Abstract:
A circuit for selectably generating a plurality of preselected digital output signals which are precise and accurate, and among which the circuit can switch rapidly. In one embodiment, the outputs are connected to analog circuit signals, and drive a optical laser such that choice of the particular outputs of the circuit permits controllable switching of the laser output wavelength.

Description:
BACKGROUND 
     The increasing speed with which electro-optical devices can operate has created the need for correspondingly fast devices to control them. A notable example are wavelength tunable, single-mode, semiconductor lasers, which are key components in the rapidly expanding field of fiber optics, the applications of which include wavelength division multiplexing based communication systems, measurement applications, sensor applications, and sophisticated optically controlled microwave systems. An intense effort has been devoted to optimizing and manufacturing such tunable lasers, but the predictability and performance of such lasers, as well as that of the circuit drivers needed for them, have not been explored carefully. An important example of such a laser is the tunable super structure grating distributed Bragg reflector laser, whose tremendous speed in switching from one output wavelength to another offers the scope for applications which the optics community is only beginning to explore. However, to operate a super structure grating distributed Bragg reflector laser, one needs external circuitry to separately generate four control currents, the so-called front and back currents which control the effective cavity size of the laser, a phase section to control the phase of the laser&#39;s optical field, and the laser&#39;s gain current, which turns the laser on and controls its output power. To exploit the promise of this laser, one needs circuitry that can generate these currents with speed comparable to the laser itself, circuitry that can generate these currents accurately to ensure precise wavelength control, and circuitry that is re-programable, to permit updating circuit information as the operating parameters of the laser vary over time. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide a circuit which can output highly accurate and precise signals. 
     Another object is to provide a circuit which is very fast, and thus can controllably switch between preselected outputs rapidly. 
     Another object is to provide such a circuit whose output parameters are continuously updatable. 
     Another object is to provide a laser system whose output wavelength is controlled by such a fast, accurate, and reprogrammable control circuit. 
     In accordance with these and other objects made apparent hereinafter, the invention concerns a signal generator, having a data input, and a buffer which has a digital storage device which can store a plurality of digital words in a corresponding plurality of storage addresses. The data input and the buffer are co-operatively disposed to permit the data input to selectably address one or more of the plurality of storage addresses so as to cause the buffer to output one or more selected digital words. 
     Because the circuit is digital, its output, unlike that of an analog circuit, has fixed values, and hence is inherently more immune to noise. Furthermore, its accuracy is an arbitrary choice determined by the bit resolution one selects. Being digital, the outputs of the circuit are updatable simply by reading new values into the digital storage device, e.g. new settings corresponding to the drive currents for a laser whose operating parameters have drifted over time. Finally, being digital, the response of such a circuit can be extremely fast, and, moreover, computer controllable. 
     These and other objects are further understood from the following detailed description of particular embodiments of the invention. It is understood, however, that the invention is capable of extended application beyond the precise details of these embodiments. Changes and modifications can be made to the embodiments that do not affect the spirit of the invention, nor exceed its scope, as expressed in the appended claims. The embodiments are described with particular reference to the accompanying drawing, wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a circuit diagram of a system according to the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a circuit device according to the invention, in which data input  10 , data converter and conditioner  12  (hereafter, data converter), buffer  14 , and digital to analog current driver  19 , ultimately produce control currents which tune the output wavelength of laser  18 . EEProm  28  in buffer  14  contains a plurality of digital addresses, in which are stored a plurality of digital values, each value corresponding to the magnitude of a control signal for output to laser  18 . Data converter  10 , for example a process computer or manual control panel, is disposed to forward a digital word  13  to data input device  20 . Two bits of digital word  13  are called respectively a latch bit  15  and a jump bit  17 , for reasons discussed below. The remaining bits of digital word  13  contain information (hereafter called the “address data” of word  13 ) coded to identify an address in EEProm  28  in which resides a desired control signal for laser  18 . Upon receipt of digital word  13  from data input  10 , device  20  passes word  13  to differential receiver  22 . Word  13  is coded with an address in EEProm  28 , a latch bit, and a jump bit. 
     Receiver  22  is preferably an interface which coverts input from data input  10  from differential logic to TTL, or other conventional, logic, or other which permits use of particularly fast digital devices based on TTL. Members  10  and  20  preferably use differential logic, which is more resistant to noise, permitting members  10  and  20  to be remotely located from generator  12  and buffer  14 , and still minimize the amount of noise to which the circuit is exposed. Upon data input  20  forwarding latch bit  15  to differential receiver  22  forwards the address data of word  13  to flop-flop  24 , and, thereafter, upon receiving jump bit  17 , forwards an enabling pulse to one-shot multivibrator  32 , causing it to generate a responsive clock pulse  17 ′. 
