Method and apparatus for controlling the bias current of a laser diode

1 2

Therefore, the optical output intensity is initially

METHOD AND APPARATUS FOR CONTROLLING greater than the desired output optical intensity. Con

THE BIAS CURRENT OF A LASER DIODE versely, when the ambient temperature is high or after

the laser diode has been operating for a period of time FIELD OF THE INVENTION 5 and the operating temperature has correspondingly This invention relates generally to a method and increased, the low level optical output intensity is less apparatus for maintaining a desirable extinction ratio for no data transmission and greater during data transanal data burst response of laser diodes used in an optical mission. Therefore, during high temperature operation transmitter of an optical communication system or opti- the optical output intensity is substantially lower than cal interconnects and, more particularly, to a method 10 the desired optical output intensity in response to the and apparatus for controlling the bias current" of the" initial data burst.

laser diode to maintain the trough level of the optical Further, it should also be appreciated that the extinc

output power at a preselected constant level. tion ratio of these prior art mean level control circuits is

DESCRIPTION OF THE RELATED ART is ProPor?°nal to *e °PeratinS temperature unless some

1J form of temperature compensation is employed. More

Laser diodes are commonly used in optical communi- particularly, as the operating temperature of the laser cation systems for high speed data transmission. How- diode increases its extinction ratio increases. It should ever, laser diodes typically demonstrate strong nonlin- be appreciated that any noise present in the optical ear characteristics for variations in operating tempera- transmission system impacts the data transmission accuture. Thus, the optical output intensity of the laser diode 20 raCy more significantly if the extinction ratio is larger, is difficult to regulate during variations in the operating jn particular, increased shot noise arising from the detetemperature. riorated extinction ratio increases the bit error rate of

Typically, prior art automatic power controls supply tne optical receiver, a bias current that is set at approximately the threshold ^ apparatus of the invention overcomes the above current level of the laser diode. In this manner, when a " problems by providing a control circuit for a laser diode pulse current is added to the bias current, the: laser diode that demonstrates a„ improved predictability in reis effectively switched between a high and low optical e tQ a ^ bufst ... adversel the

output intensitycorresponding to the wave form of the extinction rati which is sim le ^ reliable m con. pulse current. The supplied bias current is controllably ... ^ tion. varied to be approximately at the threshold current 30 r

level of the laser diode, particularly when variations in SUMMARY OF THE INVENTION

the threshold current level occur because of corre- TM , ,. , . „ ,

,. ... ... . _ . The laser diode receives current from a pulse current

sponding variations in the operating temperature of the , , ,. „ . „ ^ ^

laser diode supply and a bias current supply. First means receives a

In order'to maintain the bias current at this desired 35 P°rtion the opti^ output of the !fer diode. ""J current level, the prior devices have typically employed deh^ers a fir,st ^ haYlnS a magnitude proportional a mean (time-averaged) level or peak level detection t0 the °Ptlcal lntensltv °{th*laser dl°de optical output, and control circuit. These mean level detection circuits Second means recelves the &st "8"^. detects the magordinarily monitor the optical output intensity of the mtude corresponding to the minimum magnitude of the laser diode, determine the average optical output inten- 40 optical output, and delivers a second signal having a sity, and vary the bias current supply so as to maintain magnitude corresponding to the minimum magnitude of the optical output intensity centered about a preselected the optical output. Finally, the apparatus includes third average level. These mean level detection circuits tend means for comparing the magnitude of the second sigto operate well when data is actually being delivered, nal to a preselected magnitude and altering the magnibut suffer from poor response to data burst, as well as 45 tude of said bias current in response to second signal deteriorated low/high optical power ratio (extinction differing from the preselected magnitude, ratio) for increased operating temperatures. That is to In another aspect of the present invention, an apparasay, when data is not being delivered over the optical tus is provided for controlling the optical output intencommunication system, the optical output intensity of sity of a laser diode. The laser diode receives current the laser diode is allowed to sink to a preselected low 50 from a pulse current supply and a bias current supply, level that does not necessarily correspond to the low The apparatus includes first means for receiving a porlevel optical output intensity that occurs during data tion of the optical output of the laser diode and delivertransmission. Since the low optical output intensity ing a first signal having a magnitude proportional to the during data transmission is linked to the preselected optical intensity of the laser diode optical output. Secaverage level and the amplitude of the optical output 55 ond means receives the first signal, detects magnitude signal, it can be seen that it does not necessarily corre- corresponding to the minimum magnitude of the optical spond to the preselected low value that occurs when output, and delivers a second signal having a magnitude there is no data signal and, consequently, the amplitude corresponding to the minimum magnitude of the optical of the optical output signal is zero. output. Third means compares the magnitude of the

Accordingly, during the transition period between 60 second signal to a preselected magnitude and delivers a

when data is not being delivered and when data is first third signal having a magnitude corresponding to the

being delivered there is a short period of time where the magnitude of the difference between the second signal

operation of the laser diode is dependent upon the oper- and the preselected magnitude. Finally, the apparatus

ating temperature. For example, when the ambient tern- includes fourth means for receiving the third signal and

perature is low or the laser diode is initially operated 65 altering the magnitude of the bias current correspond

and the operating temperature is relatively low, the low ing to the magnitude of the third signal,

level optical output intensity is typically higher for no In another aspect of the present invention, a method

data transmission than when data is being transmitted. is provided for controlling the optical output intensity 3 4

of a laser diode. The laser diode receives current from a The output of the integration circuit 13 corresponds

pulse current supply and a bias current supply. The to the time-averaged optical output intensity of the laser

method includes receiving a portion of the optical out- diode 11. Similarly, the output of the integration circuit

put of the laser diode and delivering a first signal having 16 corresponds to the time-averaged value of the digital

a magnitude proportional to the optical intensity of the 5 input data pattern, such as mark/(mark+space). The

laser diode optical output. The method also includes outputs of the integration circuits 13, 16 are compared

detecting the magnitude of the first signal correspond- by a differential amplifier 17 and the difference of these

ing to the minimum magnitude of the optical output and two voltages is amplified and delivered to an integration

delivering a second signal having a magnitude corre- circuit 18. The integration circuit 18 is necessary be

sponding to the minimum magnitude of the optical out- 10 cause of the possible timing difference between the two

put. The magnitude of the second signal is compared to inputs of the differential amplifier 17. This timing differ

a preselected magnitude and the magnitude of the bias ence is principally caused by the difference between the

current is altered in response to the second signal differ- time constants of the integration circuits 13, 16.

ing from the preselected magnitude. lt shouId be appreciated that the data delivered to

15 data input terminal 14 is serial in nature and can take on

BRIEF DESCRIPTION OF THE DRAWING the form of any combination of ones and zeros. For

Other advantages of the invention will become appar- example, FIG. 5 illustrates one such typical serial data

ent upon reading the following detailed description and ,. <Ve" ">M»">»): If the transmission system were

upon reference to the drawing in which: hnated to transmitting a simple 101010 pattern, then the

FIG. 1 is an electrical schematic of a prior art auto- 20 level ^fter 15 and mtegration circuit 16 would be un

matic power control for a laser diode; necessary. Rather, the level shifter 15 and integration

FIG. 2 is a graphical representation of laser diode Clr°mt 16 coul(* ^ rePlaced bv a Prelected reference

drive current versus optical output intensity of a laser voltage, since the average signal would be known,

diode operating at various preselected temperatures and „ However, where thei serial transmission of data can take

controlled by the mean optical output level; 25 foTM' *hen ^ average optical output intensity

T?t7- i ■ i * *• r.t 1 J- J will be a function of the serial data bemg transmitted.

FIG. 3 is a graphical representation of the laser diode ,-, . „,„ . T-,t„ _ . , , . , .

. . . , i_ 4.-4.-r For example, at time d in FIG. 5, the serial data being

burst signal response characteristics for various prese- t e. . . ^ ' _. - °

t f J f t . transmitted is two consecutive zeros. This, of course,

CCJi^ e/npera J"*8. . causes the average value of the data signal to decrease;

FIG. 4 is an electrical schematic of the mstant appara- 3Q ... at ^ v> of FIG 5 ^ ^ data being

_. ,. , . , „ transmitted consists of two consecutive ones, thereby

FIG. 5 is a graphical representation of the output of causin the a e ical QUt mtensity t0 mcrease

thel^aser diode during a data burst; and ^ the levd shifter 15 ^ mtegration circuit 16

FIG. 6 is a graphical representation of laser diode constantly adjust the average optical output intensity to

drive current versus optical output intensity of a laser 35 correspond t0 the particular data being transmitted at

diode operating at various preselected temperatures and tnat jnstan£

controlled by the trough current level. Referring again to FIG. 1, the output of the integraWhile the invention is susceptible to various modifi- tion circuit 18 drives a bias current supply 19, which cations and alternative forms, specific embodiments controls the bias current delivered to the laser diode 11. thereof have been shown by way of example in the 40 Data delivered to the data input terminal 14 is also used drawing and will herein be described in detail. It should to drive a pulse current supply 20. The output current be understood, however, that there is no intention to 0f the bias current supply 19 and the pulse current suplimit the invention to the particular forms disclosed, but piy 20 combine to drive the laser diode 11. The circuit on the contrary, the intention is to cover all modifica- controls the bias current of the laser diode 11 so that the tions, equivalents, and alternatives falling within the 45 time-averaged optical output intensity of the laser diode spirit and scope of the invention as defined by the ap- \\ is approximately proportional to the mark/(mark-|-spended claims. pace) of the digital input data. In other words, the pulse DESCRIPTION OF THE PREFERRED current supply 20 consistently delivers a controllable EMBODIMENT dutv cvc^e signal that has a constant amplitude, but this

50 constant amplitude variable duty cycle signal is shifted

Turning now to the drawing and referring first to m magnitude according to the magnitude of the bias

FIG. 1, an electrical schematic of a typical prior art current provided by the bias current supply 19. Thus,

automatic power control is shown. Such prior art de- the bias current supply 19 supplies a DC offset to the

vices ordinarily control the laser diode bias current via variable duty cycle constant magnitude pulse current. It

a mean (time-averaged) level detection and control 55 should be appreciated that the pulse width correspond

system. ing to any single bit of data is constant, but consecutive

A photodiode 10 receives a portion of the optical bits of data give the appearance of a variable duty cycle

output of a laser diode 11 and detects the intensity of signal.

that optical output. The output current of the photodi- A better appreciation of the operation of the prior art ode 10 is delivered through a pre-amplifier 12 and time- 60 device and its shortcomings may be had by reference to averaged by an integration circuit 13. the graphic representation illustrated in FIG. 2. FIG. 2 Digital data supplied to a data input terminal 14 is shows three characteristic curves of optical output inconverted to a signal that has a suitable ratio for the tensity versus drive current for three respective tempersignal low/high level analogous to the optical extinc- atures, low "u," medium "v," and high "w." It can be tion ratio. This conversion is accomplished by a level 65 seen that as the temperature of the laser diode increases shifter 15. An integration circuit 16 time-averages the from the low temperature characteristic curve "u" to level shifted signals in substantially the same manner as the high temperature characteristic curve "w," the the integration circuit 13. same time-averaged optical output intensity can only be