Title: Laser modulator
Abstract: An integrated device is provided having both laser and modulator sections. The laser section generates a primary light signal using a distributed feedback process. The modulator section influences the primary light signal by means of an electroabsorption process. The laser and modulator sections are electrically separated from each other with respect to anode and cathode areas. As a result, a single supply voltage can be used to operate the device. An optical transmitter contains, in addition to the integrated device, a protocol converter which receives an information signal to be transmitted and generates a control signal which is received by a modulator driver. Based thereon, the modulator driver produces a modulating signal, which causes a modulated light signal representing the information signal to be output. A laser bias control unit controls the output power level of the primary light signal based on a leak signal from the laser section.
Patent Number: 6,859,479 Issued on 02/22/2005 to Svenson
| Inventors:
|
Svenson; Lars-Gote (Sollentuna, SE)
|
| Assignee:
|
Optillion Operations AB (Stockholm, SE)
|
| Appl. No.:
|
492651 |
| Filed:
|
April 14, 2004 |
| PCT Filed:
|
November 22, 2002
|
| PCT NO:
|
PCT/SE02/02142
|
| 371 Date:
|
April 14, 2004
|
| 102(e) Date:
|
April 14, 2004
|
| PCT PUB.NO.:
|
WO03/04705 |
| PCT PUB. Date:
|
June 5, 2003 |
Foreign Application Priority Data
| Current U.S. Class: |
372/50 |
| Intern'l Class: |
H01S 005//00 |
| Field of Search: |
372/9,26,43-50
|
References Cited [Referenced By]
U.S. Patent Documents
| 4503541 | Mar., 1985 | Weller et al. | 372/50.
|
| 5383216 | Jan., 1995 | Takemi | 372/50.
|
| 5798856 | Aug., 1998 | Suzuki et al. | 398/183.
|
| 6191464 | Feb., 2001 | Barnard | 257/427.
|
| 6574260 | Jun., 2003 | Salvatore et al. | 372/50.
|
| Foreign Patent Documents |
| 0 749 154 | Dec., 1996 | EP.
| |
| 0 809 129 | Nov., 1997 | EP.
| |
| 0 917 260 | May., 1999 | EP.
| |
| 1 073 168 | Jan., 2001 | EP.
| |
Other References
Copy of International Search Report for PCT/SE02/02142, dated Feb. 27,
2003.
|
Primary Examiner: Leung; Quyen
Attorney, Agent or Firm: Alston & Bird LLP
Claims
What is claimed is:
1. An integrated device for emitting a modulated light signal, comprising a
laser section and a modulator section, the laser section generating a
primary light signal by means of a distributed feedback process in a first
quantum well structure, the modulator section influencing the primary
light signal by means of an electroabsorption process in a second quantum
well structure to produce the modulated light signal, wherein the laser
section and the modulator section are electrically separated from each
other such that a laser anode area is electrically isolated from a
corresponding modulator anode area and a laser cathode area is
electrically isolated from a corresponding modulator cathode area.
2. An integrated device according to claim 1, wherein
the laser section comprises at least one externally accessible laser anode
contact and at least one externally accessible laser cathode contact, and
the modulator section comprises at least one externally accessible
modulator anode contact and at least one externally accessible modulator
cathode contact.
3. An integrated device according to claim 1, wherein the device comprises
a semi-insulating region between the laser anode area and the modulator
anode area which electrically separates these areas from each other.
4. An integrated device according to claim 1, wherein the device comprises
a proton bombarded region between the first quantum well structure and the
second quantum well structure, the proton bombarded region extending
through respective doped layers of a first doping polarity below the
quantum well structures such that also the doped layers are electrically
separated from each other into a first doped layer and a second doped
layer.
5. An integrated device according to claim 4, wherein the device comprises
two mutually reverse biased diodes, the diodes separating the laser
cathode area electrically from the modulator cathode area.
6. An integrated device according to claim 5, wherein the diodes are
represented by a separation layer below the second doped layer, the
separation layer having a doping polarity which is opposite to the second
doping polarity.
7. An integrated device according to claim 5, wherein the separation layer
forms a diode bias area between the anodes of the diodes, the diode bias
area being externally accessible via a bias contact.
8. An integrated device according to claim 4, wherein the device comprises
a semi-insulating layer below the first doped layer, the semi-insulating
layer separating the laser cathode area electrically from the modulator
cathode area.
9. An integrated device according claim 1, wherein the primary light signal
represents a substantially continuous light, whereas the intensity of the
modulated light signal varies over time.
10. An optical transmitter for producing a modulated light signal that
carries information, comprising
a modulator driver receiving a control signal and in response thereto
delivering a modulating signal,
a controllable distributed feedback laser unit
receiving a bias current and in response thereto generating a primary light
signal, and
receiving the modulating signal and in response thereto influencing the
primary light signal such that the modulated light signal is generated,
and
a laser bias control unit generating the bias current to the laser unit for
controlling the output power level of the primary light signal, wherein
the laser unit comprises an integrated device comprising:
a laser section and a modulator section, the laser section generating a
primary light signal by means of a distributed feedback process in a first
quantum well structure, the modulator section influencing the primary
light signal by means of an electroabsorption process in a second quantum
well structure to produce the modulated light signal, wherein the laser
section and the modulator section are electrically separated from each
other such that a laser anode area is electrically isolated from a
corresponding modulator anode area and a laser cathode area is
electrically isolated from a corresponding modulator cathode area.
11. An optical transmitter according to claim 10, wherein the transmitter
comprises a protocol converter receiving at least one signal that
represents the information f and in response thereto generating the
control signal.
