A digitally tuned optical frequency synthesizer includes a laser control arrangement for tuning the laser over a range of frequencies. The laser is tunable in response to different magnitudes of injected bias current. A digital processor determines bias current values which produce the desired different operating frequencies corresponding to a set of Fabry-Perot resonant frequencies. Those current values are stored in the digital processor for ready retrieval. During operation, one of the bias current values is retrieved and is applied to the laser. If the laser characteristic curve has drifted, an incremental bias current is determined and is added to the retrieved bias current value. For future use, this new bias current value is stored in the digital processor in place of the originally retrieved bias current value. The digital processor additionally calculates new bias current values for all of the desired frequencies of operation and stores those new values for subsequent selection.

This invention relates to a laser which is arranged for selectable 
frequency tuning. 
BACKGROUND OF THE INVENTION 
There are known arrangements for tuning a laser to operate at selected 
frequencies throughout a wide range of frequencies. Frequency-tunable 
semiconductor lasers are attractive devices for optical frequency division 
multiplex transmission systems. In a tunable arrangement, a distributed 
Bragg reflector laser provides a very large tuning range when different 
values of injection, or bias, current are injected into the phase control 
and the distributed Bragg reflector regions of the laser. 
In the prior art, changes of frequency have been made by selecting the 
values of bias current from continuously variable analog functions. 
Although the tuning range is very wide, there is a problem when the laser 
is changed from a first frequency to a second frequency and thereafter is 
to be returned to the exact first frequency. In returning the laser to the 
first frequency, it is very difficult to select the injection current 
value which will produce the exact first frequency. The exact frequencies 
are defined by the resonances of a Fabry-Perot resonator. There is no 
provision in the prior art to measure the drift of the operating 
characteristic of the laser. 
SUMMARY OF THE INVENTION 
This problem and others are solved by a digitally tuned optical frequency 
synthesizer including a laser control arrangement for tuning the laser 
exactly to a desired resonant frequency selected from a range of 
frequencies. The laser is tunable in response to different values of 
control signal. A digital processor determines control signal values which 
produce different operating frequencies. Those control signal values are 
stored in the digital processor for ready retrieval. During operation, one 
of the control signal values, representing a desired operating frequency, 
is retrieved from storage and is applied to the single frequency laser. If 
the laser characteristic curve has drifted, a control signal error is 
determined by a control circuit part of the feedback loop and is added to 
the retrieved control signal value. The digital processor measures the 
value of the control signal error and derives a new total control signal 
value that is stored in place of the originally retrieved control signal 
value. 
When the digital processor determines that the laser characteristic has 
drifted and causes a error control signal, the digital processor 
additionally calculates new control signal values for all of the desired 
frequencies of operation and stores those new control signal values in the 
appropriate locations in the digital processor for subsequent selection 
and use. 
As a result, the laser is controlled by readily selectable control signal 
values which produce the exact desired operating frequencies. An optical 
transmitter using this arrangement can be applied advantageously in an 
optical frequency division multiplexed transmission system.

DETAILED DESCRIPTION 
Referring now to FIG. 1, there is shown a block diagram of an optical 
frequency division multiplex arrangement for transmitting information from 
a set of transmitting stations 20, 21, 22 and 23 to a set of receiving 
stations 26, 27, 28 and 29. 
Optical fibers 30, 31, 32 and 33 carry the optical signals, respectively, 
from the transmitting stations 20, 21, 22 and 23 to an optical star 
coupler 35. Each of those optical signals includes information modulated 
on an optical carrier. For example, the transmitting stations 20, 21, 22 
and 23 each transmit information on a different optical carrier frequency 
selected from some number N (where, e.g., N =forty) of available optical 
carrier frequencies. By choice of an operator, those forty optical carrier 
frequencies are available from each of the transmitting stations 20, 21, 
22 and 23. As a result of interactive controls, only one station at a time 
can select any one of the available optical carrier frequencies. 
Concurrently, the other stations can be operated at different ones of the 
carrier frequencies. The transmitting stations are synchronized with one 
another by a scheme, such as the one disclosed in a copending patent 
application, Ser. No. 059.973, filed in my name on June 9, 1987. The 
teaching of that patent application is incorporated herein by reference. 
All of the concurrently selected optical carrier frequencies are 
multiplexed together within the optical star coupler 35. From the star 
coupler, all concurrently transmitted carriers are forwarded through all 
of the fibers 36, 37, 38 and 39 to the receiving stations 26, 27, 28 and 
29 which are also operated in synchronism. 
