Abstract:
A particularly important factor limiting the reliability of diode lasers is the catastrophic mirror damage which occurs when the radiated output exceeds a critical value. Experience has shown that this occurs as a result of fluctuations in the drive current, especially in high efficiency diodes. To avoid such damage, a portion of the output radiation from a diode laser, located in the collector circuit of a common emitter drive transistor, is coupled back to a photodetector, located in the base-emitter circuit of the transistor. The resulting negative feedback tends to stabilize the overall operation of the device and maintain the maximum radiated power within safe limits.

Description:
This invention relates to arrangements for stabilizing the power output from a diode laser. 
     BACKGROUND OF THE INVENTION 
     The output power from a diode laser is known to vary as a function of temperature, natural aging, and drive current. As materials and fabrication techniques have improved, aging has become less of a problem. Temperature variations can be handled either by placing the laser in a controlled environment (oven or refrigerator), or by programming the drive source to correct for temperature variations. This type of control, however, will either require individual adjustment or extremely tight tolerances on design parameters which, in turn, will lead to reduced yield and higher cost. 
     The third factor referred to hereinabove, i.e., drive current, is particularly important because of the ease with which catastrophic mirror damage occurs in diode lasers when the radiation output power exceeds the critical value. This can occur as a result of spurious fluctuations in the drive current which, in the more efficient diodes, need not be very big. 
     It is, accordingly, the broad object of the present invention to stabilize the output power from diode lasers. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention, the output power from a diode laser is stabilized by means of a fast acting negative feedback circuit comprising: a photodetector; and a drive transistor connected in the common emitter configuration. A portion of the output energy radiated by the laser, which is located in the transistor collector circuit, is coupled back to the photodetector, which is located in the transistor base circuit. Photocurrent generated in the photodetector by the incident laser radiation reduces the base input current, thus providing the negative feedback. 
     It is shown that the total laser output in such an arrangement is independent of the laser slope efficiency to first order. The feedback control circuit thereby provides substantial immunity from changes in this parameter either from device to device, or from within the same device. 
     It is further shown that the sensitivity of the laser output to changes in the threshold current, the drive current, and the drive transistor current gain is greatly reduced. All of these improvements tend to stabilize the overall operation of the device and minimize the opportunity for catastrophic mirror damage. 
     It is an advantage of the present invention that the feedback circuit is sufficiently fast acting to be capable of responding on a pulse-by-pulse basis in a digital system. 
    
    
     These and other objects and advantages, the nature of the present invention, and its various features, will appear more fully upon consideration of the various illustrative embodiments now to be described in detail in connection with the accompanying drawings. 
     BRIEF DESCRIPTION 
     FIG. 1 shows a first embodiment of a diode laser drive circuit with feedback in accordance with the present invention; 
     FIG. 2 shows a typical diode laser output power -vs- input current characteristic; and 
     FIG. 3 shows a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, FIG. 1 shows a laser output stabilization circuit in accordance with one embodiment of the present invention. Basically, the circuit includes a drive transistor 10, connected in the common emitter configuration, and a photodetector 11. The latter, which is back-biased by means of a direct current voltage source 12, is connected between the base electrode and the emitter electrode of transistor 10. 
     The diode laser 13 is connected between the collector electrode and the emitter electrode of transistor 10. Means, not specifically shown, are provided for coupling a portion of the laser output power onto the photodetector. This radiation feedback can be done by means of totally and partially reflecting mirrors which deflect a portion of the output power back towards detector 11, or by making both laser cavity mirrors partially transmissive, and using the output from one of the two laser mirrors as the useful output, and the output from the other laser mirror as the feedback signal. 
     A driver signal source 14 is connected between the base and emitter electrodes of transistor 10. 
     In the absence of an input signal from driver source 14, there is no output signal from laser 13 and the feedback loop between laser 13 and photodetector 11 is open. When the drive signal is applied, the loop remains open until the collector current exceeds the laser threshold current I T , at which point the laser turns on, and the photodetector is illuminated. 
     In order to analyze the circuit operation, several simplifying assumptions will be made. First, the relationship between the laser power output and the laser injection current, illustrated graphically in FIG. 2, is approximated by 
     
         L = O for I.sub.L ≦ I.sub.T                         (1) 
    
     and 
     
         L = n (I.sub.L - I.sub.T) for I.sub.L ≧ I.sub.T,    (2) 
    
     where 
     L is the total laser output power; 
     I t  is the laser threshold current; 
     I l  is the laser current; 
     and 
     n is a constant called the differential slope efficiency. 
     The reverse bias on the photodetector is made high enough so that the detector appears essentially as an open circuit to the driver source, which itself has a high output impedance. When illuminated, however, the photodetector becomes a current source whose current I pc  is related to the incident illumination L pc  by 
     
         I.sub.pc = KL.sub.pc ,                                     (3) 
    
     where 
     K is the photodetector efficiency constant. 
     The photodetector only produces current when it is illuminated and, hence, only when transistor 10 is &#34;on.&#34; In this &#34;on&#34; condition, the forward biased base-emitter junction of the transistor presents a low impedance to the detector and essentially all of photodetector current flows through this junction. The net base current I b  is, therefore, 
     
         I.sub.b = I.sub.d - I.sub.pc,                              (4) 
    
     where 
     I d  is the drive source current. 
     Designating the transistor current gain as β, the laser current I L , which in the embodiment of FIG. 1 is equal to the collector current I c , is given by 
     
         I.sub.L ␣ I.sub.c = β (I.sub.d - I.sub.pc). (5) 
    
     Designating the fraction of the total laser output intercepted by the photodetector as, f, we derive from equations (3) and (4) that 
     