12. An optical transmitter according to claim 10, wherein the transmitter
comprises a monitor diode detecting a leak signal of the primary light
signal and on basis thereof delivering a feedback signal to the laser bias
control unit, the feedback signal forming a basis for the bias current.
13. An optical transmitter according to claim 12, wherein the modulating
signal comprises:
a varying modulator voltage component which represents the information, and
a substantially fixed modulator bias voltage component.
14. An optical transmitter according to claim 10, wherein the modulator
driver comprises a terminated output delivering the modulating signal to
the laser unit according to a single-ended format.
15. An optical transmitter according to claim 10, wherein the modulator
driver comprises an output which is differentially coupled to the laser
unit, the output delivering the modulating signal to the laser unit
according to a differential format.
16. An optical transmitter according to claim 15, wherein the modulator
driver comprises a separate bias input for receiving a reference voltage
level which provides a drive voltage to a modulator section in the laser
unit.
17. An optical transmitter according to claim 13, wherein the transmitter
comprises a duty cycle control unit to compensate for an exponential
transfer function of a modulator section in the laser unit.
18. An optical transmitter according to claim 17, wherein the duty cycle
control unit is included in an application specific integrated circuit.
19. An optical transmitter according to claim 18, wherein the application
specific integrated circuit additionally comprises at least one of the
protocol converter and the modulator driver.
20. An optical transmitter according to claim 15, wherein the modulator
driver comprises at least one terminated output which delivers a
respective low-frequency modulating signal to the laser unit which
predominantly contains spectral components that represent relatively low
frequencies, whereas the differential outputs deliver wide-frequency
modulating signals which contains spectral components that represent both
relatively high frequencies and relatively low frequencies.
21. An optical transmitter according to claim 20, wherein the modulator
driver comprises at least one low pass filtering chain receiving a
respective primary modulating signal and in response thereto generating
the respective low-frequency modulating signals.
Description
FIELD OF THE INVENTION
The present invention relates generally to the production of modulated
light signals for transmission of information. More particularly the
invention relates to an integrated device for emitting a modulated light
signal according to the preamble of claim 1 and an optical transmitter
according to the preamble of claim 10.
THE BACKGROUND OF THE INVENTION AND PRIOR ART
Optical communication systems transport information in the form of
modulated fight signals. A semiconductor laser is normally used to
accomplish these signals (laser=light amplification by stimulated emission
of radiation). A direct modulated laser, however, inevitably shifts the
wavelength of the primary flight signal to some degree when the signal is
modulated. This causes dispersion, i.e. degradation of the signal due to
the fact that the various wave components experience different propagation
velocities. The severity of the dispersion problem increases with
increased bitrate and fiber length. Today, direct modulated lasers of 1300
nm wavelength can be used at 10 Gbit/s for transmission over relatively
short optical fibers. Nevertheless, at higher bitrates and/or longer
distances a distributed feedback laser with an electroabsorption modulator
(DFB-EA) can be used to generate modulated light signals around the
wavelength 1550 nm. A DFB-EA modulator represents a combination of a
continuous light emitting laser with a narrow spectral width and an
electroabsorption modulator. The laser here operates at a constant
current, which results in a relatively low chirp (i.e. undesired rapid
changing of the frequency/wavelength).
Various efforts have, of course, already been made to improve the
performance of the optical transmitters. For instance, the document EP,
A2, 0 809 129 describes a semiconductor optical modulator, which includes
a separately mounted electroabsorption-type optical modulator for
modulating an optical signal. The modulator may accomplish a high-speed
transmission of an optical signal with a low insertion loss and a low
chirp. However, the design is comparatively bulky and expensive. Moreover,
a complex calibration of two sets of optical fibers and lenses is
required.
U.S. Pat. No. 6,191,464 discloses a solution which aims at avoiding
modulation and chirp of the laser when there is no information signal
present at the modulator input. The suggested solution involves electrical
separation of semiconductor components integrated within an integrated
opto-electronic device. A complete DFB-EA modulator is fabricated on the
same substrate. The anode of the laser diode and the anode of the
modulator are electrically separated by an isolation region, Nevertheless,
the laser cathode and the modulator cathode are connected to a common
ground potential.
The documents JP, A, 5 524 751; EP, A1, 0 917 260 and EP, A1, 1 073 168
describe other examples of optical modulators with a quantum well laser
and an electroabsorption modulator being integrated on the same substrate.
Also here, the laser and the modulator are electrically separated with
respect to the anodes. However, the components are electrically connected
via a common substrate, which functions as a cathode for both of them.
As can be seen from the above examples it is common practice in DFB-EA
modulator design to arrange the laser section and the modulator section on
a single substrate with a common cathode connection for both the laser and
the modulator. FIG. 1 shows a typical laser-modulator structure of this
kind and FIG. 2 illustrates an exemplary optical transmitter, which
includes such laser-modulator 250.
A protocol converter 210 receives an information signal X and produces in
response thereto a control signal C.sub.X, which typically has a serial
format, whereas the information signal X might have a parallel format. In
any case, the control signal C.sub.X has a relatively large bandwidth. A
modulator driver 220 receives the control signal C.sub.X and generates in
response thereto a modulating signal M.sub.X, which is adapted to a
modulator section 252 in the laser-modulator 250. A laser bias control
unit 230 generates a bias current I.sub.B to a laser section 251 in the
laser-modulator 250. The bias current I.sub.B controls the output power
level of the primary light signal L.sub.1, which is generated in a first
quantum well structure MQW-1 in the laser section 251, such that the power
of the primary light signal L.sub.1 lies within a desired range for the
specific application. A control loop determines what is an appropriate
value of the bias current I.sub.B. The control loop contains a monitor
diode 240, which detects a leak signal L'.sub.1 of the primary light
signal L.sub.1 from the laser section 251. Based on the leak signal
L'.sub.1, the monitor diode 240 delivers a feedback signal I.sub.ph to the
laser bias control unit 230. The feedback signal I.sub.ph (or
photocurrent) is proportional to the primary light signal L.sub.1. The
control unit 230 assigns a suitable bias current I.sub.B depending on the
feedback signal I.sub.ph. The modulating signal M.sub.X influences (or
more precisely attenuates) the primary light signal L.sub.1 from the laser
section 251, when this signal passes through a second quantum well
structure MQW-2 in the modulator section 252, in such manner that an
outgoing modulated light signal L.sub.2 is produced, which represents the
information signal X. A resistor 253, of say 50 .OMEGA., may also be
connected in parallel with the diode in the modulator 252 in order to
terminate signal reflections.
Returning to FIG. 1, the laser section 251 contains a separate anode area
of a positive doping polarity p (e.g. InP), accessible via a first anode
contact 101 and a first anode lead 101a. Correspondingly, the modulator
section 252 contains another separate anode area p+ (e.g. InGaAs), also of
positive doping polarity, which is accessible via a second anode contact
103 and a second anode lead 103a. A semi-insulating area SI (e.g. InP)
separates the laser anode area p from the modulator anode area p+. A
sub-layer p of anode area p+ in the modulator section 252 may extend below
the un-doped area SI to contact the laser anode area p. However in any
case, both the laser section 251 and the modulator section 252 share a
common cathode area of negative polarity, for example in the form of a
layer n- (InP) and a substrate n (InP). The cathode area is accessible via
a cathode contact 102, which normally is connected to the ground
potential. As a result thereof, the laser section 251 will have to be
biased by a positive voltage +Y.sub.cc on its anode 101, whereas the
modulator section 252 must be biased by a negative voltage -V.sub.ee on
its anode 103.
Consequently, a DFB-EA modulator according to this design requires both a
positive supply voltage +V.sub.cc, and a negative supply voltage
-V.sub.ee. This in turn, gives rise to a comparatively large, complex and
expensive circuitry. Moreover, the modulator demands a high voltage swing
from the modulator driver in; order to obtain a sufficiently high
extinction ratio (i.e. the ratio of the two optical power levels used to
represent information on a binary format). Typically, the required voltage
swing is higher than what can be accomplished by means of a high-speed
CMOS process (CMOS=complementary metal-oxide semiconductor), which of
course, places severe restrictions and demands on the modulator driver.
SUMMARY OF THE INVENTION
The object of the present invention is therefore to provide a solution for
the production of information carrying (i.e. modulated) light signals,
which alleviates the problems above and thus offers a power and size
efficient design of moderate circuit complexity.
According to one aspect of the invention the object is achieved by an
integrated device for emitting a modulated light signal as initially
described, which is characterized in that the laser section and the
modulator section are electrically separated from each other, such that a
laser anode area is electrically isolated from a corresponding modulator
anode area. A laser cathode area is also electrically isolated from a
corresponding modulator cathode area.
This separation of the laser and the modulator sections is very
advantageous, since it is thereby possible to operate the device with a
single supply voltage, even though the laser requires a positive bias
voltage and the modulator requires a negative bias voltage.
According to a preferred embodiment of this aspect of the invention, the
laser section comprises at least one externally accessible laser anode
contact plus at least one externally accessible laser cathode contact.
Correspondingly, the modulator section comprises at least one externally
accessible modulator anode contact as well as at least one externally
accessible modulator cathode contact. The advantage with the thus
accessible contacts is that they facilitate the connection of relevant
groundings and supply voltages.
According to another preferred embodiment of this aspect of the invention,
the integrated device comprises a semi-insulating region between the laser
anode area and the modulator anode area, which in an uncomplicated manner
electrically separates these areas from each other.
According to yet another preferred embodiment of this aspect of the
invention, the integrated device comprises a proton bombarded region
between the first quantum well structure, and the second quantum well
structure. The proton bombarded region extends through a first doped layer
having a first doping polarity below the quantum well structures.
Moreover, a modulator cathode contact is connected to the first doped
layer.
Hence, the proton bombarded region accomplishes two regions in the first
doped layer, which are electrically isolated from each other. An advantage
of this is that the proton bombarded region per se may accomplish the
electrical separation between the laser and modulator cathode areas.
According to a first preferred alternative embodiment of: this aspect of
the invention, the integrated device comprises two mutually reverse biased
diodes, which separate the laser cathode area electrically from the
modulator cathode area. The diodes may be represented by a separation
layer below the first doped layer, which has an opposite doping polarity
to the first doping polarity, i.e. that of the first doped layer below the
quantum well structures. Preferably, a diode bias area between the anodes
of the diodes is also externally accessible via a bias contact.
This first preferred alternative embodiment is advantageous in many ways,
since it allows a straight-forward attachment of a laser cathode directly
to the substrate of the device.
According to a second preferred alternative embodiment of this aspect of
the invention, the integrated device instead comprises a semi-insulating
layer below the first doped layer. This layer, in combination with the
proton bombarded region, thereby separates the laser cathode area
electrically from the modulator cathode area.
This second preferred alternative embodiment is very advantageous, since it
on one hand results in a comparatively compact device structure, and on
the other hand does not require a diode bias supply.
According to another aspect of the invention the object is achieved by an
optical transmitter as initially described, which is characterized in that
the laser unit comprises the above proposed integrated device. Hence, the
transmitter only needs a single supply voltage, which of course is
advantageous due to the reasons stated initially.