Receiving stations 26, 27, 28 and 29 may be operated in either of two 
different ways. The first way to operate is to assign each of the 
receiving stations a fixed predetermined one of the forty optical carrier 
frequencies. Such predetermined frequency assignments limit the 
flexibility of the system to the extent that each transmitting station 
must select the optical carrier frequency which can be received by the 
desired receiving station. A second way to use the optical carrier 
frequencies in the receiving stations, is to provide each of the receiving 
stations with an arrangement which at any time will enable the operator of 
each receiving station to select for reception any one of the forty 
optical carrier frequencies being transmitted. The second way to operate 
the receiving stations is described more completely in another patent 
application Ser. No. 347,121, filed concurrently herewith in my name. The 
teaching of that patent application is incorporated herein by reference. 
All of the transmitting stations are equipped with a tunable laser 
arrangement wherein the laser can selectively produce any one of the forty 
optical carrier frequencies at the option of the transmitting station 
operator. 
The transmitting stations 20, 21, 22, and 23 of FIG. 1 have been arranged 
to compensate, or correct, automatically for any variation, or drift, in 
the device or circuit characteristics. By thus compensating for drift, the 
transmitting station operator is assured that any selection of operating 
carrier frequency for the operator's transmitting station will result in 
that transmitting station operating at the specifically selected optical 
carrier frequency rather than some other frequency to which the station 
might otherwise tend to drift. 
A description of the automatic control arrangement for the transmitting 
stations is presented in greater detail hereinafter with reference to 
FIGS. 2 through 7. 
Referring now to FIG. 2, there is shown a diagram of a tunable optical 
frequency synthesizer arrangement 50. This laser frequency selection and 
control arrangements 50 can be used for each one of the transmitting 
stations 20, 21, 22 and 23 of FIG. 1. In FIG. 2, the arrangement 50 
includes a distributed Bragg reflector laser 51 which produces a tunable 
single frequency output signal that is coupled into an optical fiber, or 
guide 52. A single frequency laser is a laser that produces essentially a 
single longitudinal mode. An optical coupler 53 taps a fraction of the 
optical signal from the guide 52 to be applied to an opto-electronic 
arrangement 54 that stabilizes a series of spaced optical output 
frequencies of the laser 51 by using the resonant characteristics of a 
Fabry-Perot resonator. The arrangement 54 phase modulates the tapped 
sample of the laser output in a phase modulator 55 in response to a 
modulation signal F1. The Fabry-Perot resonator 56, in response to the 
output of the phase modulator 55, produces a selectable set of phase 
modulated resonance frequencies that are applied to a photodiode 57. The 
photodiode 57 detects the time derivative of the phase modulation as the 
optical signal frequency drifts across a resonance of the Fabry-Perot 
resonator 56. Resulting photocurrent is correlated with an appropriately 
phase-adjusted version of the phase modulation signal F1 in a balanced 
mixer 58. A quadrature phase shift device 61 makes such adjustment. The 
output of the balanced-mixer 58 is filtered and amplified through a filter 
and amplifier 59 into an error signal on a lead 60. Thus the arrangement 
54 generates, on the lead 60, an error signal that is used for locking the 
laser oscillator frequency to the selected operating frequency. A more 
detailed description of the arrangement 54 is presented in the 
aforementioned copending patent application, Ser. No. 059,973, filed in my 
name. There are other well known techniques for producing an error signal, 
as produced by the opto-electronic arrangement 54. See for example, R. V. 
Pound, Radiation Laboratory Series 16, pages 342-343. 
In the system of FIG. 2, there is a need to guarantee that the lasers of 
the transmitters will be controlled to operate at the desired resonance 
frequencies. 
Briefly, FIG. 3 presents a frequency domain curve 62 of the output of the 
Fabry-Perot resonator 56. The Fabry-Perot resonant frequencies occur at 
the frequencies f1, f2, f3, f4 and f5 where the curve 62 peaks in FIG. 3. 
Desired optical carrier signal frequencies for the output of the 
synchronizer 50 of FIG. 2 coincide with the Fabry-Perot resonant 
frequencies shown in FIG. 3. 
In FIG. 4, there is a curve 63 representing an error signal which makes 
both positive and negative polarity excursions between the resonance 
frequencies established by the Fabry-Perot resonator. Desired optical 
carrier signal frequencies coincide with the resonant frequencies of the 
Fabry-Perot resonator. It is noted that the error signal 63 has zero 
crossings which coincide with the resonance frequencies f1, f2, f3, f4 and 
f5 of the Fabry-Perot resonator. Also it is noted that the amplifier 
inserts an offset voltage to the error signal 63 so that, between the 
peaks, the flat portions of the error signal are slightly above zero. The 
reason for this offset voltage will become clear subsequently. During 
regular operation of the synthesizer 50 of FIG. 2 and from a keyboard 79, 
the digital processor 75 is given a desired frequency selection from among 
the frequencies f1, f2, f3, etc., and it applies an appropriate bias 
current or control signal to the laser 51. A set of such appropriate bias 
currents is determined for the set of desired operating frequencies and is 
stored in the digital processor. A process for determining the set of bias 
currents is described subsequently. 
Referring now to FIG. 5, there is shown a laser frequency vs bias current 
characteristic curve 65. Heavy dots on this curve represent the values of 
laser bias current required to operate the laser 51 of FIG. 2 at the 
Fabry-Perot resonant frequencies f1, f2, f3, f4 and f5 within a very wide 
band. 
When the tunable laser control arrangement 50 of FIG. 2 is first operated, 
it operates along the laser characteristic curve 65 of FIG. 5. Values of 
laser drive current i1, i2, i3, i4 and i5 for the desired Fabry-Perot 
resonant frequencies f1, f2, f3, f4 and f5 are determined and are stored 
for subsequent use as the values of laser bias current for determining the 
desired optical operating frequencies. 
For any selected laser, there is a known operating curve. Similarly the 
Fabry-Perot resonator has known resonant frequencies. Information 
representing the laser operating curve is stored in the digital processor 
75. A set of specific addresses is reserved for storing the laser bias 
current values which produce the desired operating frequencies. The 
relevant laser bias current values for producing those frequencies are 
determined as follows in a calibration process. 
A first desired operating frequency, e.g., frequency f1, is selected. From 
the known laser characteristic curve, an initial laser bias current or 
control signal is selected. The value of that initial laser bias current 
is selected to produce a free-running frequency slightly below the first 
operating frequency f1. From the keyboard 79 the value is put into the 
digital processor 75 and is read out to a power supply 80 which applies 
the appropriate value of bias current or control signal by way of a lead 
81 to an input of adder 82. That bias current is divided appropriately by 
resistors 83 and 84 and the resulting portions are applied respectively to 
the phase control and distributed Bragg reflector regions of the laser 51. 
Power supply 80 also directly supplies a constant bias current through a 
lead 87 to the active region of the laser 51. 
Since the free-running laser frequency is below the desired Fabry-Perot 
resonance frequency f1, the feedback loop produces an error signal on the 
lead 60. On a lead 89, the active integrator 70 produces an incremental 
laser bias current or control signal error .delta.i1 which is applied to 
another input of the adder 82. This incremental laser bias current 
.delta.i1 is added to the initially selected laser bias current and is 
applied through the resistors 83 and 84 to the laser 51 driving it to 
oscillate at the desired Fabry-Perot resonant frequency f1. 
The digit processor 75 measures the incremental laser bias current in the 
lead 89, adds it to the initial laser bias current, and stores the total 
as a new value of laser bias current at the address for the desired 
resonant frequency f1. 
Similarly an appropriate laser bias current is determined for each of the 
other desired resonant frequencies f2, f3, f4 and f5. The values of those 
bias currents are stored by the digital processor in reserved addresses 
for subsequent selected retrieval. 
During subsequent operation, the operator selects the desired resonant 
frequency by inputting a selection through the keyboard 79 to the digital 
processor 75. The processor then initializes the active integrator 70 and 
through a lead 90 requests the appropriate value of laser bias current 
from the power supply 80, which supplies that value of laser bias current 
through the adder 82 and resistors 83 and 84 to the laser 51. As long as 
the laser characteristic has not drifted, the laser operates at the 
desired Fabry-Perot resonant frequency, and no error signal is generated 
on the lead 60. No incremental laser drive current .delta.i is generated 
on the lead 89 by the active integrator 70. 
As a result of device aging or as a result of environmental changes, the 
laser characteristic curve may change to a different position, for 
example, as presented by an alternative dotted line characteristic curve 
67 of FIG. 6. When the environment of the laser 51 shifts the operating 
characteristic to the alternative characteristic curve 67, applying any of 
the predetermined values of bias current i1, i2, i3, i4 and i5 from the 
original laser characteristic curve 65 will cause an incorrect laser 
frequency to be selected. Because such changes are expected to occur in 
the position of the laser characteristic over any long duration of 
operation in the field, the circuitry included in the laser feedback 
control loop operates to correct for those changes. 
In FIG. 2 a servo-control circuit includes the active integrator 70 which 
responds to the error signal on the lead 60 and to signals from the 
digital processor 75. This servo-control circuit includes a switch 76 
placed across a capacitor 78. Charging and discharging of the capacitor 78 
is controlled by signals to the switch from the digital processor 75. 
Activation of frequency locking is initiated by closing the switch 76 and 
discharging the capacitor 78. Once the capacitor 78 is discharged, the 
frequency locking operation is commenced by opening the switch 76. For a 
selected laser biasing current that produces a laser free-running 
frequency, the laser frequency varies during the locking process in 
response to the error signal on the lead 60. As shown by the curve 63 of 
FIG. 3, the error signal on the lead 60 crosses zero at the resonant 
frequencies f1, f2, f3, etc. From either side of each resonance frequency, 
the polarity of the error signal is appropriate to drive the laser to the 
desired Fabry-Perot resonance frequency. In response to the error signal 
on the lead 60, the active integrator circuit 70 commences to charge the 
capacitor 78 and develop the incremental bias current needed on the lead 
89. This incremental bias current is added by the adder 82 to the selected 
laser bias current on the lead 81. The resulting total laser bias current 
drives the laser 51 to oscillate at the desired Fabry-Perot resonant 
frequency. The frequency function of curve 63 is the frequency of the 
laser determined by the bias current selected from the power supply 80 
plus the integral of the error signal and the offset voltage. 
Optical output from the laser 51 is coupled into the optical fiber 52 as an 
output signal, or carrier wave, from the tuned optical frequency 
synthesizer 50. An information source 100 modulates that carrier wave with 
information signals and applies them onto an optical fiber 102. 
When the free-running frequency of the laser is equal to the desired 
Fabry-Perot resonant frequency, no error signal is produced. The digital 
processor measures, reads, or otherwise determines the value of the 
incremental laser bias current in the lead 89, adds it to the selected 
value of bias current, and stores the new total bias current in place of 
the selected bias current for subsequent retrieval for that Fabry-Perot 
resonant frequency. 
Inclusion of the active integrator 70 of FIG. 2 extends the pull-in range 
of the circuit beyond the steep slope between peaks of the error signal 63 
of FIG. 3 at the zero-crossing of frequency f1. The extended range 
includes the entire range between the frequencies of the pairs of the 
other zero crossings 64. Because of the offset current, the frequency 
locking arrangement avoids frequency sticking, or very long pull-in time 
which can otherwise be caused by the low amplitude of a non-offset error 
current. 
Each Fabry-Perot resonance has a capture range that equals the range 
between zero-crossings, such as zero-crossings 64, which are centered 
unsymmetrically around a resonance because of the offset current. 
Importantly, the just described feedback loop of FIG. 2 informs the 
digital-processor 75 of any drift of the laser operating characteristic. 
For instance, by reference to FIG. 6, the characteristic may drift from 
the position of the initial characteristic curve 65 to the position of the 
alternative characteristic curve 67. When the incremental laser bias 
current occurs in the lead 89 causing the digital processor 75 to 
recalculate the value of bias current i1, e.g.,i1', for the selected 
operating frequency, e.g., f1, the displacement of the curve 67 shows that 
all of the other stored values of laser bias currents i2, i3, i4 and i5 
would fail to produce the desired freerunning laser frequencies f2, f3, f4 
and f5, respectively. Using the stored characteristic curve data and the 
known incremental laser bias current, the digital processor 75 calculates 
and stores new values of laser bias currents i2', i3', i4' and i5' for the 
frequencies f2, f3, f4 and f5, respectively. Thereafter when any desired 
frequency is selected for use, the digital processor 75 will cause the 
laser bias current supplied to the laser to be very close to the correct 
value for the desired resonant frequency. 
The foregoing describes an embodiment of a tuned optical frequency 
synthesizer. This embodiment together with others, which are obvious in 
view thereof, are within the scope of the appended claims. FIG. 7 shows 
the result of a recomputation of the values of laser bias currents.