         I.sub.L = β [I.sub.d - fKn (I.sub.L - I.sub.T)].      (6) 
    
     Solving for I L  yields ##EQU1## 
     Assuming that one-half the total output power is used in the feedback loop, i.e., f = 1/2, and using typical values for K, β n, of K = 0.5 mA/mW, β = 100 and n = 0.6 mW/mA, the term fKβn in the denominator is approximately equal to 16. Being much greater than unity, the one in the denominator can be neglected, in which case equation (7) reduces to 
     
         I.sub.L ≈ I.sub.T + I.sub.d /fKn.                  (8) 
    
     Substituting equation (7) for I L  in equation (2), and making the same approximation as in equation (8), we obtain for the total laser output power ##EQU2## 
     The first thing to note in equation (9) is that the laser output power is to a first order approximation independent of the laser slope efficiency n. The feedback circuit thus provides substantial immunity from changes in this parameter either from laser to laser, or from within a given laser. 
     More accurately, the total laser output power is given by ##EQU3## 
     Using this more accurate expression, we find that the sensitivity of the laser output power to changes in n with feedback is given by ##EQU4## whereas without feedback it is ##EQU5## which is larger by the factor (βfKn) 2   &gt;&gt;|. 
     Secondly, it is noted that the sensitivity of the laser output to changes in the laser threshold current, to changes in the drive current, and to changes in the transistor current gain is, in each case, significantly reduced. For example: 
     a. The ratios of the changes in output power, ΔL, to the change in threshold current, ΔI T , are given by ##EQU6## and ##EQU7## 
     For the parameters given hereinabove, these ratios are -4×10 -   2  mW/mA and -60×10 -   2  mW/mA, respectively, corresponding to a fifteen fold improvement. 
     b. The ratios of the change in output power, ΔL, to the change in drive current, ΔI d , are given by ##EQU8## and ##EQU9## 
     For the same numerical values, these ratios are 4 mW/mA and 60 mW/mA, respectively, thus showing a similar fifteen fold reduction. 
     c. The sensitivities of the laser output to changes in the transistor gain are given by ##EQU10## and ##EQU11## 
     For a laser with a threshold current of 100 mA, nominally operation 10 percent above threshold, we obtain for I d  without feedback 1.1 mA, and for equations (14) and (15) values for ΔL/Δβ of 0.04 mW and 0.66 mW, respectively. Thus, with feedback the sensitivity of the output power to changes in β is reduced by a factor of approximately 16. 
     Expressed in terms of the drive current, the output power from the laser is given by 
     
         L = nβ (I.sub.d - I.sub.T /β) Without feedback)  (19) 
    
     and 
     
         L = (1/fK) (I&#39;.sub.d - I.sub.T /β) (With feedback).   (20) 
    
     To get the same output power with and without feedback, we equate equations (19) and (20) and obtain 
     
         I&#39;.sub.d = nβfKI.sub.d - (nβfK-1) (I.sub.T /β). (21) 
    
     For the conditions specified above, we find that I&#39; d  = 2.5 mA, or that for the same output, the drive current with feedback is 2.27 times the drive current without feedback (I d  = 1.1 mA). However, what is more significant is the reduced sensitivity of the laser output to changes in driver current and other circuit parameters. 
     FIG. 3 shows the embodiment of FIG. 1, modified to take into account two matters of practical consideration. While both matters are taken into account in the illustrative embodiment, the inclusion of either one or the other, or both modifications in any specific case will, of course, depend upon the particular application. 
     The first of these modifications is a prebiasing circuit comprising a direct current voltage source 20, a resistor 21 and a r.f. choke 22. The prebiasing circuit is connected across laser diode 13 and serves to maintain a minimum bias current I o  flowing through the diode. The prebias current, which is less than the threshold current, is provided so as to reduce the laser turn-on time. 
     Also included in the embodiment of FIG. 3 is a second diode 23, such as a Schottky barrier diode, connected across the base-emitter junction of transistor 10. This diode is included to prevent reverse-bias breakdown of the base-emitter junction. It will be noted that both photodetector 11 and the base-emitter junction of transistor 10 are reverse-biased by direct current voltage source 12. The latter can be as large as 100 volts or greater, which voltage will be divided between the photodetector and the base-emitter junction. To avoid having too large a reverse-bias voltage develop across the latter, diode 23, poled in the forward-bias direction, is connected across the base-emitter junction. This clamps the reverse-bias voltage across the junction at some well-defined, low value. Alternatively, a resistor can be used instead of a diode. The base-emitter voltage in this second case will be determined by the leakage current through the photodetector and the magnitude of the added resistor. If the output impedance of the signal source dividing transistor 10 is low enough, it will effectively clamp the base-emitter, junction voltage at a safe, low value, in which case no separate resistor need be added. 
     The operation of the embodiment of FIG. 3 is substantially the same as described in connection with FIG. 1 except that the equations (7), (8) and (9) are now given by ##EQU12## and ##EQU13## respectively. 
     The sensitivity equations are unaffected except for equation (17) wherein (I T  -I o ) is substituted for I T . 
     One assumption implicit in the previous analysis is the timely application of the feedback (i.e., photodetector) current to the base of the transistor. This implies that the total loop delay is at least equal to or faster than the rate at which the output power builds up in the laser diode. 
     The required rapid response is achieved, in accordance with the present invention, by locating the photodetector in the base-emitter circuit of the laser drive transistor. In addition, one would advantageously use a fast responding photodetector, and might also shape the drive current pulse to further control the power build up in the laser. 
     Thus, in all cases it is understood that the above-described arrangements are illustrative of only a small number of the many possible specific embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can readily be devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.