     Flip flop  26  is disposed to receive the address data of digital word  13  from flip flop  24  and pass the address data in word  13  to EEProm  28 , both EEProm  28  and flip-flop  26  being clocked along line  32 ′ by one-shot multivibrator  32 . Enablement of EEProm  28  causes EEProm  28  to receive the address data of word  13  from flip flop  26 , and to make available to buffer driver  30  the digital signal stored in EEProm  28  at the address corresponding to the address data of digital word  13 . Multivibrator  32 , responsive to enablement of jump pulse  17 ′, drives inverter  34  which in turn sends a disable signal to driver  30 . Because of inverter  34 , driver  30  is disabled when jump bit  17  is enabled, and re-enabled when jump bit  17  reverses. 
     The digital portion of the circuit of FIG. 1, members  12  and  14  (or, depending on one&#39;s particular application, members  20  or  10  also), are preferably placed closely packed on one circuit board, with a common ground plane isolated from the non-digital portion of the circuit, namely member  38 ,  18 , etc., to increase circuit reliability and increase circuit speed by minimizing the distances over which its signals must travel. 
     In operation, member  10  sends word  13  to device  20 , which automatically forwards the address data of word  13  to differential receiver  22 , along with by latch bit  15  and then jump bit  17 . If the value of latch  15  is enabling, the address data of word  13  is forwarded after a short delay to flip flop  24 . Similarly, if the value of jump bit  17  is enabling, differential receiver  22  strobes one-shot multivibrator  32 , which clocks the address data from flip flop  24  to flip flop  26  and enables EEProm  28 . Upon enablement, EEProm  28  receives word  13  from flip flop  26 , locates the digital control signal at the address corresponding to the address data in digital word  13 , and makes the control signal available to driver  30 . Simultaneously, multivibrator  32  and inverter  34  disable driver  30 , until multivibrator  32 &#39;s one-shot pulse has finished, whereupon driver  30  becomes enabled and receives the control signal from EEProm  28 , and forwards the signal to digital to analog converter  36 , and ultimately to laser  18 . By maintaining driver  30  disabled for a short time, the output to digital to analog converter  18  is nulled for a short time, and thus the control signal to laser  18  is nulled also. This is advantageous to prevent hysterisis effects in laser  18  from causing a corresponding hysterisis in laser  18 &#39;s wavelength as members  12  and  14  switch laser  18  from one output mode to another. 
     Digital to analog converter  36  converts the digital output of driver  30  into analog form, which in turn controls current driver  38 , producing a control current for laser  18 . Converter  36  is typically a voltage source, and current source driver  38  converts the voltage output of converter  19  to a current signal. Laser  18  requires three such current signals plus a current driver for the gain section, as illustrated by lines  40 ,  42 ,  44  and  46 . In practice, each of these signals would be produced by three separate circuits of the kind shown in FIG. 1 as comprising members  12 ,  14 ,  36 ,  38 . Preferably, however, there would only be one digital word  13 , whose address data would be interpretable by each of the three circuits to data input launching of the correct digital output signal from each. This means that in practice there would be one member  10  and one member  20 , which would forward digital word  13  in parallel to each of the three circuits. Laser  18  also may have a conventional cooling circuit  45  to stabilize laser  18  and minimize wavelength drift. 
     Because the circuit of FIG. 1 employs digital components, particularly those based on TTL logic, and is placed on one circuit board with the digital components closely packed, the circuit is inherently very fast, and thus can switch control signals to laser  18  very quickly. Because the output of buffer  14  is a digital signal, rather than an analog signal, current output is inherently very stable, making the signals in member  36 ,  38 , etc. similarly stable. This permits one to dispense with complicated and inherently slow phase locked loops which may be used to keep a corresponding analog control system within tolerances. One could, for example, use for driver  38  a straightforward operational amplifier driving a high current capacity transistor, a textbook scheme which is both inexpensive and inherently quick. 
     The precision of the output from member  19  is limited only by the number of bits employed in EEProm  28 , and hence can be made as precise as one&#39;s application demands within currently available technology. Because address  13  is clocked sequentially across generator  12  and buffer  14 , interstage noise such as one would associate with an analog system is minimized. Such noise as is present, most notably ringing as circuit components are switched, is further minimizable by making the response time of downstream components in the circuit longer than the ringing time of upstream components, by proper grounding of the circuit, and by proper isolation of the digital portion of the circuit of FIG. 1, from the analog portion. In this manner, the circuit of FIG. 1 provides an output signal that is precise, fast, and robust against noise. Furthermore, because the output signals are stored as digital words in EEProm  38 , one can readily recalibrate the system as laser  18 &#39;s operating parameters change by simply reading new values into EEProm  28 . These new values would typically come from calibration trials on laser  18 , in which one tests the laser to determine which sets of drive currents produce which laser modes. Having done this, one simply digitizes the critical values of drive current and reads them into EEProm  28 . 
     The invention has been described in what is considered to be the most practical and preferred embodiments. It is recognized, however, that obvious modifications to these embodiments may occur to those with skill in this art. Accordingly, the scope of the invention is to be discerned from reference to the appended claims, wherein: