Wavelength stabilization of laser source using fiber Bragg grating feedback

A laser source with an extremely stable output is provided. A laser diode has an output intensity centered at a peak wavelength which is responsive to a control signal. First and second fiber Bragg gratings are coupled to the laser diode. The first fiber Bragg grating having a reflectivity centered about a first wavelength and the second fiber Bragg grating having a reflectivity centered about a second wavelength different from the first wavelength. Each of the first and second fiber Bragg gratings generates a feedback signal responsive to the reflectivity of the fiber Bragg grating and the output intensity of the laser diode. A controller connected to the laser diode generates the control signal responsive to the feedback signals from the first and second fiber Bragg gratings so that the peak wavelength of the laser diode is maintained at a fixed wavelength between the first and second wavelengths.

BACKGROUND OF THE INVENTION 
The present invention is related to the field of fiberoptic network laser 
sources and, more particularly, to laser diode sources having very high 
spectral stability. 
In many fields of optics, such as precision optical instruments, optical 
telemetry (remote) sensing systems, high performance optical sensors and 
the like, laser sources having a high degree of spectral stability are 
very desirable. With such sources, the wavelength (or frequency) of the 
laser light output varies little with changing conditions. Perhaps the 
field with the most pressing need, at least in terms of numbers, for a 
laser source with a stable spectral output is the Dense WDM (DWDM) 
fiberoptic network. 
In WDM (Wavelength Division Multiplexed) networks, the wavelength of an 
optical signal is used to direct the signal from its source to its 
destination. Each network user typically has a laser source operating at a 
specific wavelength which is different from those of other laser sources. 
Hence a stable laser source having a fixed output wavelength is desirable. 
As the number of users on a WDM network increases, a larger number of 
laser sources are required for signal generation. The large bandwidth 
networks, such as DWDM networks, increase the demand for highly stabilized 
laser sources. 
To increase the bandwidth and the number of communication channels in WDM 
networks, the ITU, the International Telecommunications Union, has 
proposed the Dense WDM, or DWDM. The separation between communication 
channels in the DWDM is only 0.8 nm, or 100 GHz in frequency. Thus a light 
source for such a network must also have a very narrow output linewidth, 
i.e., the wavelength of the output signal must be concentrated in a very 
narrow portion of the optical spectrum, and the wavelength of the source 
must be extremely stable to avoid drifting into the wavelength range of 
another channel. 
In present laser sources, such as DFB (Distributed Feedback), DBR 
(Distributed Bragg Reflectors) or Fabry-Perot laser diode laser sources, 
the output wavelength changes in varying degrees with changes in the bias 
current of the laser diode and changes in temperature. FIG. 1, for 
instance, illustrates the changes in spectral output in response to 
changes in the bias current for a modern DFB laser diode. Various 
techniques are used to stabilize the bias current and temperature of the 
laser diode. However, conventional bias current and temperature 
stabilization are inadequate for the stringent requirements for many 
optical systems, such as DWDM networks. 
The present invention provides for such a laser source with an output which 
is very stable. 
SUMMARY OF THE INVENTION 
The present invention provides for a laser source with an extremely stable 
output. The laser source has a laser diode which is connected to an output 
optical fiber. The laser diode has an output intensity centered at a peak 
wavelength which is responsive to a control signal. First and second fiber 
Bragg gratings are coupled to the laser diode with the first fiber Bragg 
grating having a reflectivity centered about a first wavelength and the 
second fiber Bragg grating having a reflectivity centered about a second 
wavelength different from the first wavelength. Each of the first and 
second fiber Bragg gratings generates a feedback signal responsive to the 
reflectivity of the fiber Bragg grating and the output intensity of the 
laser diode. A controller connected to the laser diode generates the 
control signal responsive to the feedback signals from the first and 
second fiber Bragg gratings so that the peak wavelength of the laser diode 
is maintained at a wavelength between the first and second wavelengths.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
It should be noted that the same reference numerals are sometimes used for 
elements in a different drawings to emphasize that the elements have 
substantially the same function or operation to better explain the 
different aspects of the present invention. 
FIG. 2A is a schematic representation of a laser source according to one 
embodiment of the present invention. The output light from a laser diode 
10 is passed to an output fiber 12 through an optical isolator 11. The 
laser source also has an optical coupler 13 which diverts a small portion 
of the output light from the output fiber 12 toward a second coupler 14. 
For example, an output ratio of 9:1 in favor of the output fiber 12 has 
been found to work effectively. Such couplers are known in optical fiber 
practice. For example, optical fibers with unevenly stretched cores and 
claddings may be twisted together to form the coupler. On the other hand, 
the optical coupler 14 splits the received light evenly into two optical 
fibers 15 and 16, each of which has fiber Bragg gratings 17 and 18, 
respectively. A so-called "3 dB" twisted pair coupler may be used for the 
optical coupler 14. 
A photodiode 21 receives light which has passed through the fiber Bragg 
grating 17 and a photodiode 22 receives light which has passed through the 
fiber Bragg grating 18. The output of these photodiodes 21 and 22 is sent 
to an amplifier/controller unit 20, which generates a feedback control 
signal for the laser diode 10. Responsive to the differences in output 
signals from the photodiodes 21 and 22, the control signal works in a 
negative feedback mode to maintain the output from the diode 10 centered 
at the desired wavelength. 
The FIG. 2B graph of reflection intensity versus wavelength illustrates an 
idealized performance of the two fiber Bragg gratings 17 and 18. Fiber 
Bragg gratings are created by a periodic or near periodic variation in the 
index of refraction in the core of an optical fiber and may have extremely 
sharp and narrow peaks in reflectivity with respect to wavelength. As 
shown in the graph of FIG. 2B, the grating 17 has a reflectivity peak at 
wavelength .lambda..sub.1 and grating 18 has a reflectivity peak at 
wavelength .lambda..sub.2. The gratings 17 and 18 are designed such that 
the peak wavelengths .lambda..sub.1 and .lambda..sub.2 are separated so 
that the intensity of the light received by the photodiodes 21 and 22 as 
modified by the fiber Bragg gratings 17 and 18 are responsive to different 
parts of the optical spectrum. Hence, as shown in FIG. 2B, the peak 
wavelengths .lambda..sub.1 and .lambda..sub.2 are separated to define an 
intermediate center wavelength .lambda..sub.C where the reflection 
intensities of the two gratings 17 and 18 are equal. At this output 
wavelength of the laser diode 10, the photodiodes 21 and 22 detect equal 
power. The difference in the two reflection intensities about the central 
wavelength .lambda..sub.C is used to lock in the output of the laser diode 
10. For example, if the signal of the photodetector 21, which is connected 
to one (positive) of the input terminals of the amplifier/controller unit 
20, is stronger than the signal from the photodetector 22, which is 
connect to the other (negative) input terminal, then the output wavelength 
of the laser diode 10 is shorter than .lambda..sub.C. The 
amplifier/controller unit 20, operating as a comparator, generates a 
positive signal to the laser diode 10 to increase its output wavelength. 
Likewise, if the output wavelength is longer than .lambda..sub.C, then the 
amplifier/controller unit 20 generates a negative signal to the laser 
diode 10 to decrease its output wavelength. By this feedback loop, the 
amplifier/controller 20 operates to maintain the output of the laser diode 
10 so that the signals received from the photodiodes 21 and 22 are 
substantially equal. 
Note that while the gratings 17 and 18 are discussed in terms of reflection 
intensities, they also form transmission intensities versus wavelength as 
shown in FIG. 2C. In other words, as shown in the arrangement of FIG. 2A, 
the actual light intensity as received by each of the photodiodes 21 and 
22, have minimums at spaced apart wavelengths, specifically .lambda.1 and 
.lambda.2. The two transmission intensities of the gratings 17 and 18 are 
equal at .lambda..sub.C where the laser diode 10 is locked. As shown in 
FIG. 2C, actual wavelengths for fiber gratings are indicated. In this 
case, the reflection intensities are centered about 1531.5 nanometers and 
1531.85 nanometers. 
