamplifier coupled with the photodiode;
a second regenerator coupled with the preamplifier; and an output
signal interface coupled with the second regenerator; and
74. The transceiver of claim 73, wherein the reflector is selected
from the group consisting of Mangin mirror, parabolic reflector
coupled with a corrector lens, and mirror having a general conic or
aspheric optical surface and coupled with at least one corrector
lens.
75. The transceiver of claim 73, wherein the transceiver is capable
of communicating broadband signals through free space across a
distance of at least eight kilometers in favorable weather
conditions.
76. The transceiver of claim 73, wherein the transceiver is capable
of communicating broadband signals through free space across a
distance of at least approximately two kilometers in foggy
conditions according to a London, England fog environment with 99%
availability.
77. The transceiver of claim 73, wherein the regenerator includes a
first clock and data recovery circuit, and the first clock and data
recovery circuit may be switched between one of a plurality of
clock frequencies.
78. The transceiver of claim 77 further comprising a background
rejection filter adjacent to the focal point of the reflector.
79. The transceiver of claim 78, wherein the background rejection
filter is an optical hemispherical interference filter having a
nominal center wavelength of approximately 1550 nanometers.
80. The transceiver of claim 79, wherein the hemispherical
interference filter has a narrow bandwidth of approximately 100
nanometers.
81. The transceiver of claim 73, wherein the reflector has an
f-number of about 0.67.
82. An apparatus for efficiently driving a laser diode, the
apparatus comprising: a signal source providing an input signal; a
laser diode having a characteristic impedance; and a power
amplifier with a low output impedance suited to drive the laser
diode; wherein the power amplifier is operated as a
voltage-controlled current driver for the laser diode.
83. The apparatus of claim 82, further comprising a voltage
amplification stage between the signal source and the power
amplifier.
84. The apparatus of claim 83, wherein the voltage amplification
stage includes a non-linear limiting amplifier.
85. The apparatus of claim 82, wherein the power amplifier includes
a broadband RF power field effect transistor.
86. The apparatus of claim 82, wherein the broadband RF power field
effect transistor is operated with a low supply voltage.
87. The apparatus of claim 82, wherein the supply voltage of the
power amplifier is approximately equal to or less than 12
volts.
88. The apparatus of claim 82, wherein the supply voltage of the
power amplifier is approximately 5 volts.
89. The apparatus of claim 85, wherein the power field effect
transistor is selected from the group consisting of MOSFET, silicon
FET, and GaAs FET.
90. The apparatus of claim 85, wherein the broadband RF power field
effect transistor is capable of operating at a minimum frequency of
1 MHz or less.
91. The apparatus of claim 82, wherein the power transistor
provides output current of at least 100 mA to the laser diode.
92. The apparatus of claim 82, wherein the power transistor
provides output current of at least 200 mA to the laser diode.
93. The apparatus of claim 91, wherein the input signal is
characterized by a data rate of at least 10 Mbits/second.
94. The apparatus of claim 91, wherein the input signal is
characterized by a data rate of at least OC-3 bandwidth.
95. The apparatus of claim 82, wherein the input signal is a signal
selected from a group of protocols consisting of: TCP/IP, IPX, Fast
Ethernet, SONET, and ATM
96. The apparatus of claim 82, wherein the laser diode has a
characteristic dynamic impedance of between approximately 2 and 5
ohms.
97. The apparatus of claim 82, further comprising a thermoelectric
cooler in thermal communication with the laser diode.
98. The apparatus of claim 82, wherein the laser diode is
stabilized against temperature fluctuations.
99. The apparatus of claim 82, wherein the power amplifier is
stabilized against supply voltage fluctuations.
100. The apparatus of claim 99, further comprising a zener diode
for stabilizing power supply voltage against voltage
fluctuations.
101. The apparatus of claim 82, further comprising an attenuator
between the signal source and the power amplifier.
102. The apparatus of claim 101, wherein the attenuator is
adjustable and is used to control the amplitude of the input signal
to the power amplifier.
103. The apparatus of claim 82, further comprising: a temperature
sensor for sensing temperature of the laser diode, a thermoelectric
cooler in thermal communication with the laser diode, and a
thermoelectric cooler power amplifier, wherein the thermoelectric
cooler power amplifier is operated as a controlled current source
to supply current to the thermoelectric cooler at near-perfect
efficiency when maximum cooling is required.
104. The apparatus of claim 103, wherein the temperature sensor is
a thermistor.
105. The apparatus of claim 104, wherein the voltage drop across
the thermistor at a given temperature is compared to a reference
voltage corresponding to the thermistor voltage when it is operated
at a desired setpoint temperature.
106. The apparatus of claim 103, wherein the power amplifier is a
power FET.
107. A method for efficiently driving a laser diode, the method
comprising the steps of: providing a wideband input signal,
providing a power amplifier with a low output impedance suited to
drive a laser diode; generating a wideband output current from the
wideband input signal to modulate the laser diode, operating the
power amplifier as a voltage-controlled current driver for the
laser diode.
108. The method of claim 107, further comprising the steps of
selecting minimum, maximum, and average power levels for the laser
diode; supplying bias current to the laser diode to operate the
laser at the selected average power level supplying wideband
modulation to cause the laser output to vary between selected
minimum and maximum output power levels.
109. The method of claim 107, wherein the communication input
signal is characterized by a rate of at least 10 Mbits/second and
the power amplifier provides output current of at least 100 mA to
the laser diode.
110. The method of claim 107, wherein the power amplifier is
operated as a voltage-controlled current source by DC biasing the
power amplifier with a gate voltage to provide linear modulation of
the laser drive current.
111. The method of claim 108, wherein modulation of the power
amplifier output causes the laser drive current to swing from
nearly off to the desired output power with an optical power
extinction ratio of at least 10:1.
112. The method of claim 107, further comprising the step of
providing adaptive control of the output power of the laser
driver.
113. The method of claim 107, further comprising the step of
controlling the laser output power in multiple discrete steps.
114. The method of claim 113, wherein the step of controlling the
laser output power is accomplished by simultaneously controlling
the power amplifier gate bias voltage, bias current of the laser
diode, and modulation current of the laser diode using an input
signal.
115. The method of claim 113, wherein the power amplifier output
power is controlled in multiple discrete steps with a digital
control input signal characterized by at least 2 bits.
116. The method of claim 113, wherein an attenuator is provided,
and the digital control input signal is used to attenuate the
modulation signal.
117. The method of claim 108, further comprising the step of
imposing a narrowband modulation on the laser drive current.
118. The method of claim 117, wherein the narrowband modulation is
a telemetry signal.
119. The method of claim 117, wherein the narrowband modulation is
a tracking tone
120. The method of claim 117, wherein the frequency of the
narrowband modulation is between 50 Hz and 50 kHz.
121. The method of claim 108, further comprising the step of
monitoring laser bias current.
122. The method of claim 107, further comprising the step of
monitoring peak-to-peak amplitude of the laser modulation
current.
123. A method for operating a thermoelectric cooler, the method
comprising the steps of: providing a temperature sensor, a
thermoelectric cooler control circuit including a power amplifier,
a low voltage power supply, and a thermoelectric cooler, sensing
the temperature of a item to be temperature-controlled, operating
the power amplifier as a controlled current source to supply
current to the thermoelectric cooler at near-perfect efficiency
when maximum cooling is required.
124. The method of claim 123, wherein the sensor is thermistor
125. The method of claim 123, further comprising step of providing
a voltage divider circuit and a regulated supply voltage, wherein
the thermistor is part of the voltage divider circuit.
126. The method of claim 123, wherein the thermoelectric cooler has
a characteristic impedance and an optimal operating current, and
the item to be temperature controlled has a maximum cooling
requirement, the method further comprising the steps of: selecting
optimal operating current of the thermoelectric cooler to
correspond with the maximum cooling requirement of the item to be
temperature controlled, and selecting the impedance of the
thermoelectric cooler to drop substantially all of the supply
voltage when the thermoelectric cooler is operated to provide the
maximum cooling requirement of item to be temperature
controlled.
127. The method of claim 123, wherein the power supply voltage is
approximately 5 volts or less.
Description
BACKGROUND OF THE INVENTION
[0001] The field of the present invention is laser
communications.
[0002] High quality video and audio signals and high bandwidth data
signals (called "broadband" signals) are becoming increasingly
desirable in today's digital world. A significant challenge is
getting high bandwidth communications to end users, or reaching the
so-called "last mile" market segment. Most U.S. metro centers are
serviced by multiple providers over SONET fiber optic rings, with
fiber to certain major buildings. Many, if not most, buildings, are
not on fiber rings, however, and laying fiber can be time consuming
and prohibitively expensive. In some instances, it may be
practically impossible to obtain property rights-of-way to provide
a high-bandwidth connection to the desired location.
