Semiconductor light source system having an optimized setting for driving a laser diode

Abstract: A semiconductor light source system includes a laser diode (11a - 11d) that is operated in a variable temperature environment and a driver circuit (12, 13), wherein the driver circuit sets a magnitude of a signal current supplied to the laser diode below an upper limit level that corresponds to a maximum operational temperature of the laser diode. The driver circuit simultaneously sets a relative magnitude of the signal current with respect to a bias current such that a sufficient extinction ratio is achieved in an output optical beam and further sets a relative magnitude of the signal current with respect to the bias current such that a sufficiently small oscillation delay is achieved in the laser diode.

Description: BACKGROUND OF THE INVENTION This invention relates to a method for driving a laser diode that is operated in a variable temperature environment wherein an operational temperature of the laser diode changes and according to which the laser diode is biased by a bias current for producing an output optical beam in response to a signal current that is superimposed on said bias current, for optimizing the operational condition of the laser diode according to claim 1. Laser diodes are used extensively in the human society particularly in the field of optical telecommunication and optical storage of information. Further, various application of laser diodes are studied for example in the field of optical information processing such as optical computers. In laser diodes, it is well known that the operational characteristics, particularly the threshold of laser oscillation, change with environmental temperature. Thus, in order to compensate for such a temperature-induced variation of the operational characteristics, the laser diode used for long range telecommunication purposes such as the devices used for optical submarine cables, use a temperature regulation system such that the laser diode is held at a constant temperature. On the other hand, there exists other applications of laser diode wherein use of such a temperature regulation system is not possible or preferable. For example, the laser diodes used in the optical wiring of supercomputers have to be accommodated in a very limited space and the room for the temperature regulator is generally not available. Similarly, temperature regulation is not practical for the laser diodes that are used for the local area network (LAN), laser printers, and the like, because of the increased cost. In order to avoid the unwanted temperature-induced fluctuation in the operation of laser diodes, conventional semiconductor optical sources use a feedback control of optical beam called automatic power control (APC) wherein the bias current is controlled such that the power of the output optical pulse of the laser diode is held constant. On the other hand, such an APC control has a problem in that, associated with the temperature-induced variation of the threshold of laser oscillation, the extinction ratio of the laser diode is deteriorated with increasing temperature. FIG.1 shows the conventional APC control applied to laser diodes, wherein the horizontal axis represents the drive current while the vertical axis represents the corresponding output optical power. Further, the line designated as P L represents the output characteristics of the laser diode at a low temperature T L , while the line designated as P H represents the output characteristics of the same laser diode at a high temperature T H . It will be noted that the characteristic line P L indicates a threshold level I TH (T L ) at the temperature T L , while the characteristic line P H indicates a threshold level I TH (T H ) at the temperature T H . Referring to FIG.1, the drive current is supplied to the laser diode in the form of a current pulse I d , wherein the current pulse I d is biased at a level I bL that is set equal to the threshold level I TH (T L ) at the temperature T L , such that the laser oscillation occurs with a minimum threshold power level when no current pulse I d is supplied. Further, the magnitude of the current pulse I d is set such that a predetermined output power is achieved in response to the drive current pulse I d at the foregoing low temperature. In the illustrated example, the magnitude of the current pulse I d is set to 5 mA. In the APC control, the biasing I b of the current pulse I d is subjected to a feedback control that is achieved in response to the output power of the laser diode, such that a predetermined output power such as 0.5 mW is maintained even when the temperature of the laser diode changes. More specifically, the bias current I b added to the current pulse I d is changed such that the laser diode produces the foregoing predetermined output power in response to the current pulse I d . When the biasing of the drive current pulse I d is set as such, there occurs a problem, when the temperature rises, in that the laser diode produces the optical output with a substantial power even when the current pulse I d is not supplied. In the illustrated example, the current pulse I d is biased at the level I bH of 20 mA at the temperature T H , and the laser diode produces the output optical power of about 0.25 mW in the absence of the input current pulse I d . Thereby, the extinction ratio of the output optical beam is inevitably deteriorated. Associated with the degradation of the extinction ratio, the S/N ratio of the optical information transmission achieved by the laser diode is deteriorated. In order to maintain the output power of the optical beam constant irrespective of variation of the temperature, it is also possible to change the magnitude of the drive current pulse I d in response to the variation of temperature. For example, one may fix the level of the bias current at I TH (T L ) and increase the magnitude of the current pulse I d with temperature such that the output power of 0.5 mW is secured even when the temperature increases. According to this approach, one can maintain a large extinction ratio. However, the foregoing approach has a drawback in that there tends to occur a delay in the timing of the optical pulse in correspondence to the interval necessary for the drive current to increase from the level I TH (T L ) to the desired level such as 25 mA. In the illustrated example, it should be noted that the laser diode does not cause oscillation until the level of the drive current pulse I d reaches the level of 15 mA, and a time lag occurs in the optical pulse produced as a result of the laser oscillation. In the applications where the use of expensive APC or automatic feedback control of the bias current is not desirable, therefore, one has to optimize the bias condition as well as the magnitude of the drive current pulse such that the problem of the degradation of the extinction ratio as well as the problem of the oscillation delay of laser diode is minimized. FIGS.3(A) and 3(B) show the eye patterns that are obtained as a result of the biasing shown in FIG.2, wherein, in FIG.2, it will be noted that the bias current I b is set between the level I TH (T L ) and the level I TH (T H ), and the magnitude of the drive current pulse I d is set such that the overall drive current I, defined as a sum of the bias current I b and the drive current pulse I d (I = I b + I d ), exceeds the threshold I th (T H ). Referring to FIG.3(A) showing the eye pattern at the temperature T L , the vertical axis represents the optical output power, while the horizontal axis represents the time. There, it will be noted that the laser diode produces the output optical power P 0 in correspondence to the binary signal "0" wherein the drive current is set equal to the bias current I b . Further, in correspondence to the binary signal "1" wherein the drive current is set equal to the sum of the bias current I b and the drive current pulse I d , it will be noted that the laser diode produces the output optical power at the level P 1 . With continuous operation of the laser diode, the temperature of the laser diode increases gradually from T L and reaches T H . Thus, the eye pattern shown in FIG.3(A) shifts gradually to the right with time as shown in FIG.3(B), and there appears a delay t d in the timing of the optical pulse as indicated in FIG.3(B). When such a delay t d occurs, the time slot represented by the open area in the eye pattern inevitably decreases. It will be noted that the foregoing problem becomes particularly acute in the optical systems wherein a laser diode array including a large number of laser diodes are accommodated in a small space without temperature regulation as in the case of the foregoing optical wiring, optical local area network, optical print head of laser printers, and the like. In the optical systems of the foregoing type, mere increase of the drive current pulse I d for increasing the extinction ratio may invite unwanted temperature increase that in turn results in an unwanted deterioration of the eye pattern characteristics. From the Patent Abstracts of Japan, vol. 9, no. 136 (E-320) (1859), 12 June 1985, there is known a drive circuit that changes the bias current and the pulse current to be supplied to a laser diode with temperature. With this known construction the optical output of the laser diode is continuously monitored for adjustment of the bias current and pulse current in response to the temperature change. This known drive circuit produces the pulse current as a sum of the current component that changes proportionally to the bias current and the current component that changes inversely proportional to the bias current. This drive circuit thereby sets the bias current in the vicinity of the threshold of laser oscillation. The Patent Abstracts of Japan, vol. 8, no. 198 (E-265) (1635), 11 September 1984, describes a drive circuit that changes the bias current according to the operational temperature of a laser diode such that a constant optical output is maintained. From Patent Abstracts of Japan, vol. 8, no. 91 (E-241) (1528), 26 April 1984, there is known a pulse width modulation (PWM) system for a laser diode. According to this known system a modulation current is superimposed upon a current pulse having a predetermined duty cycle to produce a drive current such that the peak level of the drive current is held at a predetermined level. In order to achieve the desired pulse width modulation the oscillation delay is modulated in response to the modulation current. When the modulation current is small, there appears a large oscillation delay, while when the modulation current is large and close to the threshold, the oscillation delay decreases. SUMMARY OF THE INVENTION Accordingly, it is a general object of the present invention to provide a novel and useful method for driving a laser diode, wherein the foregoing problems are eleminated. Another and more specific object of the present invention is to provide a method for driving a laser diode with which a relative magnitude of a signal current with respect to a bias current can be set such that a sufficient extinction ratio is achieved in an output optical beam and also such that a sufficient small oscillation delay is achieved in the laser diode. According to the present invention this object is solved by the features of the claim 1. Further improved embodiments of the inventive method are disclosed within the subclaims 2 and 3. According to the present invention, one can optimize the operational condition of the laser diode that forms the semiconductor optical source, wherein the temperature regulation of the laser diode is employed such that the laser diode provides a satisfactory extinction ratio and a satisfactory oscillation delay even when the operational temperature of the laser diode changes as a result of the operation or as a result of the variation of the environmental temperature. Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a diagram showing the relationship between an optical output power and a drive current of a typical laser diode at different temperatures; FIG.2 is a diagram showing a general setting of bias current and drive current pulse for driving a laser diode; FIGS.3(A) and 3(B) are diagrams showing the eye pattern of the laser diode that is driven in accordance with the setting shown in FIG.2; FIG.4 is a diagram showing the principle of the present invention for optimizing the operational condition of a laser diode; FIG.5 is a diagram showing schematically the construction of a semiconductor optical source according to an embodiment of the present invention; and FIG.6 is a diagram showing an example of the optimization of the operational condition in the semiconductor optical source of FIG.5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG.4 shows the principle of the optimization of the operational condition of the laser diode according to the present invention. In FIG.4, it will be noted that the vertical axis represents the magnitude of the drive current pulse I d while the horizontal axis represents the magnitude of the bias current I b . Referring to FIG.4 representing the optimum operational condition by a hatched region, the upper boundary of the hatched region is defined by an upper limit (I d ) max of the drive current pulse I d . There, the foregoing upper limit (I d ) max is defined from the condition that the operational temperature of the laser diode has to be set below the highest tolerable operational temperature T H above which the normal operation of the laser diode is not guaranteed. As the operational temperature of the laser diode is represented as a sum of the environmental temperature T OH and the temperature increase δT of the laser diode due to the operation, the foregoing condition is represented as T OH + δT(I d ) < T H where it will be noted that the temperature increase δT is represented as a function of the drive current pulse I d . In FIG.4, the boundary represented by the inequality (1) is represented as L 3 . On the other hand, in order to secure a sufficient extinction ratio that is at least equal to or larger than a predetermined extinction ratio R, one has to set the bias current I b and the drive current pulse I d to satisfy the relationship (I b + I d - I th (T L ))/(I b - I th (T L )) ≥ R wherein the predetermined extinction ratio R is specified from the system requirement of a system that cooperates with the semiconductor optical source. In FIG.4, the relationship of Eq.(2) is represented by a line L 1 . It should be noted that the foregoing Eq.(2) holds true in the region where the bias current I b exceeds the threshold current I th (T L ) for the temperature T L . On the other hand, the drive current pulse I d has to be set such that the sum of the bias current I b and the drive current pulse I d exceeds the threshold I th , in order that the laser diode cause the laser oscillation. In other words, the drive current I d has to be set larger than the characteristic line that is designated in FIG.4 as L 1 ' and is represented as I d = -I b + I th (T L ) in the region where the current I b is smaller than I th (T L ). Further, the drive current I d has to be set larger than the characteristic line that is designated in FIG.4 as L 1 " and is represented as I d = -I b + I th (T H ) The hatched region of FIG.4 is further defined by a characteristic line L 2 , wherein the line L 2 represents the condition that the delay τ 0 in the laser oscillation becomes equal to or smaller the oscillation delay that is tolerated in the system that cooperates with the semiconductor optical source. Generally, the delay τ is represented as τ = τ s log I d /[I d - {I TH (T H ) - I b }] where τ s represents the carrier lifetime, and the condition corresponding to the line L 2 is represented as τ ≤ τ 0 . Thereby, the condition (4) is represented as I d ≥ (I th (T H ) - I b )·exp(τ 0 /τ s )/[exp(τ 0 /τ s ) - 1] From the inequalities (1), (2) and (5), it will be understood that the optimum operational condition that provides a satisfactory extinction ratio R and a satisfactory oscillation delay τ while simultaneously prohibiting the excessive temperature rise of the laser diode, is represented by the hatched region of FIG.4. Particularly, the combination of the optimized drive current pulse (I d ) op and the optimized bias current (I b ) op as shown in FIG.4 is most preferable in view point of the smallest heat generation that is advantageous for extending the lifetime of the laser diode. It should be noted that the line L1" and the line L2 have a common interception I th (T H ) on the I b axis, while the gradient of the line L2 always exceeds the gradient of the line L1". In other words, there holds a relationship -exp(τ 0 /τ s )/[exp(τ 0 /τ s ) - 1] < -1, indicating the lines L1' and L1" locating always below the line L2. Thereby, the condition pertinent to the lines L1' and L1" does not affect the foregoing optimization of the laser driving condition. FIG.5(A) shows the construction of a semiconductor optical source according to a first embodiment of the present invention, wherein it will be noted that the semiconductor optical source includes a substrate 10, and the substrate 10 carries thereon a number of laser diodes 11a - lld forming a laser diode array 11. The laser diodes lla - lld are driven by a biasing circuit 12 that supplies a bias current I b via respective inductances La - Ld, and a drive current pulse I d , produced by a driver circuit 13 shown in FIG.5(B), is superposed on the bias current I b via respective capacitors Ca - Cd. Thereby, the biasing circuit 12 and the driver circuit 13 supply the bias current I b and the drive current pulse I d with the respective magnitude set to fall in the hatched region of FIG.4. In the construction of FIG.5(A), the substrate 10 acts as a heat sink for maintaining the laser diodes 11a - 11d forming the array at substantially the same temperature, while the illustrated construction lacks any means for regulating the temperature of the laser diode array 11. In other words, the temperature of the laser diode array 11 changes with the environmental temperature as well as with the heat that is generates with the operation of the laser diode array 11. FIG.5(B) shows the construction of a semiconductor optical source of FIG.5(A) schematically, wherein each laser diode 11a - 11d is driven by the common biasing circuit 12. There, it will be noted that there are provided a plurality of the drive circuits 13a - 13d, and each driver circuit produces the drive current pulse I d in response to a drive signal that is supplied to the input terminal of the circuit 13. Thereby, a drive current is supplied to the laser diode La - Ld as a sum of the drive current pulse I d and the bias current I b . In an example, a laser diode operable up to about 80 °C is used in the semiconductor optical source, and the semiconductor optical source is used in the variable temperature environment wherein the environmental temperature changes in the range between 25 °C - 65 °C. In this case, a temperature rise δT of about 15 °C is tolerated for the laser diode, and this magnitude of the tolerance specifies the maximum drive current pulse of about 55 mA, based upon the relationship of Eq.(1). FIG.6 shows the maximum drive current pulse thus obtained by the line L 3 . The lines L 1 and L 2 are determined, on the other hand, based upon the externally specified parameters T L , T H , R, τ 0 and τ s that are determined as follows. T L = 25 °C T H = 80 °C R = 10.0 τ 0 = 400 psec τ s = 2 nsec Based upon the setting given above, the equation that specifies the line L 1 is represented as I d = 9(I b - 3), while the equation that specifies the line L 2 is represented as I d = 5.5 (8 - I b ), wherein the threshold bias current I th has a value of 3 mA at 25 °C (I th (T L ) = 5 mA) and a value of 8 mA at 80 °C (I th (T H ) = 8.0 mA). Thereby, a triangular region shown in FIG.6 by hatching is obtained as the region in which the operational condition of the laser diode is optimized. In particular, an operational point, defined at the intersection of the lines L 1 and L 2 and circled in FIG.6, represents the operational point that satisfies the foregoing three conditions simultaneously and wherein the drive current I d is minimized. In other words, the operational point circled in FIG.6 and represented by (I d ) op and (I b ) op , minimizes the heat generation of the laser diode and provides the longest lifetime of the laser diode associated with the reduced heat generation. In the illustrated example, the optimized drive current pulse (I d ) op takes a value of 17.1 mA and the optimized bias current (I b ) op takes a value of 4.9 mA. Generally, the laser diode to which the present invention is applicable and is operational in the temperature range above 40 °C, is supplied with an optimized bias current of which magnitude falls in the range between 1 mA and 10 mA and further with an optimized drive current pulse of which magnitude falls in the range between 10 mA and 60 mA. In the foregoing example, the optimized bias current is set to about 5 mA, while the optimized drive current is set to about 20 mA. Further, the present invention is not limited to the embodiments described heretofore, but various variations and modifications may be made without departing from the scope of the invention.

Claims: A method for driving a laser diode that is operated in a variable temperature environment wherein an operational temperature of said laser diode changes, said laser diode being biased by a bias current for producing an output optical beam in response to a signal current that is superimposed on said bias current, said method optimizing the operational condition of said laser diode by the steps of: setting a magnitude of said signal current to satisfy a first relationship T OH + δT(I d ) < T H where T OH represents the temperature of an environment in which said laser diode is operated, δT(I d ) represents a temperature rise of said laser diode upon activation with said signal current having said magnitude, I d represents said signal current, and T H represents a maximum operational temperature of the laser diode; setting a relative magnitude of said signal current with respect to said bias current to satisfy a second relationship (I b + I d - I th (T L ))/(I b - I th (T L )) ≥ R where I b represents said bias current having a first magnitude, I th (T L ) represents a threshold bias current of said laser diode at the lowest operational temperature, and R represents a predetermined extinction ratio that is required for said optical signal; and setting a relative magnitude of said signal current with respect to said bias current to satisfy a third relationship I d ≥ (I th (T H ) - I b )·exp(τ 0 /τ s )/[exp(τ 0 /τ s ) - 1] where I th (T H ) represents a threshold bias current of said laser diode at said temperature limit, τ s represents a carrier lifetime in an active layer of said laser diode and τ 0 represents a tolerable oscillation delay of said laser diode. A method as claimed in claim 1, characterized in that said first magnitude of said bias current is set within a range between 1 mA and 10 mA, and that the magnitude of said signal current is set within a range between 10 mA and 60 mA. A method as claimed in claim 2, characterized in that said first magnitude is set to about 5 mA and said signal current is set to about 20 mA.

Inventors: ODAGAWA TETSUFUMI, WAKAO KIYOHIDE, ODAGAWA, TETSUFUMI, WAKAO, KIYOHIDE, Odagawa, Tetsufumi, c/o FUJITSU LIMITED, Wakao, Kiyohide, c/o Fujitsu Limited
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1993
Main classification: H01S3
IPC codes: H01S5