FIG. 3A illustrates a laser diode source in greater detail, according to 
another embodiment of the present invention. The laser diode source is 
formed by a laser diode module 30, an optical feedback module 31 generates 
a control signal from the output light of the module 30, and the 
amplifier/controller block 20 which receives the control signal from the 
optical feedback module 31 to control the output of the laser diode module 
30. 
The laser diode module 30 has the laser diode 10 with a lens system 33 
which collimates the output light from one facet of the laser diode 10 for 
an optical isolator 32. A second lens system (not shown) focusses the 
collimated light from the isolator 32 into the output fiber 12. On the 
other side of the laser diode 10, a second lens system 34 focuses the 
output light from the second facet of the laser diode 10 into an optical 
fiber section 26 in which a fiber Bragg grating 19 is formed. The narrow 
reflection bandwidth of the fiber Bragg grating 19 narrows the output of 
the laser diode 10 such that the reflection peak of the fiber Bragg 
grating 19 defines the output wavelength of the laser diode 10. 
Also part of the laser diode module 30 is a temperature-control unit 27 to 
which the optical fiber section 26 is fixed. Despite changes in the 
ambient temperature of the module 30, the temperature control by the unit 
27 controls the effective index refraction of the core of the section 26, 
the expansion of the section 26 and the fiber Bragg grating period 
.LAMBDA.. Changes to the period .LAMBDA. causes shifts in the wavelength 
of reflectivity peaks of the fiber Bragg grating. Thermoelectric modules 
have proved to be effective as temperature-control units in providing such 
temperature control of optical fiber sections and fiber Bragg gratings. 
The optical feedback module 31 has the optical isolator 11 which is 
connected to the optical fiber section 26 from the module 30. The optical 
feedback module 32 also has optical couplers 23, 24 and 25, and 
photodiodes 21 and 22. The optical isolator 11 is connected to the optical 
coupler 23 which, in turn, is connected to the optical couplers 24 and 25. 
The optical coupler 24 is connected to an optical fiber 37 which contains 
the fiber Bragg grating 17, and to an optical fiber 39 which is connected 
to the photodiode 21. The optical coupler 25 is connected to an optical 
fiber 38 which has the fiber Bragg grating 18, and to an optical fiber 29 
which is connected to the photodiode 22. The ends of the optical fibers 37 
and 38 are respectively terminated by end sections 35 and 36, which are 
slanted at an angle and coated with antireflection material. Light which 
is not reflected by the fiber Bragg gratings 17 and 18 is transmitted 
through the terminals 35 and 36, rather than being reflected back. The 
operation of the couplers 24 and 25 is such that upon reflection by the 
fiber Bragg gratings 17 and 18, light is passed back to the fibers 39 and 
29, and to the photodiodes 21 and 22 respectively. The optical isolator 11 
prevents any light which is also reflected back into the coupler 23 from 
reaching the laser diode module 30. 
The portions of the optical fibers 37 and 38 which hold the fiber Bragg 
gratings 17 and 18 are also fixed to a temperature-control unit 28. As 
discussed previously with respect to the fiber Bragg grating 19, the 
temperature-control unit 28 controls the performance of the fiber Bragg 
gratings 17 and 18, despite variations in ambient temperature. 
The feedback signals from the photodiodes 21 and 22 are connected back by 
lines 71 into the amplifier/controller block 20. The block 20 then adjusts 
the bias current to the laser diode 10 (and its cooling since the laser 
diode 10 is also mounted to a temperature-control unit(not shown)) and 
controls the operation of the temperature-control unit 27 for the fiber 
Bragg grating 19. The block 20 communicates to the module 30 through lines 
70. The block 20 also controls the operations of the temperature control 
units 28 through a line 72 to set the temperatures for the fiber Bragg 
gratings 19 and 17 and 18. It should be noted that in the drawings lines 
carrying optical signals, i.e., optical fibers, are represented by solid 
lines and lines carrying electrical signals are represented by dashed 
lines. 
The amplifier/controller block 20 controls many elements which can affect 
the output wavelength of the laser diode 10. The laser output is 
maintained at a constant wavelength in spite of changing conditions. 