[0003] While wireless radio frequency (RF) systems can provide data
rates of 155 Mbps, there is limited spectral bandwidth available,
communication licenses are generally required, the possibility for
mutual interference exists, and the requisite equipment is
expensive. Extending to higher data rates is difficult for RF
frequencies with good atmospheric propagation characteristics.
[0004] Atmospheric laser communication provides a potential
alternative for wireless point-to-point communications of high
bandwidth signals. For instance, laser transceivers are capable of
sending high bandwidth signals through the atmosphere. However,
commercially available laser systems capable of transmitting high
bandwidth signals across distances longer than a small city block
are prohibitively large and extremely expensive. Moreover, several
challenges must be overcome to facilitate high bandwidth laser
communications over significant distances. One consideration is
ensuring reliable communications despite varying atmospheric
conditions. Since conditions such as fog in particular are
difficult for low power laser beams to penetrate, ensuring
uninterrupted atmospheric laser communications requires the use of
high power lasers. A second design consideration is preventing high
power laser beams used in an atmospheric laser system from causing
eye or tissue damage if received by people. At short wavelengths,
non-eyesafe power levels can permanently damage the eye before the
victim becomes aware, because the retina has no pain sensors
[0005] Further complicating the use of atmospheric lasers is a
phenomenon called scintillation that causes the random fading of
signals transmitted through the atmosphere. It is understood that
the atmosphere is not homogeneous, in that the index of refraction
of air is not constant due to wind or turbulence. The transmission
of a beam of light through the atmosphere is subject to these
variations in the index of refraction such that the beam may be
momentarily deflected from a straight path. With such deflection,
an observer of the beam perceives the source to be flickering. Such
flickering is highly disruptive to data transmission. A solution
may be found in aperture averaging, by increasing the size of the
apertures of the receiving unit. The intensity of the source can,
to a certain extent, mitigate losses in transmission where the
sensitivity of the receiver is not correspondingly decreased.
Often, however, physical and practical limitations detract from
such solutions.
[0006] To significantly overcome the effect of scintillation,
spatial diversity transmitters have been constructed which employ
multiple diode lasers arranged to produce displaced parallel beams.
As these beams diverge, they overlap one another. A receiver
displaced from the transmitter thus receives uncorrelated light at
the receiver when aligned with the beams. As it is unlikely that
all beams will be simultaneously diverted, the receiver is able to
receive uninterrupted data from at least some of the plurality of
transmitters. It has been found that the normalized standard
deviation of the intensity at the receiver is reduced by the square
root of the number of transmitting elements when properly
separated. Reference is made to W. M. Bruno, R. Mangual, & R.
F. Zampolin, Diode Laser Spacial Diversity Transmitter, pp.
187-194, SPIE.: vol. 1044, Optomechanical Design of Laser
Transmitters and Receivers (1989), the disclosure of which is
incorporated herein by reference.
[0007] One structural application of the very principles presented
in the foregoing publication is found in U.S. Pat. No. 5,777,768,
the disclosure of which is also incorporated herein by reference.
Transceivers using spaced multiple laser transmitters are used for
two-way communication.
[0008] Another example of laser transceivers used for
communications purposes may be found in application Ser. No.
09/434913, filed Nov. 5, 1999, for a Portable Laser Transceiver,
the disclosure of which is further incorporated herein by
reference. The portable laser transceiver disclosed therein is
capable of transmitting near-broadcast quality video, audio, and
Ethernet signals.
[0009] For broadband fiber optic applications, a number of
pre-fabricated integrated circuits are available for driving lasers
at data rates of 1 gigabit per second (Gbps) or more. These laser
drivers are used to drive the now-common fiber optic networks.
These integrated circuits, however, are inadequate for high power
lasers used in atmospheric laser communications, as they typically
provide drive current capability of only 50 to 75 mA. Such low
drive current is insufficient to overcome the effects of
atmospheric scintillation at distances beyond of approximately the
length of a laboratory.
[0010] When a high power laser is used, one method that has been
used to achieve the high drive current needed to overcome
atmospheric scintillation effects is the use of a RF Bias-Tee. This
method typically uses a 50 ohm bias tee, thus coupling the
broadband signal into a 50 ohm load--typically consisting of a 47
ohm matching resistor in series with a 3 ohm laser diode--to
achieve a broadband match. The RF bias-tee approach, however, is
not practical for high drive currents because the majority of the
output power is wasted in the matching resistor. For example, a 700
mA drive current typically results in 5.8 watts of power
dissipation in the 50 ohm bias tee.
[0011] A high current 4:1 broadband RF transformer may be used with
a bias-tee approach to double the output drive current and
transform the 50 ohm into a 12.5 ohm source, as seen by the load.
However, this alternative approach still requires a 9 ohm resistor
to match the source to a 3 ohm laser diode. Thus, the majority of
the drive power is still wasted in the matching resistor. A
transformer with a higher ratio could theoretically solve the lost
power problem, but high ratio transformers capable of handling
currents in excess of 200 mA and having a broadband response of up
to 1 Ghz are not available.
[0012] U.S. Pat. No. 5,521,933 discloses a method of positioning
the laser diode remotely from the driver circuit to reduce the
effect on the laser diode of heat generated by the driver circuit.
This method still uses a matching resistor, located remotely from
the laser diode, which again causes power loss in the output drive
current.
[0013] Regardless of the driving frequency, when driving a laser
diode at high power, it is frequently desirable to use a
thermoelectric cooler (TEC) to maintain the temperature stability
of the laser diode. In many applications, the temperature stability
of the laser diode may be important to maintain the output signal
of the laser diode within a specified set of parameters. The
cooling function of a TEC is controlled by a TEC controller
circuit. Typical implementations of TEC controllers are either
pulse-width-modulated (PWM) or proportional controllers. PWM
controllers are undesirable for use in communication systems with
sensitive receivers because PWM controllers tend to generate
unwanted noise. Proportional controllers such as the
proportional-integral-differe- ntial (PID) type are therefore
commonly used in such communication systems.
[0014] PID controllers, however, tend to dissipate the most heat
when maximum cooling at the laser diode is required. The heat
dissipation occurs because PID controllers function as a current
source, having a compliance voltage that is significantly less than
the supply voltage. For example, a PID controller operating off a
5V supply at its maximum rated current output typically has a
useable compliance voltage of about 3V. The difference between the
supply voltage and the useable compliance voltage tends to be
dissipated in the controller as heat. Thus, in the most demanding
cooling conditions such as hot weather, a PID controller tends to
generate even more heat.
[0015] A need, therefore, exists for small and efficient, yet
powerful laser transceivers that are capable of transmitting and
receiving high power and high bandwidth signals across distances
greater than a single city block. A need also exists for a means to
efficiently cool high powered laser transmitters used in such
transceivers.
SUMMARY OF THE INVENTION
[0016] The present invention is directed to transceivers using
laser light as the carrier and methods for laser communication for
transmitting broadband signals. Transmitter and receiver modules
are contemplated. Use of a high power, high frequency laser driver
facilitates free space communication across distances of at least
eight kilometers in favorable weather conditions, and at least
approximately two kilometers in foggy conditions according to a
London, England fog environment with 99% availability.
[0017] In a first separate aspect of the invention, a high power,
high frequency laser driver includes a power amplifier with a low
output impedance suited to drive a laser diode. The power amplifier
is operated as a voltage-controlled current driver for the laser
diode. The laser driver is capable of providing very high current
modulation, at least 100 mA, at high data rates, at least 10 Mbps,
to a laser diode.
[0018] In a second separate aspect of the invention, a laser
transceiver includes an input, a regenerator, a splitter receiving
signals from the regenerator, a plurality of high power and high
data rate laser drivers, and a plurality of lasers transmitting
high bandwidth signals of the splitter. At least one digital signal
may be transmitted.
[0019] In a third separate aspect of the invention, a transceiver
having a regenerator includes a clock and data recovery circuit
having multiple switchable digital data rates.
[0020] This switching may be performed by software.
[0021] In a fourth separate aspect of the invention, a laser
transceiver includes a fast reflector characterized by a low
f-number, a long wave pass background rejection filter adjacent to
the focal point of a the reflector, and a photodiode having a
predictable responsivity roll-off at a wavelength above the long
wave pass frequency. The resulting combination effectively operates
like a bandpass filter.
[0022] In a fifth separate aspect of the invention, a high power
laser diode is stabilized against temperature fluctuations with
highly efficient thermoelectric cooling system and method. Aided by
a temperature sensor, a thermoelectric cooler in thermal
communication with a laser diode is coupled with a power amplifier
that is operated as a controlled current source to supply current
to the thermoelectric cooler at near-perfect efficiency when
maximum cooling is required.