According to a preferred embodiment of this aspect of the invention, the
optical transmitter comprises a protocol converter, which receives at
least one signal that represents the information to be transmitted, and in
response thereto generates the control signal. An advantage with the
protocol converter is that it may transform the actual information signal
into a signal format which is adapted to the modulator driver and the
laser unit in the transmitter.
According to a preferred embodiment of this aspect of the invention, the
optical transmitter comprises a monitor diode that detects a leak signal
of the primary light signal. Based on the detected signal, the monitor
diode delivers a feedback signal to the laser bias control unit, which in
turn forms a basis for the a bias current fed to the laser unit. An
advantageous effect of this control loop is that the average output power
from the laser can thereby be maintained at a desired level.
According to another preferred embodiment of this aspect of the invention,
the modulating signal comprises a varying modulator voltage component,
which represents the information to be transmitted by the optical
transmitter. The modulating signal also comprises a substantially fixed
modulator bias voltage component, which is adapted to a value being
optimal with respect to the application in question. A lowered bias
voltage (i.e. a more negative value) results in a decreased optical output
power level, which generally is undesired. At the same time, the lowered
bias voltage decreases the modulator's chirp, which is a desired effect.
In any case, the proposed signal format is advantageous, since it makes it
possible to generate an modulator signal which is optimal with respect to
the modulator's characteristics.
According to a first preferred alternative embodiment of this aspect of the
invention, the modulator driver comprises a terminated output, which
delivers the modulating signal to the laser unit according to a
single-ended format. A single-ended signal format is desirable in many
applications, particularly where a relatively low frequency output is
demanded.
According to a second preferred alternative embodiment of this aspect of
the invention, the modulator driver comprises a terminated output, which
is differentially coupled to the laser unit. This output thus delivers the
control signal to the laser unit according to a differential format.
Preferably, the modulator driver also comprises a separate bias input for
receiving a reference voltage level, which provides a drive voltage to the
modulator section in the laser unit.
A differential format signal format is often desirable, since the swing of
the modulator signal can thereby be equal to the entire potential
difference between the supply voltage and the ground voltage. This, in
turn, means that a power efficient low-voltage driver can be used to
provide the supply voltage. Furthermore, a differential signal is less
sensitive to disturbances and the signal itself generally causes less
disturbances to other signals. This is due to the fact that the total
average current is substantially constant.
According to yet another preferred embodiment of this aspect of the
invention, the optical transmitter comprises a duty cycle control unit,
which compensates for the exponential transfer function of the modulator
section in the laser unit. The advantage of such non-linear compensation
is, of course, that the control and tuning of the modulator driver can be
made much simpler than otherwise.
According to an additional preferred embodiment of this aspect of the
invention, the modulator driver comprises at least one terminated output,
which delivers a respective low-frequency modulating signal to the laser
unit. The modulating signal from at least one terminated output thus
predominantly contains spectral components that represent relatively low
frequencies. The differential outputs, on the other hand, deliver
wide-frequency modulating signals, which contain spectral components that
represent both relatively high frequencies and relatively low frequencies.
The at least one low-frequency modulating signal is preferably produced by
a respective low pass filtering chain, which receives a respective primary
modulating signal.
The advantage obtained by this spectral separation of the modulating
signals is that the high-frequency signals can use a well terminated
driver with small capacitors, while the terminated outputs need only be
used for delivering the low-frequency signals.
The invention offers a highly efficient and flexible solution for
generating modulated light signals. Simultaneously, the invention vouches
for power efficient designs of comparatively small sizes and moderate
circuit complexity. Naturally, the invention will therefore provide a
competitive edge to any communication system where optical transmitters
are utilized for the transmission of information.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention is now to be explained more closely by means of
preferred embodiments, which are disclosed as examples, and with reference
to the attached drawings.
FIG. 1 shows a per se known integrated structure including a laser section
and a modulator section,
FIG. 2 shows a block diagram over a known optical transmitter which
utilizes the structure shown in FIG. 1,
FIG. 3a shows an integrated laser-modulator structure according to a first
alternative of a first embodiment of the invention,
FIG. 3b shows an integrated laser-modulator structure according to a second
alternative of the first embodiment of the invention,
FIG. 3c shows an integrated laser-modulator structure according to a third
alternative of a first embodiment of the invention,
FIG. 3d shows an integrated laser-modulator structure according to a fourth
alternative of the first embodiment of the invention,
FIG. 4 shows a block diagram over a proposed optical transmitter including
a single-ended modulator driver,
FIG. 5a shows a block diagram over an optical transmitter according to a
first embodiment of the invention, which includes a differential modulator
driver,
FIG. 5b shows a block diagram over an optical transmitter according to a
second embodiment of the invention, which includes a differential
modulator driver,
FIG. 6 shows a block diagram over a modulator driver in CMOS-technology
according to an embodiment of, the invention, which may be included in the
proposed optical transmitter shown in FIG. 5b,
FIG. 7a shows an integrated laser-modulator structure according to a first
alternative of a second embodiment of the invention, and
FIG. 7b shows an integrated laser-modulator structure according to a second
alternative of the second embodiment of the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
FIG. 3a shows an integrated laser-modulator structure according to a first
alternative of a first embodiment of the invention. A laser section here
contains a separate anode area of a positive doping polarity p.sub.1 (e.g.