Conversely, the elements, under the control of the block 20, can be used 
to vary the output wavelength. One example of this type of operation is 
disclosed below. 
FIG. 3B illustrates the reflection intensity versus wavelength for two 
fiber Bragg gratings which may be used for the gratings 17 and 18. As 
shown, the peaks of the reflection intensities of the two fiber Bragg 
gratings are separated by 0.45 nanometers between which a wavelength 
.lambda..sub.L to which the output wavelength of the laser diode is locked 
by the feedback operations of the module 32 and amplifier/controller block 
20. 
FIG. 4 illustrates a block diagram of the amplifier/controller block 20. 
The block 20 is formed by amplifiers 40-42, 44-49 and a comparator 43. As 
shown, the photodiode 21 is connected across a resistor 51 and to one 
input terminal of the amplifier 41. Likewise, the photodiode 22 is 
connected across a resistor 52 to an input terminal of the amplifier 42. 
The outputs of the amplifiers 41 and 42 are fed into the terminals of a 
comparator amplifier 43 whose output is connected to the input of a main 
amplifier 44. The output of the amplifier 44 is then connected to the 
input terminals of buffer amplifiers 45, 47 and 49. Each of these 
amplifiers 45, 47 and 49 has a second terminal connected to a reference 
voltage. The output of the amplifier 45 is connected to a driver amplifier 
46 which, in turn, drives the bias current to the laser diode 10. The 
output of the buffer amplifier 47 is connected to a driver amplifier 48 
whose output terminal is connected to the temperature-control units 27 and 
28. The amplifier 49 has its output connected to the driver amplifier 40 
which drives other wavelength controller functions. For example, in the 
embodiments illustrated in FIGS. 3A and 6A, besides bias current and 
temperature, the output of the driver amplifier 40 could be used to 
stretch or compress the fiber Bragg gratings 19, 69A, 69B and 69C by 
piezoelectric transducers to control the wavelength of the laser diode 
output. 
As discussed above, the reflection intensities of the two fiber Bragg 
gratings 17 and 18 used in the feedback loop have extremely narrow 
bandwidths. If, by chance, the output of the laser diode 10 falls outside 
of the range between the peaks of these two fiber Bragg gratings 17 and 
18, there is a problem of returning the output of the laser diode 10 into 
the center or locked wavelength. Rather than two fiber Bragg gratings, 
four feedback fiber Bragg gratings are used to address this problem. 
The arrangement shown in FIG. 5A illustrates the operation of this aspect 
of the present invention. In this embodiment, the output of the laser 
diode 10 is again received by the optical isolator 11 which is connected 
to the output fiber 12. The optical coupler 13, which taps off a small 
amount of the laser diode 10 output, sends the tapped output intensity to 
the optical coupler 14, which splits the two light intensities into 
optical fibers 15 and 16. The optical fiber 15 is in turn connected to a 
coupler 51 which splits the output further into two optical fiber sections 
which each contain one of two fiber Bragg gratings 53 and 54. Likewise, 
the optical fiber 16 is connected to a coupler 58 which splits the 
incoming light into two optical fiber sections which each contain one of 
two fiber Bragg gratings 57 and 58. The outputs through the fiber Bragg 
gratings 53 and 54 are received by photodiodes 55 and 56 respectively. The 
output signals from the photodiodes 55 and 56 are fed into the 
amplifier/control block 60. The outputs through the fiber Bragg gratings 
57 and 58 are received by the photodiodes 59 and 61 which, in turn, send 
their outputs to the amplifier/control block 60. The amplifier/control 
block 60 then sends back a feedback signal back to the laser diode 10. 