[0023] In a sixth separate aspect of the invention, a stand-alone
radio frequency transceiver is operated with at least one laser
transceiver in overflow mode. A router is used to monitor and
distribute incoming data signals between the laser and RF
transceivers to promote enhanced total system bandwidth capability
and low switching latency.
[0024] In a seventh separate aspect of the invention, a laser
transceiver is intended for outdoor use and included within a
protective enclosure, the apparatus further including an
environmental control system to maintain appropriate conditions for
the transceiver to operate.
[0025] In a eighth separate aspect of the invention, a laser diode
is thermally coupled with a thermoelectric cooler and then enclosed
within a housing having integral heat sinks to draw heat away from
the diode by way of the thermoelectric cooler.
[0026] In an ninth separate aspect of the invention, a transceiver
has a plurality of laser diodes, each of which is coupled with a
lens for receiving and collimating the laser output into a beam,
with the lenses subject to beamwidth adjustment. Beamwidth
adjustment between 0.3 and greater than 3.5 mrad is provided.
[0027] In a tenth separate aspect of the invention, a laser
transceiver that is independent of particular digital protocol and
is switchable between different data rates is provided. The
transceiver may operate on different physical layers including
STS-3, STS-12, OC-3, and OC-12, and support various digital signal
protocols including TCP/IP, IPX, Fast Ethernet, SONET, and ATM.
[0028] In an eleventh aspect of the invention, any of the foregoing
aspects are contemplated in combination for additional
advantage.
[0029] Accordingly, it is a principal object of the present
invention to provide an improved transceiver using laser
communications, and an improved laser driver. Other and further
objects and advantages will appear hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] In the figures, wherein like numbers reflect elements having
similar function:
[0031] FIG. 1 is a schematic drawing of a laser communication
system;
[0032] FIG. 2 is a front view of a transceiver according to a first
embodiment;
[0033] FIG. 3 is a side view of the transceiver in FIG. 2;
[0034] FIG. 4 is a schematic diagram of a transceiver and
associated devices according to a second embodiment;
[0035] FIG. 5 is a side sectional view of a transceiver and
associated housing according to a third embodiment.
[0036] FIG. 6A is a front perspective view of a front portion of
the housing depicted in FIG. 5.
[0037] FIG. 6B is a rear perspective view of a front portion of the
housing depicted in FIG. 5.
[0038] FIG. 7A is a front perspective view of a rear portion of the
housing depicted in FIG. 5.
[0039] FIG. 7B is a front perspective view of a rear portion of the
housing depicted in FIG. 5.
[0040] FIG. 8 is a schematic diagram of a transceiver according to
a fourth embodiment.
[0041] FIG. 9 is a schematic diagram of a laser driver that may be
used with the transceivers of FIG. 5 or 8;
[0042] FIG. 9A is a first magnified portion of the schematic
diagram depicted in FIG. 9.
[0043] FIG. 9B is a second magnified portion of the schematic
diagram depicted in FIG. 9.
[0044] FIG. 9C is a third magnified portion of the schematic
diagram depicted in FIG. 9.
[0045] FIG. 10 is a schematic diagram of a complementary circuit
for the laser driver of FIG. 9.
[0046] FIG. 10A is a first magnified portion of the schematic
diagram depicted in FIG. 10.
[0047] FIG. 10B is a second magnified portion of the schematic
diagram depicted in FIG. 10.
[0048] FIG. 10C is a third magnified portion of the schematic
diagram depicted in FIG. 10.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0049] Turning in detail to the drawings, FIG. 1 illustrates a
laser communication system 10 comprising a first laser transceiver
12 and a second laser transceiver 14, each transceiver 12, 14
having a digital signal input 20 and digital signal output 22 that
carry the communication signals being transmitted and received. The
signal input 20 and the signal output 22 may carry either
electrical or optical signals of various types. Compatible
electrical signals may be carried on electrical cables such as
Category 5 cable with RJ 45 connectors, for example, and the
signals may conform to protocols such as TCP/IP, IPX, Fast
Ethernet, or others known in the art, operating on physical layers
such as STS-3, STS-12, or others known in the art. Compatible
optical signals may be carried on fiber optic cables, and the
signals may conform to protocols such as SONET, ATM, or others
known in the art, operating on physical layers such as OC-3, OC-12,
or others known in the art.
[0050] Electronics 16, 18 within each transceiver 12, 14 use the
digital signal input 20 to drive the outgoing laser signals and
convert the incoming laser signals into appropriate digital signal
output 22. Preferably, the laser communication system 10 is capable
of transmitting and receiving laser signals a bandwidth of at least
155 Megabits per second over distances of at least eight kilometers
in favorable weather conditions, and at least approximately two
kilometers in foggy conditions according to a London, England fog
environment with 99% availability. More preferably, the system is
capable of transmission across like distances and conditions at a
bandwidth of at least 622 Megabits per second.
[0051] Preferably, each laser has a nominal output wavelength of
1.5 .mu.m. One of the principal advantages of 1.5 .mu.m wavelength
is that are eyesafe intensity levels are approximately 50 times
greater than at 0.8 .mu.m. This is because for near-infrared
wavelengths longer than 1.4 .mu.m, the light is absorbed by the
lens and cornea, and is not focused onto the retina. For 1.5 .mu.m,
the eyesafe power levels are the same as levels for heating of the
skin, as it is the same effect. Also, at short wavelengths,
non-eyesafe power levels can permanently damage the eye before the
victim becomes aware, because the retina has no pain sensors. At
1.5 .mu.m, if non-eyesafe power levels are encountered, the
sensation of heat (or even pain) can be felt on the surface of the
eyeball, and the natural blink reflex is induced.
[0052] The use of 1.5 .mu.m wavelength has other benefits. Since
the eyesafe intensity level at 1.5 .mu.m is 50 times greater than
at 0.8 .mu.m, and since the intensity is inversely proportion to
the square of the aperture diameter, this means that for a given
power level, the diameter of the transmitting aperture can be seven
times smaller at 1.5 .mu.m than 0.8 .mu.m. This facilitates the use
of multiple transmitting apertures for scintillation reduction.
Additionally, atmospheric attenuation is reduced at 1.5 .mu.m
relative to shorter wavelengths. The 1.5 .mu.m wavelength region
also corresponds to the low loss region for modern fiber
optics.
[0053] FIGS. 2 and 3 illustrate one embodiment of a transceiver 50
directed to laser communication. FIG. 2 shows a front view of the
transceiver 50 having a front plate 52 with a large centrally
located receiving aperture 54 and four laser apertures 56 spaced
about the receiving aperture 54. The diameter of the receiving
aperture 54 is approximately twenty (20) centimeters. A narrow
support beam 58 crosses the receiving aperture 54 and supports a
photodiode 60, a background filter 61, and, if needed, one or more
field correcting lenses 63, all of which are centrally located in
relation to the receiving aperture 54.
[0054] High power laser transmitters 62 are disposed within each of
the laser apertures 56 such that the laser transmitters 62 emit
beams that are substantially parallel to each other. Each laser
diode 62 responds to an amplified laser data signal to generate an
intensity modulated light signal. Each laser 56 preferably produces
an average power of at least eighty (80) milliwatts and has a
nominal wavelength of 1,550 nanometers (1.5 .mu.m), within the
eyesafe region. As the laser diode 62 tends to emit a wide angle
signal of about 30.degree. cone angle, a lens 55 receives the laser
output. The lens design is flexible in setting the beamwidth; that
is, the beamwidth may be adjusted during production by adjusting
the spacing between the lens 55 and the laser diode 62. This
adjustment may be made by way of a threaded connection, or, more
preferably, by using a vacuum chuck and micrometer positioner to
position the lens. Once the desired lens position is reached, the
lens 55 may be epoxied in place. After the epoxy is cured, the
vacuum is released, and the micrometer positioner is removed. The
range of beamwidth (i.e. beam divergence angle) adjustment provided
by the lens 55 ranges from 0.3 mrad to greater than 3.5 mrad. If
the laser diode 54 has a spatially elliptical output, a
circularizing optic (not shown), which may be a cylindrical lens or
prism pair, may be added. As noted before, the lasers 54 are all
aligned such that the collimated beams are parallel.
[0055] A sighting scope 64 may be located along an outer portion of
the front plate 52 for aiming the transceiver 50 at a second
transceiver (such as the transceiver 14 shown in FIG. 1). As the
sighting scope 64 is intended for initial alignment of a
transceiver pair (e.g., the laser transceivers 12, 14 shown in FIG.
1), the sighting scope 64 may be removed after this alignment is
attained. Alternatively, a CCD (charge coupled device) sensor 65
may be added to a transceiver 50 and used to facilitate alignment
of a transceiver pair. Connection of the CCD 65 (or CCD 256 in FIG.
8) with a controller 250 is illustrated schematically in FIG. 8.
When the CCD 65, 256 is coupled with a controller 250, a frame
grabber 257 extension to the controller 250 may be used to capture
images of what is within the line of sight of the transceiver 12.
Such images may be transmitted or downloaded from the controller
250 to a remote location, with an external interface operating a
protocol such as SNMP, Ethernet, or another protocol known in the
art.
[0056] Referring now to FIG. 3, the front plate 52 is connected to
a rear plate 66 using support rods 68. The rear plate 66 supports a
focusing reflector 70, which is centrally aligned with the
receiving aperture 54 such that the photodiode 60 is positioned at
the focal point of the focusing reflector 70. The focusing
reflector 70 is approximately equal in diameter to the aperture 20
and facing in a parallel direction with the lasers 62.
[0057] The receiver portion of the transceiver 50 includes a
receiving aperture 54, a focusing reflector 70, a photodiode 60, a
background rejection filter 61, and electronics 72. The focusing
reflector 70 focuses the incoming optical beam onto a photodiode
receiver 60. The fast design enables the large aperture 54 for
collection efficiency, yet provides a very short focal length--both
for compactness, and to achieve the largest field-of-view for a
given detector diameter. The preferred f-number of the reflector 70
is approximately 0.67. As the f-number of the focusing reflector 70
decreases, off axis optical aberrations increase.
[0058] Mangin mirror or parabolic reflector approaches may be used
for the focusing reflector 70 according to separate embodiments. A
catadioptric design, the Mangin mirror is a negative meniscus lens
with a mirrored rear surface which combines a compact overall
design with a short focal length. The presence of both reflective
and refractive elements in a Mangin mirror provides sufficient
degrees of freedom to keep the amount of optical aberration within
acceptable limits over the entire optical field of view, such that
no additional optical corrector lens is necessary.
[0059] A parabolic reflector may be used for the focusing reflector
70 according to an alternative embodiment. A parabolic reflector
has a single reflective surface, and requires one or more field
corrective lenses to sufficiently correct aberrations. The
parabolic reflector may therefore comprise one or more optical
elements 63 to control the amount of aberration near the edges of
the field of view. In placing the corrector lenses 63 near the
focal plane, the corrector lenses 63 may be mounted in the same
mechanical assembly that holds the detector 60. Multiple corrector
lenses may be placed thusly to provide a higher degree of
aberration control.
[0060] In a third reflector embodiment, the focusing reflector 70
comprises a mirror having a more general conic or aspheric optical
surface and one or more corrector lenses (not shown). A conic or
aspheric mirror provides a limited amount of aberration control,
thus reducing the number of corrector lenses needed to properly
focus the incoming signals.
[0061] To reduce interference and background noise from ambient
light and other sources, the optical path of the receiver
advantageously includes a flat or hemispherical background
rejection filter 61. The filter 61 may be comprised of alternating
layers of dielectric material deposited on a glass or crystalline
substrate. This construction results in a peak transmission at band
center of over 65%. The filter 61 may be located at the receiving
aperture 54. However, such a configuration requires a large and
costly filter, adding weight and size to the assembly 50.
Preferably the filter 61 is located between the focusing reflector
70 and the photodiode 60. A bandpass filter nominally centered at
1500 nanometers may be used. However, if the bandpass filter is a
flat multilayer dielectric filter, when receiving light rays from
both small and large angles of incidence with respect to normal
characterized by a low f-number optical system, then the passband
is shifted for the different angles of incidence. To correct this,
then, a much larger passband for a flat filter is to be used (about
a 400 nanometer filter), or a hemispherical filter may be used
since all of the incident rays are normal to the filter surface. A
hemispherical filter centered at the nominal laser wavelength of
1,550 nanometers and having a passband width of 100 nanometers or
less may be used.
[0062] A bandpass filter inherently requires the use of multilayer
dielectric technology. A long wave pass approach may be used in
conjunction with a detector 60 having predictable responsivity
roll-off to create an effective bandpass filter. In one embodiment,
the detector responsivity rolls off significantly between 1500 and
1700 nm, and is effectively zero beyond 1700 nm. Coupling this
detector 60 with a long wave pass filter transmitting 1500 nm and
above wavelength, the resulting combination provides an effective
bandpass filter. A long wave pass filter, as used in conjunction
with a detector as described above, may be dielectric, and flat or
hemispherical in shape. As an alternative to a long wavepass
multi-layer dielectric filter, the filter 61 may be an absorptive
filter.
[0063] If a hemispherical filter 61 is used, its center of
curvature is located near the focal point of the reflector 70.
Using a standard TO-8 size detector package results in a
hemispheric filter dome having an outer diameter of approximately
twenty-two (22) millimeters and a thickness of 2.5 millimeters. In
an embodiment utilizing one or more corrector lenses 63, the
corrector lenses 63 may be used to create an a focal region in
front of the photodiode 60 in which a filter 61 having a flat shape
would be placed.
[0064] The transceiver may also include a sealed protective
enclosure 74 as shown in FIG. 3. The protective enclosure 74
protects the transceiver from weather conditions and provides an
enclosed environment in which the transceiver may operate. At the
front cover of the protective enclosure 74 is an acrylic filter 76
covering the large aperture 54. The acrylic filter 76 is
transparent to the operational wavelength of the laser transmitters
62, but limits transmission of visible light to prevent
introduction of noise and heat from this light. Further, a stray
light baffle 75 is preferably placed between the transceiver 12 and
the acrylic filter 76 to reduce interference from stray light, and
electromagnetic and RF sources. The baffle 75 is preferably a
honeycomb of thin aluminum, approximately three inches in
thickness, with a face-to-face hex cell size of approximately
{fraction (11/32)} inch.
[0065] FIG. 4 is a schematic representation of a laser transceiver
100 according to another embodiment. The transceiver 100 includes a
combined laser transmitter/laser driver module 110, a receiver
module 120, and associated electronics 130. Starting with the
receiver module 120, an incident laser beam is reflected by a
reflector 122 and focused through a background rejection filter 123
onto a photodiode 124 located at the focal point of the focusing
reflector 122. From the photodiode 124, the signal is carried to a
preamplifier 125 preferably contained within the laser receiver
module 120, and then sent to signal conditioning electronics 131.
After conditioning, the signal is provided to an output signal
interface 132, which may include a fiber transmitter for
transmitting an optical signal to a fiber optic cable. Electrical
signals may alternatively be output, such as via a Category 5
cable. A switching device 140, which may include a switch or router
portion to enable transmission of various electrical or optical
signals including TCP/IP, IPX, Fast Ethernet, SONET, ATM, or other
signal types, on various physical layers such as STS-3, STS-12,
OC-3, or OC-12) receives the output signal.
[0066] In the illustrated embodiment, the switching device 140 is
coupled to a computer 142 having a network interface card 143. The
personal computer 142 may further have an audio/video interface
card 144 for receiving audio and video signals. The switching
device may be connected with various electrical or optical input
types, such as Category 5 cable using RJ-45 connectors, or fiber
optic cable. An optional stand-alone RF (radio frequency) backup
transceiver 146 may further connect to the switching device
140.
[0067] On the input/transmitter side, high bandwidth digital input
signals are provided to the transceiver 100 via a switching device
140. A high bandwidth signal is conventionally considered to be a
signal of 10 Mbps or greater. One skilled in the art will recognize
that any high bandwidth signal may be used as input to the
transceiver 100, so long as appropriate electronics are provided,
when necessary, to convert the high bandwidth input signal into a
high bandwidth digital signal. Input signals may be provided
through the switching device 140 to the transceiver 100 by way of
an input signal interface 134, which in one embodiment may include
a fiber optic receiver for receiving digital signals from a fiber
optic cable. The signal is then provided to signal conditioning
electronics 135, and thereafter provided to a splitter 136. The
splitter 136 splits the signal into four identical signals at
one-quarter of the power. Each signal emerging from the splitter
136 is provided to a laser driver circuit 111 and ultimately
provided to a laser diode 115. The laser driver circuit 111
includes a DC bias circuit 112 and a high power, high bandwidth
power amplifier 114 for driving the laser diode 115. A high power
laser driver is conventionally considered to be a laser driver
operating at a nominal level of 100 mA or greater. The high power,
high bandwidth laser driver circuit is discussed in detail
hereinafter, in connection with FIGS. 9-10C. The output of the
laser diode 115 is provided to a lens 116, and may be monitored
with a fiber optic segment 117 and fed to a photodiode 118 for
monitoring the output of the laser diode 115. A thermoelectric
cooler 119 and its associated controller 113 are provided to
control the temperature of the laser diode 115. The controller 113
is coupled with the thermoelectric cooler 119, which is in thermal
communication with the laser diode 115. The controller 113 and the
thermoelectric cooler 119 act in conjunction with a temperature
sensor (not shown) to stabilize the temperature, and thus the
output power, of the laser diode 115. The optimum operational
temperature of each laser diode 115 depends on the specifications
for the particular laser diode used, as provided by the laser diode
manufacturer.
[0068] Preferably, output of the laser diode 115 is monitored. One
method of monitoring the laser diode 115 is by monitoring its input
current, but this method is indirect. A better method is to monitor
the transmitted light signal from the laser diode 115. One method
of monitoring the light signal is by way of a fiber optic element
117 that may extend into the transmitted light signal from the
laser diode 115. This fiber optic element 117 is coupled with a
photodiode 118. More preferably, the laser diode 115 is provided
with an integral monitor photodiode (not shown), which eliminates
the need for a separate fiber optic element 117. The output of the
photodiode may be coupled with the modulation signal amplifier 114
to use the sampled laser diode output for controlling the amplifier
114 and in turn the signal strength, thus permitting operation in
constant optical power mode. Thus, the modulation signal amplifier
114 may be made responsive to the output of the photodiode 118.
[0069] FIGS. 5, 6A, 6B, 7A, and 7B illustrate an alternative
compact and high power laser transceiver 150 and its associated
housing 151. The housing 151 is designed to both mount the
operative components and serve as a protective enclosure suitable
for outdoor use. Only three parts, namely, a front portion 152, a
rear portion 153, and a snout 182, are designed to interconnect to
form the housing 151 to fully enclose the transceiver 150. Of these
three parts, the optical payload mounts to only one: the front
portion 152. This construction promotes precise positioning of the
optical components, since in comparison to the embodiment of FIGS.
3-4, this approach limits the adverse effects of additive
dimensional tolerances for multiple structural parts. The front
portion 152 includes prominent annular heat sinks 184 to dissipate
heat from laser diodes (such as the laser diodes 115 shown in FIG.
4) and from laser drive circuits and associated electronics (such
as shown in FIG. 4) contained within the housing. More
specifically, an inner surface of each heat sink 184 is in thermal
communication with at least one thermoelectric cooler (such the
pair of thermoelectric coolers shown in FIG. 8), which in transmit
heat from the laser diodes (such as shown in FIGS. 4, 8) to the
heat sinks 184. Preferably, an annular thermoelectric cooler
disposed around the laser diode 115 is used.
[0070] The front portion 152 is preferably cast from a lightweight
aluminum alloy, but may alternatively be manufactured by various
techniques from a variety of suitably durable, strong, and
thermally conductive materials, including steel. The housing 151 is
designed to mate with the optical payload at four flat interior
payload mating surfaces 179 adjacent to the laser apertures 156,
preferably by way of screws and tapped holes. The payload mating
surfaces 179 further serve as a primary contact surface for
transferring heat from laser diodes to the heat sinks 184 by way of
thermoelectric coolers. The cast housing 151 is preferably machined
along the payload mating surfaces 179 to ensure that all four
payload mating surfaces 179 are commonly flat. The front portion
152 is preferably painted to reduce effects of corrosion when
subjected to outdoor use. The rear portion 153 does not contain any
dedicated heat sinks 184, and therefore does not necessarily need
to be fabricated from a thermally conductive material. The rear
portion 153 may be manufactured from a plastic suitable for outdoor
use, but could also be manufactured using other techniques and
other durable materials including metals.
[0071] The front portion 152 and rear portion 153 mate along a
common surface 155 to form the housing 151 that encloses a
reflector 170, four laser transceivers (not shown), a photodiode
160, a background rejection filter 161, one or more field corrector
lenses 163, and various electronics 172. The photodiode 160,
background rejection filter 161, and one or more field corrector
lenses 163 are disposed at the focal point of the reflector 170,
and supported by way of a narrow support rod 158 placed across a
primary aperture 154. Adjacent and connected to the front portion
152 of the housing 151 is a snout 182 across the primary aperture
154. The snout 182 preferably includes a honeycomb baffle 175 and
acrylic filter 176 for the same reasons discussed above in another
transceiver embodiment. The snout further includes a protruding
hood 183 that further reduces interference from incident light and
provides a measure of weather protection for the front cover
177.
[0072] Wired (electrical or optical) data and power signals to and
from the transceiver 150 are carried to the transceiver 150 by a
conduit 186. The transceiver 150 may be supported from below by way
of a mounting element 187. The mounting element 187 preferably the
angular position of the transceiver 150 to be adjusted to
facilitate aiming the transceiver 150 at a similar transceiver
located remotely.
[0073] FIG. 8 provides a schematic representation of a wideband
laser transceiver 200 according to a fourth embodiment. On the
input side, the transceiver 200 includes various input signal
electronics 202, a laser driver circuit 220, and a laser
transmitter module 225 with associated thermoelectric coolers 226,
227. On the output side, the transceiver 200 includes a laser
receiver module 229 and various output signal electronics 230. In
the illustrated embodiment, the transceiver 200 further includes an
associated controller 250 in communication with an environmental
control system 252, at least one temperature sensor 254, a CCD
sensor 256, and an interface for diagnostics, monitoring, network
management, and/or control 258. The controller 250 may further
include a frame grabber 257, may receive signal level information
from the receiver module 229, and may receive monitoring
information from the laser diode.
[0074] Turning to the input/transmitter side, a first input signal
is provided to an input signal interface 204. The input signal is a
wideband digital signal, either electrical or optical, according to
a variety of signal types (including TCP/IP, Fast Ethernet, SONET,
ATM, or other signal types known in the art, on various physical
layers such as STS-3, STS-12, OC-3, or OC-12). Various electrical
or fiber optic cable connectors may be received by the input signal
interface 204 depending on design requirements. Upon receipt of the
digital input signals, these signals are provided to a regenerator
206. The regenerator 206 preferably includes either a limiting
amplifier 207 or automatic gain control 208 to provide gain, as
necessary, and smooth out variations in input signal amplitude. The
regenerator 206 further includes a first clock/data recovery
Circuit 210 to provide protocol independence. The first clock and
data recovery circuit 210 includes at least one oscillator 212,
such as a crystal, to regulate the clock frequencies. When more
than one oscillator is provided, the integrated circuit utilizes
two phase locked loops to permit modulation as different
frequencies, so as to accommodate more than one data transmission
protocol, such as TCP/IP, Fast Ethernet, SONET, and ATM. Thus, a
single transceiver 200 is enabled to transmit and receive more than
Dne of the aforementioned protocols. Switching between clock
frequencies may be performed by manually substituting an existing
oscillator 212 with an oscillator of a different frequency. More
preferably, as shown in FIG. 8, multiple oscillators 212, 213 may
be built into the transceiver 200, and a switch 214 may accomplish
switching between the oscillators 212, 213. This switching may be
controlled manually, or, more preferably, by a controller 250, such
as by using transistor-transistor logic. Either oscillator 212, 213
provides a clock signal to the data recovery electronics 215.
[0075] Upon regeneration, the digital signal may be provided to an
optional multiplexer 216 to be combined with at least one
additional signal from a second input signal source 203. The
additional signal may be of various types, such as T1, Ethernet,
telemetry information, tracking information, or other signals known
to those skilled in the art. Thereafter, the combined signal is
provided to a splitter 218 that divides the digital signal input
into four substantially equal components comprising the same signal
input at one-quarter of the power. The four laser data signals are
transported to a plurality of high power, high frequency laser
driver circuits 220, each of which modulates a laser diode
contained within the laser transmitter module 225. Only one of the
four laser drivers 220 is illustrated. The remaining three laser
drivers 220 and laser diodes (within the laser transmitter module
225) are preferably identical to those depicted. The laser diodes
within the transmitter modules 225 are displaced from one another,
aligned, and facing in parallel directions. As previously
described, each diode preferably has a collimating lens (not
shown).
[0076] In addition to modulating the laser diode within the laser
transmitter module 225, the laser driver circuit 220 controls
thermoelectric coolers 226, 227 associated with the laser diode to
stabilize the temperature, and thus the output power, of the laser
diode. Further details regarding control of thermoelectric cooler
associated with the transceiver 200 will be discussed
hereinafter.