InP), which is accessible via a first anode contact 301 and a first anode
lead 301a. Correspondingly, the modulator section contains another
separate anode area p+ (e.g. InGaAs), also of positive doping polarity,
which is accessible via a second anode contact 303 and a second anode lead
303a. A semi-insulating region (i.e. un-doped area) SI-1 (e.g. InP)
separates the laser anode area p.sub.1 from the modulator anode area p+. A
sub-layer p.sub.1 of the anode area p+ in the modulator section 252,
extends below the first un-doped area SI-1 and contacts the laser anode
area pi.
The laser section has a two-layered cathode area containing a first layer
n.sub.1 - (e.g. InP) of negative doping polarity and a substrate n (e.g.
InP), also of negative doping polarity. The modulator section has a
corresponding cathode area in the form of a second layer n.sub.2 - (e.g.
InP) of negative doping polarity, which is, however, electrically isolated
from the first layer n.sub.1 - by means of two mutually reverse biased
diodes. The diodes are represented by a separation layer P.sub.2 (e.g.
InGaAs) of a positive doping polarity below the second layer n.sub.2 -.
The laser cathode area n.sub.1 -; n is externally accessible via a first
cathode contact 302 and a first cathode lead 302a, while the modulator
cathode area n.sub.2 - is externally accessible via a second cathode
contact 304 and a second cathode lead 304a. A bias contact 305 and a bias
lead 305a make the separation layer P.sub.2 (i.e. an electrical point
between the respective cathode areas n.sub.1 -; n and n.sub.2 -)
externally accessible, such that a voltage can be applied between the
anodes of the two mutually reverse biased diodes. Typically, this point is
allocated the ground potential, however technically, any other voltage may
be applied.
The integrated laser-modulator structure according to the first embodiment
of the invention also contains a first quantum, well structure MQW-1 below
the laser anode area p.sub.1 for generation of a primary light signal
L.sub.1, and a second quantum well structure MQW-2 below the modulator
anode area p+; p.sub.1, for influencing the primary light signal L.sub.1
by means of electroabsorption: (i.e. variably attenuating the primary
light signal L.sub.1). The modulator thus modulates the primary light
signal L.sub.1 and produces a resulting modulated light signal L.sub.2
depending on the voltage across the second anode contact 303 and the
second cathode contact 304, such that a relatively high voltage there over
results in a modulated light signal L.sub.2 of comparatively low power and
a relatively low voltage there over results in a modulated light signal
L.sub.2 of comparatively high power.
The first quantum well structure MQW-1 includes a layer, which includes
different impurities 310 of e.g. InP and 311 of: e.g. InGaAsP that are
distributed over the structure MQW-1 in an alternating manner. The
different materials in the impurities 310 and 311 have different
refractive indices and thus represent a grating, which determines the
wavelength of the light signal L.sub.1 being produced by the laser.
Preferably, both the impurities 310 and 311 have a positive doping
polarity.
A proton bombarded region PBR is included between the first quantum well
structure MQW-1 and the second quantum well structure MQW-2. The proton
bombarded region PBR extends through the respective negative doped layers
n.sub.1 - and n.sub.2 - below the quantum well structures MQW-1 and MQW-2,
such that also the negative doped layers n.sub.1 - and n.sub.2 - are
electrically separated from each other. Thus, a first negative doped layer
n.sub.1 - is located below the first quantum well structures MQW-1 and a
second negative doped layer n.sub.2 - is located below the second quantum
well structures MQW-2. The proton bombarded region PBR guarantees a
complete electrical separation between the laser and modulator cathode
areas n.sub.1 -; n and n.sub.2 - respectively.
FIG. 3b shows an integrated laser-modulator structure according to a second
alternative of the first embodiment of the invention. This design is
essentially the same as that illustrated in FIG. 3a. However, the
modulator is here built on a positively doped substrate p instead of a
negative n. As a consequence thereof, all other layers n.sub.2, P.sub.1 -,
P.sub.2 -, n.sub.1 ; n+ and contacts 301-305 have the opposite polarity
relative the embodiment shown in FIG. 3a. Moreover, the modulator has a
common anode (as opposed to common cathode) and the drive voltage to the
laser must be negative (instead of positive).
FIG. 3c shows an integrated laser-modulator structure according to a third
alternative of a first embodiment of the invention. This design is, with
respect to doping polarities, equivalent to the design described with
reference FIG. 3a above. However, here the modulator section is connected
to the base contact 304 while the laser section is electrically isolated
from the substrate n. All reference numerals signify the same elements as
described in relation to FIG. 3a.
Correspondingly, FIG. 3d shows an integrated laser-modulator structure
according to a fourth alternative of the first embodiment of the
invention, which is equivalent to the design described with reference FIG.
3b above.
FIG. 4 shows a first block diagram over a proposed optical transmitter,
which includes a laser unit 450 according to the invention. The laser unit
450 is here fed by a single-ended modulator driver 420. A protocol
converter 410 receives at least one signal I, which represents information
to be transmitted via a modulated optical signal L.sub.2. The protocol
converter 410 may receive the at least one signal I on a parallel format
and perform various operations with respect to the at least one signal I,
such as coding and scrambling, before a corresponding control signal
C.sub.I is delivered on the protocol converter's 410 output.
A modulator driver 420 receives this control signal C, and generates in
response thereto a modulating signal MI, which is adapted to a modulator
section 452 in the laser unit 450. The modulating signal M.sub.I format
preferably comprises a varying modulator voltage component, which
represents the information I, and a substantially fixed modulator bias
voltage component, which is adapted to an optimal value with respect to
the particular application. As mentioned earlier, on one hand, a lowered
bias voltage (i.e. a more negative value) results in a decreased optical
output power level, and on the other hand, the lowered bias voltage
additionally decreases the modulator's chirp. According to a preferred
embodiment of the invention, the modulator driver 420 is also adapted to
compensate for the exponential transfer function of the modulator section
452, such that the control and tuning of the modulator driver 420 becomes
relatively simple and straight-forward.
A laser bias control unit 430 generates a bias current I.sub.B to a laser
section 451 in the laser unit 450. The bias current I.sub.B controls the
output power level of the primary light signal L.sub.1, which is generated
in a first quantum well structure MQW-1 in the laser section 451, such
that the power of the primary light signal L.sub.1 lies within a desired
range for the specific application. A control loop determines what is an
appropriate value of the bias current I.sub.B.
The control loop contains a monitor diode 440, which detects a leak signal
L'.sub.1 of the primary light signal L.sub.1 from the laser section 451,
for example coming out a rear facet of the laser unit 450. Based on the
leak signal L'.sub.1, the monitor diode 440 delivers a feedback signal
I.sub.ph, which is proportional to the primary light signal L.sub.1 to the
laser bias control unit 430. The laser bias control unit 430 assigns a
suitable bias current is depending on the feedback signal I.sub.ph. The
modulating signal M.sub.I influences (attenuates) the primary light signal
L.sub.1 from the laser section 451 by means of an electroabsorption
process, when this signal L.sub.1 passes through a second quantum well
structure MQW-2 in the modulator section 452, in such manner that an
outgoing modulated light signal L.sub.2 is produced, which represents the
information signal I.
Analogous with the known optical transmitter shown in FIG. 2, a resistor
453 is preferably connected in parallel with the modulator section 452 in
order to terminate any undesired signal reflections. As mentioned with
reference to FIG. 3a above, the laser unit 450 may contain two mutually
reverse biased diodes 454 and 455 respectively. Hence, a voltage V.sub.com
(e.g. ground voltage) may be applied via a bias contact 305 between the
anodes of these diodes 454 and 455. Both the laser section 451 and the
modulator section 452 can thereby be fed by one and the same supply
voltage +V.sub.cc. This allows the single-ended modulator driver 420 to
deliver a modulating signal M.sub.I with a maximum swing of
.vertline.+V.sub.cc.vertline./2.
The supply voltage +V.sub.cc may, of course, be used as supply to the
modulator driver 420 and the laser bias control unit 430. Provided that
the design allows, the supply voltage +V.sub.cc could additionally drive
the protocol converter 410 (however not illustrated in the figure).
FIG. 5a shows a block diagram over an optical transmitter, which includes a
laser unit 450 according to a first embodiment of the invention. The laser
unit 450 is here fed by a differential modulator driver 420. A protocol
converter 410 receives at least one signal I, which represents information
to be transmitted via a modulated optical signal L.sub.2. The protocol
converter 410 may receive the at least one signal I on a parallel format
and perform various operations with respect to the at least one signal I,
such as coding and scrambling, before a corresponding control signal
C.sub.I is delivered on the protocol converter's 410 output.
The differential modulator driver 420 receives the control signal C.sub.I
and generates in response thereto differential modulating signals M.sub.IA
and M.sub.IB which are adapted to a modulator section 452 in the laser
unit 450. According to a preferred embodiment of the invention and in
similarity with the embodiment of the invention described with reference
to FIG. 4 above, the modulator driver 420 is also adapted to compensate
for the exponential transfer function of the modulator section 452.
A laser bias control unit 430 generates a bias current I.sub.B to a laser
section 451 in the laser unit 450. The bias current I.sub.B controls the
output power level of the primary light signal L.sub.1, which is generated
in a first quantum well structure MQW-1 in the laser section 451, such
that the power of the primary light signal L.sub.1 lies within a desired
range for the specific application. A control loop determines what is an
appropriate value of the bias current I.sub.B.
The control loop contains a monitor diode 440, which detects a leak signal
L'.sub.1, of the primary light signal L.sub.1 from the laser section 451,
for example coming out a rear facet of the laser unit 450. Based on the
leak signal L'.sub.1, the monitor diode 440 delivers a feedback signal
I.sub.ph, which is proportional to the primary light signal L.sub.1 to the
laser bias control unit 430. The laser bias control unit 430 assigns a
suitable bias current I.sub.B depending on the feedback signal I.sub.ph.
The modulating signal M.sub.I influences (attenuates) the primary light
signal L.sub.1 from the laser section 451 by means of an electroabsorption
process, when this signal L.sub.1 passes through a second quantum well
structure MQW-2 in the modulator section 452, in such manner that an
outgoing modulated light signal L.sub.2 is produced, which represents the
information signal I.
As mentioned with reference to FIGS. 3 and 4 above, the laser unit 450 may
contain two mutually reverse biased diodes 454 and 455 respectively.
Hence, a voltage V.sub.com. (e.g. ground voltage) may be applied via a
bias contact 305 between the anodes of these diodes 454 and 455. The laser
section 451 can thereby be fed directly by a supply voltage +V.sub.cc.
The same supply voltage +V.sub.cc can also be used for the modulator
section 452, however only as a derived bias voltage +V.sub.bias for
assigning a suitable voltage to a point V.sub.D on the modulator section's
452 cathode, since the modulator driver is differentially connected to the
modulator section 452 via a first coupling capacitor 425 and a second
coupling capacitor 426.