FIG. 5B illustrates the reflection intensities of the pairs of fiber Bragg 
gratings 53 and 54, and 57 and 58. The fiber Bragg gratings 53 and 54 
operate as discussed above, i.e., their peak reflection intensities are 
separated very narrowly to define a wavelength range .DELTA..lambda..sub.2 
about the center wavelength .lambda..sub.C, as shown in FIG. 5B. The fiber 
Bragg gratings 57 and 58 have their peak intensities separated by a far 
greater range .DELTA..lambda..sub.1, which is also centered around the 
center wavelength .lambda..sub.C. When the output of the laser diode 10 
falls outside the .DELTA..lambda..sub.2 range, the amplifier/control block 
60 relies upon the feedback signals from the laser diodes 57 and 58 with 
the wider wavelength range .DELTA..lambda..sub.1 to push the output of the 
laser diode 10 back toward the center wavelength .lambda..sub.C. When the 
output of the laser diode 10 returns to the wavelength range 
.DELTA..lambda..sub.2, the amplifier/control block 60 relies upon the 
feedback signals from the Bragg gratings 53 and 54 to center the laser 
diode output at .lambda..sub.C much more precisely and quickly. The 
feedback operations return to normal as have been described previously. 
FIG. 6A details an arrangement similar to the arrangement in FIG. 3A. As 
described previously, the forward output of the laser diode 10 is 
collimated by the lens system 33 for the optical isolator 32. Eventually 
the output optical fiber 12 receives the forward output. However, instead 
of a single fiber Bragg grating at the back facet of the laser diode 10, 
the laser diode module 62 has an optical fiber section 66 with three 
reflective fiber Bragg gratings 69A, 69B and 69C. The backward output of 
the laser diode 10 is focussed by the lens system 34 into an end of the 
optical fiber section 66 which has its opposite end connected to an 
optical isolator 11 of the optical feedback block 31. As described 
previously, the couplers 24 and 25 pass the light reflected by the fiber 
Bragg gratings 17 and 18 back to the fibers 39 and 29, and to the 
photodiodes 21 and 22 respectively. The outputs of the photodiodes 21 and 
22 are fed back into the amplifier/control block 20 which adjusts the bias 
current (and operating temperature) of the laser diode 10 and the 
temperatures of the fiber Bragg gratings 69A-69C through lines 73. The 
amplifier/control block 20 also controls the temperature of the fiber 
Bragg gratings 17 and 18 through the line 72. 
FIG. 6B illustrates the reflection intensities of the fiber Bragg gratings 
69A-69C. Each of the reflection intensity peaks of the fiber Bragg 
gratings 69A-69C defines an output wavelength for the laser diode 10. Each 
of the portions of the optical fiber section 66 containing the fiber Bragg 
gratings 69A-69C is fixed to its own temperature control unit 67A-67C 
respectively. Through the lines 73, the amplifier/control block 20 can set 
the temperature of each of the temperature control units 67A-67C and, 
hence, of the corresponding fiber Bragg gratings 69A-69C. As explained 
above, the period A of a fiber Bragg grating and the effective refractive 
index of the fiber core define the central wavelength of the reflection 
spectrum of the fiber Bragg grating and is affected by the temperature of 
the grating. Hence the amplifier/control block 20 can control the output 
wavelengths of the laser diode 10 separately through the lines 73 by the 
individual operation of each of the temperature control units 67A-67C. 
Overall control of the output of the laser diode 10 is effected by the 
optical feedback block 31 and its fiber Bragg gratings 17 and 18, as 
explained previously. 
To control three output wavelengths independently, three pairs of fiber 
Bragg gratings are required for a separate feedback signal for each of the 
three output wavelengths. Each fiber Bragg grating pair controls one 
output wavelength by a corresponding control of the temperature of a 
grating 69A, 69B or 69C. Three partial optical feedback blocks 31, 
connected in parallel to the optical isolator 11 to receive light signals 
from the laser diode 10, for instance, can be used for separate control of 
the temperature-control unit 67A-67C. Each feedback block 31 is modified 
so that its corresponding fiber Bragg gratings 17 and 18 are centered 
about a .lambda..sub.C which is one of the desired output wavelengths. 
While the description above provides a full and complete disclosure of the 
preferred embodiments of the present invention, various modifications, 
alternate constructions, and equivalents will be obvious to those with 
skill in the art. Thus, the scope of the present invention should be 
limited solely by the metes and bounds of the appended claims.