[0077] Turning to the output/receiver side, an incident laser beam
is received by the laser receiving module 229 and converted to a
preamplified electrical signal. The signal is provided to output
signal electronics 230, which includes several elements. First the
signal is provided to a second regenerator 232. The regenerator 232
preferably includes either a limiting amplifier 234 or automatic
gain control 235 to provide gain, as necessary, and smooth out
variations in signal amplitude. The second regenerator 232
preferably further includes a second clock/data recovery unit 236.
The second clock and data recovery unit 236 preferably also
comprises a switchable dual oscillator such as the dual oscillator
212, 213 shown in connection with the first clock and date recovery
unit 210, and operates in a similar manner. Upon regeneration, the
resulting digital electronic signal may be provided to an optional
de-multiplexer 238 to segregate at least one additional output
signal 205. Following de-multiplexing, the digital signal remaining
in the output signal electronics 230 is passed to an output signal
interface 240. An output signal interface 240 converts the
electronic signal into an appropriate output signal, such as an
electronic or optical signal. Either electrical or optical
interfaces may be provided, subject to the inclusion of appropriate
hardware as would be apparent to one skilled in the art. In one
embodiment, the output signal interface 240 includes a fiber
transmitter that converts the electronic signal to an optical
signal, and further includes a fiber optic connector for connecting
a fiber optic cable. The primary output signal is a wideband
digital signal, and may be in accordance with various protocols
known in the art, including TCP/IP, IPX, Fast Ethernet, SONET, or
ATM, and may operate on various physical layers known in the art,
including STS-3, STS-12, OC-3, or OC-12. Although depicted
separately, the output signal interface 240 and input signal
interface 204 may be combined, and if the interfaces 204, 240
include a fiber optic transmitter and a fiber optic receiver, then
an integrated fiber optic transceiver may be used.
[0078] A controller 250, preferably a microprocessor-based digital
controller or computer, is advantageously provided, primarily to
detect and monitor various aspects of the transceiver 200. This
capability may provide both real-time and historical data logging
for status monitoring, performance monitoring, network management,
and/or troubleshooting. When the transceiver 200 is first powered,
the computer 250 may sample the amount of current being supplied to
the lasers through the laser drivers 220 to ensure that the lasers
sufficient power to transmit signals. During operation, the
computer 250 monitors the temperature levels of the laser diodes
within the laser transmitter modules 225. If the temperature of any
of the diodes exceeds the optimum operational temperature as
specified by the laser diode manufacturer, then the computer 250
may take one or more steps to alleviate the overheating, such as
alerting appropriate technical personnel, shutting down one or more
of the laser transmitters 225, or any other appropriate actions.
The computer 250 preferably also monitors the voltages supplied to
the photodiodes (not shown), the modulation signal to the laser
driver amplifier within the laser driver circuit 220, and the DC
bias circuit (also part of the laser driver circuit 220). The
measured voltages are preferably converted to a current value that
is compared to pre-set values in a look-up table, customized for
the individual laser diode, to further ensure the proper
functioning of the transceiver 200. If the transmitted laser beam
signal is monitored with a photodiode, the output of the photodiode
may be coupled with the controller 250, to provide laser
performance information, or permit the lasers to be operated at
constant signal strength. The controller 250 may also monitor
signal level from the laser receiver module 229. Furthermore, the
controller 250 and may receive signals from a CCD sensor 256, and,
utilizing a frame grabber extension 257, the controller 250 may
capture images of what is within the line of sight of the
transceiver 200.
[0079] One interface that may be used by the controller 250 to
perform its functions is a RS-232 interface format utilizing SNMP
(Simple Network Monitoring Protocol), although other interface
formats, including establishment of an Ethernet connection with the
controller 250, may alternatively be used. In a preferred
embodiment, the controller 250 is a computer which is used
primarily for diagnostic purposes and may be polled remotely using
a modem, over the Internet using an (Ethernet) HTML browser, or
other such communication means as are known in the art, thus
permitting remote information monitoring and/or control. While the
controller 250 is preferably a microprocessor-based digital
computer, a dedicated digital signal processor may alternatively be
used.
[0080] Additionally, the controller 250 interfaces with
environmental controls 252 including additional temperature sensors
254, as may be provided within an enclosure surrounding the
transceiver 200, to regulate the temperature and humidity
experienced by the transceiver 200 and provide optimal
environmental conditions for the transceiver 200 to operate. Heat
sinks, such as the heat sinks 184 provided in FIGS. 5-6B may also
be added to the outside of a transmitter enclosure or housing 151
to aid with environmental control.
[0081] Referring again to FIG. 4, a stand-alone radio frequency
(RF) transceiver 146 is provided as a backup to the laser
transceiver 50 during inclement weather conditions, particularly
fog. In one embodiment directed to Fast Ethernet/OC-3 signals, the
RF transceiver 146 operates at a frequency of 2.4 GHz and is
capable of up to 11 Megabits per second (Mbps) bandwidth. One of
several different methods may be utilized to switch between the
laser transceiver 100 and the RF transceiver 146 when necessary.
First, a router 140 may be utilized to monitor the laser
transceiver 100 for repeated requests for retransmissions or packet
errors. When the requests for retransmissions or packet errors
reach a predefined level, then the router 140 switches over to the
RF transceiver 146 for a predetermined amount of time. After such
predetermined time, the router 140 switches back to the laser
transceiver 100 and again monitors the laser transceiver 100 for
requests for retransmissions or packet errors.
[0082] A second method that may be utilized to determine when to
switch from the laser transceiver 100 to the backup RF transceiver
146 is by coupling the controller 250 (as shown in FIG. 8) to the
preamplifier of the laser receiver module 229 (as shown in FIGS. 4,
8) to monitor the strength of the incoming signal. Using this
method, once the incoming signal drops below a predefined level,
the controller 250 divides the signal between the RF transceiver
146 and the laser transceiver 100. The RF transceiver 146 is used
to send the data signals while the controller 250 continues to
monitor the strength of the incoming signal from the laser
transceiver 100. Once the strength of the incoming signal from the
laser transceiver 100 returns to above the predefined level, the
computer 250 switches back to using the laser transceiver 100.
[0083] A third method, called overflow mode, has the least latency
of the methods. This method may be utilized to determine when to
switch from the laser transceiver 100 to the backup RF transceiver
146 through the use of a router 140 to monitor and distribute the
incoming data signals between the two transceivers 100, 146. All
signals are initially routed through the laser transceiver 100
until the bandwidth of the laser transceiver 100 is entirely in
use. Excess data signals are then routed through the RF transceiver
146. As the bandwidth of the laser transceiver 100 drops due to
inclement weather, more data signals will be routed to the RF
transceiver 146. Conversely, as the weather begins to clear and the
laser transceiver 100 becomes more capable of carrying data
signals, less data signals will be routed to the RF transceiver
146. This configuration has the further benefit of providing a
higher total system bandwidth capability with less switching
latency.
[0084] A laser driver 300, such as for use as the laser driver 220
depicted in FIG. 8, will now be described. Reference will be made
generally hereinafter to FIGS. 9-10C, which provide a laser driver
schematic. The laser driver 300 essentially comprises a modulation
signal amplifier to amplify the laser data signals, and a DC bias
circuit to ensure that a laser diode 301 operates in a relatively
linear range. The laser driver 300 is capable of providing very
high current modulation, in the range of 100 mA to 1500 mA, at high
data rates, in the range of 10 Mbps to 622 Mbps, to the laser diode
301. The ability to drive the laser diode 301 at such high rates
derives from a unique configuration involving a power amplifier.
The power amplifier, which is preferably a RF power FET, directly
drives the low impedance laser diode using a low voltage power
supply. A typical laser diode useful in this application has a
characteristic dynamic impedance of between approximately 2 and 5
ohms. Preferably, the power supply voltage is less than 12 volts;
more preferably, the power supply voltage is approximately 5 volts.
Higher supply voltages may also be used, such as 12V, 15V, or 28V,
but in using the higher supply voltages results in greater power
consumption and wasted thermal load. In applications where the
power consumption and thermal load is less of a concern, the higher
supply voltages may be used to attain improved gain and bandwidth.
Preferably the RF power FET is capable of operating at a minimum
frequency of 1 MHz or less, and further preferably capable of
operating at a frequency of at least 100 kHz.
[0085] FIGS. 9-10C illustrate an embodiment of the laser driver 300
in which the output stage of the RF power amplifier Q3 is coupled
with a 5V supply. The circuitry illustrated in FIGS. 9-9C comprises
the primary laser driver circuitry and is connected to the
circuitry in FIGS. 10-10C through connectors J1, J2, J3. The
circuitry illustrated in FIGS. 9-9C primarily comprises the laser
bias current circuitry and the TEC controller circuitry. This
particular embodiment of the laser driver 300 is designed to accept
a digital input signal having an amplitude of 175 mV on a 50 ohm
coaxial transmission line, with all the amplifiers operating at
least 2 dB below the -1 dB power output compression point. In
addition, this embodiment comprises three stages of amplification.
In an alternative embodiment, two stages of amplification may be
used.
[0086] As illustrated in FIGS. 9-9C, the input signal from the
coaxial transmission line is AC-coupled into the first amplifier
stage Q1, which may comprise an ERA-2SM or similar op-amp. The
signal then passes to a digital attenuator U2, which may comprise
an RF2420 or similar attenuator. The digital attenuator U2 reduces
the signal amplitude by a minimum of the 2 dB insertion loss and up
to a maximum of 30 dB or more. The signal passes from the
attenuator to the second stage amplifier Q2, which may comprise an
ERA-6SM or similar op-amp. The digital attenuator U2 may also be
located after the second stage amplifier Q2. Following the second
stage amplification, the signal is AC-coupled into the third stage
power amplifier Q3, which may comprise an F2248 or D2202UK or
device. In the embodiment shown, the power amplifier Q3 comprises a
broadband RF power MOSFET. The power amplifier used should be one
which provides the gain required as described herein and has at
least a 1 GHz bandwidth. A silicon device may be preferred in
certain instances over a GaAs device because the silicon device
meets at least the minimum requirements and typically provides a
cost savings over GaAs devices. The laser diode 301, the sense
resistors R11, R19, and the power amplifier Q3 may alternatively be
connected in series across the 5V supply voltage.
[0087] The first stage amplifier Q1 and the second stage amplifier
Q2 are broadband 50 ohm amplifier gain blocks. Alternatively,
discrete RF transistors may be used in the first and second
amplifier stages. In applications where the input signal has a
higher voltage level, or in applications where less amplification
is required to drive the laser diode, the first and second
amplifier stages may be replaced by a single amplifier which
provides the required gain for the input to the power amplifier Q3.
In an alternative embodiment, the first stage amplifier Q1 may be a
limiting amplifier that hard-limits a digital input signal to a
fixed output lever, regardless of the input signal amplitude.
Typically, this limit would be a PECL (positive emitter control
logic) output level of 700 mV.
[0088] In the embodiment depicted in FIGS. 9-9C, the signal input
to the second stage amplifier Q2 is at a higher voltage level than
the signal input to the first stage amplifier Q1. Therefore, in
order to achieve the desired overall amplification, the second
stage amplifier requires a higher supply voltage, preferably
greater than 7V. Alternatively, if the overall power consumption
needs to be reduced and if the reduced overall amplification is
sufficient for a particular application, the second stage amplifier
Q2 may be operated from the same 5V power supply as the first stage
amplifier Q1 and the power amplifier Q3. Supply voltages greater
than 5V may also be utilized, however, using higher voltages
results in proportionally greater power consumption and wasted
thermal load. In the embodiment depicted, the 5V supply voltage has
been chosen to decrease power consumption and the thermal load.
[0089] Each amplifier stage is thermally stabilized using accepted
design techniques to compensate for current and gain changes as a
function of temperature. Resistors R2, R8, and R20 are therefore
provided to the first, second, and third stage amplifiers
respectively. A DC bias voltage is provided to each amplifier stage
through one or more RF chokes in the form of inductors L4, L9, L7,
L10, L2, L3, and L8. The RF chokes present a high impedance to any
power supply noise that may range from 100 kHz to 1 GHz that could
otherwise contaminate the amplified signal. Additionally, an
AC-coupling capacitor C11 is provided to prevent the laser bias
current from interfering with the DC operating supply voltage at
the output of the power amplifier Q3.
[0090] The amplitude of the input signal to the power amplifier Q3
is adjustable, thus enabling the laser driver 300 to accommodate
laser diodes with differing drive current requirements. Adjustable
feedback is therefore provided at the second stage amplifier Q2 by
linking the output of the second amplifier Q2 to its input through
and adjustable resistor R21. Such adjustable feedback could also be
provided across the first stage amplifier Q1 or the power amplifier
Q3. Alternatively, adjustable feedback could be achieved through
the use of a voltage controlled variable gain amplifier at either
the first or second stage of amplification, wherein a potentiometer
would be used to adjust the gain of the amplifier.
[0091] The output drive current of the power amplifier Q3 to the
laser diode is adjusted to the nominal operating point of the laser
diode by adjusting the gate voltage at the input of the power
amplifier Q3. The gate voltage is controlled by a first integrated
circuit U3 and a potentiometer R6. The first integrated circuit U3
is preferably a CD4061BCM or similar device. The gate voltage is
derived from a supply voltage regulated by a zener diode D1 to
achieve a highly stable bias voltage regardless of power supply
voltage fluctuations. The power amplifier stage Q3 is AC-coupled
with the gate voltage such that the voltage waveform at the input
results in a linear modulation of the laser drive current at the
output. Therefore, the low impedance laser diode 301, being
essentially a current-controlled device that linearly converts
input current to output optical power, is driven by the voltage
controlled power amplifier Q3 which resides in the low impedance
power amplifier stage of the laser driver 300. The input and the
output of the power amplifier Q3 are AC-coupled, and the output to
the laser diode 301 is provided with an appropriate dc bias current
such that the output modulation of the power amplifier Q3 causes
the laser drive current to swing from nearly off to the desired
output power with an optical output power extinction ratio of at
least 10:1. In designing the bias circuit, consideration may be
given to selecting minimum, maximum, and average power levels for
the laser diode 301, as bias current causes the laser to operate at
a selected average power level, and the wideband signal modulation
will cause the laser output to vary between the power level
extremes.
[0092] The laser driver 300 illustrated in FIGS. 9-9C also
comprises circuitry for adaptive digital control of the output
power of the laser driver 300. Such control over the output power
may be desirable so that the output power can be reduced when high
output power is not needed. Reducing the output power may enhance
the overall life span of the laser diode by reducing operating
stress levels during periods of operation. Control over the output
power also enables the laser driver 300 to be used under a wider
variety of circumstances. For example, the output power can be
greatly reduced for communication links over short distances, thus
avoiding saturating the receivers, and can be increased to maximum
or near maximum levels in order to overcome inclement weather
conditions.
[0093] Digital control over the output power is implemented by
simultaneous control over three laser driver functions. First, the
digital attenuator comprises a digitally controllable RF attenuator
such as RF Micro Devices RF2420. A 3-bit control signal data0,
data1, data2 is used to select the attenuation of the modulation
signal. Typically, a 1 to 3 dB insertion loss is selected, however,
a 30 dB or more insertion loss may be selected as desired by the
user.
[0094] Second, the laser bias current is also to be appropriately
reduced to maintain the desired extinction ration of at least 10:1.
The laser bias current is adjusted using multiplexer U7, which is
configured so that the same 3-bit control signal data0, data1,
data2, selects a required discrete resistor value using resistors
R33, R22, R29, R30, R31, and R32. The discrete resistor value is
used to realize an appropriate laser bias current of up to 500 mA
from the output of the laser bias current FET U4. This, FET U4
preferably comprises an NDT 456 P or similar device. Potentiometer
R18 is provided as a vernier control to set the nominal bias
current required by the particular laser diode being used. The bias
current output from FET U4 is applied to the laser diode through a
broadband choke network comprising inductors L5, L6 to ensure any
RF noise that might get picked up by the bias current circuitry is
blocked and does not contaminate the laser modulation current.
[0095] The third function controlled by the 3-bit control signal is
the gate bias voltage for the power amplifier Q3. This adjustable
gate bias voltage is accomplished using multiplexor U3 configured
so that the 3-bit signal data0, data1, data2 selects a required
discrete resistor value using resistors R5, R7, R25, R9, R22, or
R26. The discrete resistor value is used to realize an appropriate
FET gate bias voltage for linear operation and minimum power
consumption in the power amplifier Q3. Potentiometer R6 is provided
as a vernier control to set the nominal bias current required by
the particular laser diode being used. Ferrite core inductor L2 is
provided as an inductive choke to prevent RF noise that may be
present on the gate bias voltage from contaminating the modulation
signal at the input to the power amplifier Q3.
[0096] The laser driver 300 may also incorporate, as an optional
feature, a low bandwidth signal into the broadband communications
signal. This narrowband signal may be used, for instance, for
telemetry or tracking. The narrowband signal is to be of a lower
bandwidth so that it does not interfere with the communications
signal. In the embodiment of the transceiver described herein, the
communications signal operates on a frequency of greater than 100
kHz, therefore, in order to avoid contaminating the communications
signal with the narrowband signal and to permit convenient
filtering of the narrowband signal from the communications signal,
the narrowband signal preferably occupies a frequency range less
than 100 kHz, and more preferably less than 50 kHz. Additionally,
because atmospheric scintillation primarily occupies the frequency
range below 200 Hz, the narrowband signal preferably occupies a
frequency range greater then 200 Hz to avoid corruption through the
AC-coupling. Such a narrowband signal may be used by the first
transceiver to communicate with the second transceiver the signal
strength being received. Thus, when the output power of the laser
diodes requires an increase or decrease, the transceivers may
communicate such a need and automatically compensate their
respective output powers using the aforementioned optional
digitally controlled power feature.