The modulator section 452 receives a first differential modulating signal
M.sub.IA via the first coupling capacitor 425 and a second differential
modulating signal M.sub.IB via the second coupling capacitor 426. A first
resistor 421 and a first capacitor 422 terminates the first differential
modulating signal MIA. The bias voltage +V.sub.bias is applied to a point
between the first resistor 421 and the first capacitor 422. A second
resistor 427 terminates the second differential modulating signal M.sub.IB
directly to ground. For example, if the bias voltage +V.sub.bias equals 2V
and the differential modulating signals M.sub.IA and M.sub.IB each has a
swing of .+-.0,5V in a push-pull relationship, the voltage in the point
V.sub.D will vary between +1V (when M.sub.IA attains its lowest value and
M.sub.IB its highest) and +3V (when M.sub.IA attains its highest value and
M.sub.IB its lowest).
The above-described differential feeding of the modulator section 452 being
accomplished by the modulator driver 420 allows the modulating signal,
i.e. the difference between M.sub.IA land M.sub.IB, to have a maximum
swing of .vertline.+V.sub.cc.vertline., which of course is more power
efficient than the single-ended driving.
FIG. 5b shows a block diagram over a proposed optical transmitter, which
includes a laser unit 450 according to a second embodiment of the
invention. This embodiment is especially adapted for producing a
relatively low-frequency outgoing modulated light signal L.sub.2 (i.e.
with a comparatively low lower cut-off frequency). All units and
components bearing the same identifiers as used in FIG. 5a here have the
same functions as described above with reference to this figure. However,
a second resistor 423 and a second capacitor 424 are added in order to
provide a sufficient termination of the higher frequencies. For the same
reason, a third capacitor 428 has been added between the second resistor
427 and the ground potential. Furthermore, both the second capacitor 424
and the third capacitor 428 are connected to the modulator driver 420. The
extra capacitors 424 and 428 are necessary because there is a physical
limit as to the highest possible value for the first capacitor 422 (and as
a consequence thereof a lowest cut-off frequency). The second resistor 423
adjusts the resistance of the first resistor 421 to a value, which is
adapted to the total capacitance of the first and the second capacitors
422; 424.
FIG. 6 shows a block diagram over a modulator driver 420, in
CMOS-technology, according to an embodiment of the invention, e.g. as
described with reference to FIG. 5b above. Here, on one hand, a set of
so-called 50w-drivers 622-625 deliver wide-frequency modulating signals
M.sub.IAW ; M.sub.IBW containing spectral components that represent a
relatively wide frequency range, and on the other hand, a set of so-called
low-impedance drivers 626-629 deliver low-frequency modulating signals
M.sub.IAL ; M.sub.IBL, predominantly containing spectral components that
represent relatively low frequencies. As mentioned earlier, the capacitors
425 and 426 may namely render the lower cut-off frequency for the
transmission path to the modulator section 452 too high. According to this
embodiment however, the low-frequency modulating signals M.sub.IAL ;
M.sub.IBL, (below this cut-off frequency) may thus nevertheless reach the
modulator section 452.
The 50 .OMEGA.-drivers 622-625 include a first P-channel transistor 622 and
a first N-channel transistor 623 connected between a supply voltage
+V.sub.cc and the ground potential GND, which receive a non-inverted
amplified control signal A.sub.CI from an amplifier 621 and generate on
basis thereof a first differential modulating signal M.sub.IA. The
amplifier 621, in turn, bases its output signals A.sub.CI and A'.sub.CI on
a control signal C.sub.I. A second P-channel transistor 624 and a second
N-channel transistor 625 connected between the supply voltage +V.sub.cc
and the ground potential GND, receive an inverted amplified control signal
A'.sub.CI from the amplifier 621 and generate on basis thereof a second
differential modulating signal M.sub.IB. The first differential modulating
signal M.sub.IA and the second differential modulating signal M.sub.IB are
fed out directly as the wide-frequency modulating signals M.sub.IAW ;
M.sub.IBW to the modulator section 452 in a laser unit, via a respective
decoupling capacitor 425 and 426.
A first low pass filtering chain containing a first lowpass filter 626 and
a following first filter amplifier 627 receives the first differential
modulating signal M.sub.IA and generates in response thereto a first
low-frequency modulating signal M.sub.IAL. Correspondingly, a second low
pass filtering chain containing a second lowpass filter 628 and a
following second filter amplifier; 629 receives the second differential
modulating signal M.sub.IB and generates in response thereto a second
low-frequency modulating signal M.sub.IBL. The first and second lowpass
filters 626 and 628 have a cut-off frequency, which corresponds to the
cut-off frequency for the signal path via the first coupling capacitor 425
and the first resistor 426 respective the second coupling capacitor 426
and the second resistor 421.
Thus, the modulator driver 420 delivers modulating signals over a
relatively broad frequency range to the modulator section !452, namely in
the form of the low-frequency modulating signals M.sub.IAL and M.sub.IBL
and the wide-frequency modulating signals M.sub.IAW and M.sub.IBW. This
arrangement is advantageous, since the spectral separation of the
modulating signals M.sub.IAL and M.sub.IBL ; M.sub.IAW ; and M.sub.IBW
means that the high-frequency signals can use a well terminated driver
with small capacitors 425 and 426, while the terminated outputs need only
be used for delivering the low-frequency signals M.sub.IAL and M.sub.IBL.
In analogy with the biasing in the FIGS. 5a and 5b, a bias voltage
+V.sub.bias allocates a suitable cathode voltage to the modulator section
452. The modulator section 452 receives a first low-frequency differential
modulating signal M.sub.IA via a first coupling capacitor 425 and a second
low-frequency differential modulating signal M.sub.IB via a second
coupling capacitor 426. A first resistor 421 and a first capacitor 422
terminates the first differential modulating signal M.sub.IA. The bias
voltage +V.sub.bias is applied to a point between the first resistor 421
and the first capacitor 422. A second resistor 427 terminates the second
differential modulating signal M.sub.IB directly to ground.
According to a preferred embodiment of the invention, the modulator driver
420 is included in an application specific integrated circuit (ASIC) 600.
It is further preferable if this ASIC 600 also includes a protocol
converter 410, which receives at least one signal I representing the
information to be transmitted. The protocol converter 410 then produces
the control signal C, based on the signal I as described above with
reference to FIGS. 4 and 5a. Naturally, it is advantageous if the
integrated protocol converter 410 additionally includes a duty cycle
control unit for compensation with respect to the exponential transfer
function of the modulator section 452.
FIG. 7a shows an integrated laser-modulator structure according to a first
alternative of a second embodiment of the invention. The laser section
here contains a separate anode area of a positive doping polarity p (e.g.
InP), which is accessible via a first anode contact 301 and a first anode
lead 301a. Correspondingly, the modulator section contains another
separate anode area p+ (e.g. InGaAs), also of positive doping polarity,
which is accessible via a second anode contact 303 and a second anode lead
303a. A semi-insulating region (i.e. un-doped area) SI-1 (e.g. InP)
separates the laser anode area p from the modulator anode area p+. A
sub-layer p of the anode area p+ in the modulator section extends below
the first un-doped area SI-1 and contacts the laser anode area p. The
laser section has a single-layered cathode area containing a first layer
n.sub.1 - (e.g. InP) of negative doping polarity, which is placed on a
semi-insulating un-doped substrate SI-2. The modulator section has a
corresponding cathode area on top of the substrate SI-2 in the form of a
second layer n.sub.2 - (e.g. InP) of negative doping polarity, which
however, is electrically isolated the first layer n.sub.1 - by means of a
proton bombarded region PBR.
The proton bombarded region PBR also separates a first quantum well
structure MQW-1 from a second quantum well structure MQW-2 overlying the
first layer n.sub.1 - respective the second layer n.sub.2 -. The proton
bombarded region PBR thereby guarantees a complete electrical separation
between the laser cathode area no- and the modulator cathode area n.sub.2
-.
The laser cathode area n.sub.1 - is externally accessible via a first
cathode contact 302, while the modulator cathode area n.sub.2 - is
externally accessible via a second cathode contact 304 and a second
cathode lead 304a. For reasons of illustration, FIG. 7a shows the first
cathode contact 302 to the immediately left of the first quantum well
structure MQW-1. However, according to a preferred embodiment of the
invention this contact 302 is instead positioned behind the first quantum
well structure MQW-1 in level with the second cathode contact 304, such
that the integrated modulator-laser structure obtains a planar vertical
end-surface on the modulator side. It is worth mentioning that this
laser-modulator structure does not require any biasing, since no diodes
are used to separate the cathode areas electrically from each other.
Hence, the structure also lacks a bias contact.
In analogy with the laser-modulator structure according to the first
embodiment of the invention, the structure according to this embodiment
also generates a primary light signal L.sub.1 in the first quantum well
structure MQW-1 below the laser anode area p.sub.1. The second quantum
well structure MQW-2 below the modulator anode area p+; p.sub.1,
influences the primary light signal L.sub.1 by means of electroabsorption
(i.e. attenuating the primary light signal L.sub.1) such that a resulting
modulated light signal L.sub.2 is produced depending on the voltage across
the second anode contact 303 and the second cathode contact 304. A
relatively high voltage over the contacts thereby results in a modulated
light signal L.sub.2 of comparatively low power and a relatively low
voltage results in a modulated light signal L.sub.2 of comparatively high
power.
The first quantum well structure MQW-1 includes a layer, which includes
different impurities 310 of e.g. InP and 311 of e.g. InGaAsP that are
distributed over the structure MQW-1 in an alternating manner. The
different materials in the impurities 310 and 311 have different
refractive indices and thus represent a grating, which determines the
wavelength of the light signal L.sub.1 being produced by the laser.
Preferably, the impurities 310 and 311 have a positive doping polarity.
FIG. 7b shows an integrated laser-modulator structure according to a second
alternative of the second embodiment of the invention, which is
essentially the same as that illustrated in FIG. 7a. However, in analogy
with the alteration of polarities between the embodiments described in
FIGS. 3a and 3b, all layers p.sub.1 -, P.sub.2 -, n; n+ and contacts
301-304 have the opposite polarity relative the embodiment shown in FIG.
7a. Likewise, the modulator has a common anode (as opposed to common
cathode) and the drive voltage to the laser must be negative (instead of
positive). Also here, the figure shows the first cathode contact 302 to
the immediately left of the first quantum well structure MQW-1, for
reasons of illustration. In the actual design, however, this contact 302
is instead preferably positioned behind the first quantum well structure
MQW-1 in level with the second cathode contact 304, such that the
integrated modulator-laser structure obtains a planar vertical end-surface
on the modulator side.
The optical transmitters described above with reference to the FIGS. 4, 5a
and 5b are primarily adapted to the laser-modulator structures shown in
the FIGS. 3a, 3c and 7a respectively. Nevertheless, the teachings of this
specification as a whole is deemed to provide sufficient guidance for the
skilled person to design corresponding optical transmitters also for the
reverse-polarity laser-modulator structures shown in the FIGS. 3b and 7b.
The term "comprises/comprising" when used in this specification is taken to
specify the presence of stated features, integers, steps or components.
However, the term does not preclude the presence or addition of one or
more additional features, integers, steps or components or groups thereof.
The invention is not restricted to the described embodiments in the
figures, but may be varied freely within the scope of the claims.
*