[0097] In FIGS. 9-10C, such a narrowband telemetry signal is
incorporated through the laser bias circuitry. The narrowband
modulation signal is imposed upon the DC laser bias current, and
therefore imposed upon the output of the laser driver to the laser
diode. The narrowband modulation input to the DC bias current is a
standard TTL interface into an analog switch U5. A potentiometer
R17 is used to set the desired ratio of the narrowband current to
the DC bias current. Preferably, the narrowband current is less
than 20% of the DC bias current. More preferably, the modulation
amplitude of the narrowband signal is less than 10% but more than
5% of the DC bias current. For example, a nominal 350 mA DC bias
current is preferably modulated by a narrowband signal having a 20
to 35 mA range. Amplifiers U3B and U3C superimpose the narrowband
modulation on the DC bias current originating at resistor R18. If
no narrowband input is provided into the analog switch U5, the DC
bias circuit provides only a steady DC bias current.
[0098] Baud rates ranging from 100 bps to 100 kbps may be
accommodated by the narrowband signal with the circuitry herein
described. For example, in one implementation the narrowband signal
may comprise a 9600 bps Manchester-coded bit stream at 19.2 kBd, a
signal which occupies a spectrum ranging from approximately 500 Hz
to 20 kHz.
[0099] A second optional feature included in the laser driver
illustrated in FIGS. 9-10C is monitoring circuitry that senses the
DC bias current ("bias-test") and the peak-to-peak amplitude of the
laser modulation current ("RF-test"). This monitoring circuitry,
when included, is preferably coupled to the aforementioned
controller 250. The DC bias current is monitored through an op-amp
U3D which senses the voltage drop across two in parallel resistors
R11, R19, providing a 0.5 ohm impedance, in series with the laser
diode drive current. The laser modulation current amplitude is
monitored by sensing the modulated voltage across the same two in
parallel resistors R11, R19, wherein the modulated voltage is
amplified by an op-amp Q4, rectified by a diode pair D7 and
integrated by a capacitor C29 and a resistor R38 at the input to
op-amp U3A. The op-amp U3A generates the monitor voltage "RF-test
out" that is linearly related to the laser modulation current
amplitude.
[0100] Yet another optional, but preferred, feature included in the
laser driver is a thermoelectric cooler (TEC) controller. A TEC is
used to stabilize the temperature of the laser diodes against
temperature fluctuations, thus reducing thermal stress. Reducing
thermal stress to the laser diode typically stabilizes laser
wavelength over time and enables the laser diode to have a longer
life. The TEC controller as described herein permits highly
efficient control over the TEC under circumstances where the laser
drivers are operating at maximum load, even in conditions of hot
weather. In fact, operating of the thermoelectric cooler power
amplifier as a controlled current source to supply the
thermoelectric cooler results in near-perfect efficiency when
maximum cooling is required.
[0101] The TEC controller illustrated in FIGS. 9-10C operates off a
5V supply. A power FET U8, preferably an NDT 456P or similar
device, is used as a voltage-controlled current source to supply
current to the TEC. The power FET U8 and the TEC TM2 are connected
across the 5V supply. The TEC TM2 used preferably (1) provides the
required maximum cooling capacity when operated at a near-optimum
current, and (2) has an impedance at this optimum operating current
that results in a 5V drop across the TEC TM2. Thus, when maximum
cooling capability is required of the TEC TM2, the 5V supply
voltage is dropped entirely across the TEC TM2 and only about 0.1V
is dropped across the power FET U8, resulting in greatly enhanced
efficiency under the most thermally stressing conditions. The power
FET U8 is controlled through four op-amps U1A, U1B, U1C, U1D, which
preferably comprise an LM2902M or similar device.
[0102] A thermistor TM1 having a 10 kohm impedance at 25.degree.
Celsius is used to sense the temperature of the laser diode. A
regulated 5V supply source is provided through a 40 kohm resistor
R11 and the thermistor TM1, resulting in a nominal 100 .mu.A
current through the thermistor TM1 and a voltage drop of
approximately 1V across the thermistor TM1 at 25.degree. Celsius.
The thermistor is advantageously operated as part of a voltage
divider circuit. The voltage drop across the thermistor TM1 changes
with temperature and is sensed by op-amp U1A. The same 5V supply
source is used to generate a 1V reference voltage using resistors
R12, R34. This 1V reference source is sensed by op-amp U1B. In
other words, the TEC control circuit compares temperature sensor
voltage drop from the thermistor with a reference voltage, which
corresponds to a voltage that would result if the thermistor were
operated at a desired setpoint temperature. The difference between
the output voltages of op-amps U1A and U1B is amplified by op-amp
U1C. The output from op-amp U1C is the error signal associated with
the desired temperature setpoint versus the actual laser
temperature. The temperature setpoint in the embodiment depicted is
25.degree. Celsius. Op-amp U1D and its associated resistors R45,
R46 provide the error signal to the power FET U8 with the
appropriate gate bias resistors R43, R44 and loop gain. The control
signal to the power FET is integrated by resistor R47 and capacitor
C8 to provide a gentle time constant so the current applied to the
TEC is not impulsive. The time constant is preferably greater than
0.5 second, and more preferably approximately 1.0 second. Such a
gentle time constant is desirable so that stress to the TEC is
minimized, thus extending the life span of the TEC.
[0103] In a preferred embodiment utilizing TECs to cool the laser
diodes, two TECs are coupled in series to cool a single laser
diode. The TECs preferably comprise Melcor CPO 0.9-21-06 or similar
devices. Such TECs operate at nearly optimum efficiency at 1.25 A
and have an impedance of 2 ohms when operated at such a current.
Thus, the total impedance of the two in series TECs is 4 ohms. When
the maximum current of 1.25 A is drawn, approximately the full 5V
supply voltage is dropped across the TECs, resulting in less than a
0.1V drop across the power FET U8 under maximum cooling
circumstances. In less demanding conditions requiring less cooling,
the power FET U8 controls the current to the TECs by dropping some
of the voltage across Vds. For example, when only 50% of the
maximum current is required to cool the laser diode, the TECs draw
approximately 0.63 A and drop 2.5V, with the remaining 2.5V
appearing as Vds. Such circumstances represent a worst case
scenario for dissipation in the controller, which is only 1.6 W.
However, this worst case scenario occurs in cooler weather when the
thermal stress on the overall system is reduced and the overall
heat load can be more readily dissipated into the surrounding
environment.
[0104] The above TEC controller circuit may be modified to use a
3.3 V supply source instead of a 5 V supply source. A 3.3 V supply
source is more appropriate for less demanding cooling requirements.
The TEC may also be modified to act as a both a cooling and heating
element within the transceiver by proving a bipolar voltage supply
source of +5v and -5V.
[0105] Notably, operation of a TEC as described above is not
limited to controlling heat transfer from laser diodes. Instead,
the TEC operation described above could apply to most any item that
needs to be temperature-controlled. Operation of a power amplifier
(preferably by way of a low voltage power source) as a controlled
current source provides near-perfect efficiency when maximum
cooling is required. When designing such a cooling system, the
maximum cooling requirement of the item to be temperature
controlled should be considered in light of the fact that a
thermoelectric cooler has a characteristic impedance and an optimal
operating current. The desired result may be achieved by selecting
the optimal operating current of the thermoelectric cooler to
correspond with the maximum cooling requirement of the item to be
temperature controlled, and further selecting the impedance of the
thermoelectric cooler to drop substantially all of the supply
voltage when the thermoelectric cooler is operated to provide the
maximum cooling requirement of the item. If maximum cooling
efficiency is desired, then lower power supply voltages are
preferable.
[0106] Accordingly, improved transceivers for transmission using
laser signals originating in digital format are disclosed. Further
disclosed are improved laser driver circuits capable of driving
lasers with high frequencies and high currents, from either digital
or analog input signals. Still further disclosed are methods and
apparatus for operating a thermoelectric cooler with high
efficiency, either with or without an accompanying laser diode.
While embodiments and applications of this invention have been
shown and described, it would be apparent to those skilled in the
art that many more modifications are possible without departing
from the inventive concepts herein. The invention, therefore, is
not to be restricted except in the spirit of the appended
claims.
* * * * *