Control system for a laser diode and a method for controlling the same

[0001] 1. Field of the Invention

[0002] This invention relates to a method for controlling a semiconductor laser diode and an apparatus for controlling the laser diode.

[0003] 2. Related Prior Art

[0004] Since an optical output and an extinction ratio of the laser diode depends on temperatures, a specific controller will be necessary to maintain the optical output and the extinction ratio thereof to a predetermined value. The laser diode emits light by supplying a bias current and a modulation current. The bias current and the modulation current determine the output power and the extinction ratio of the laser diode. Therefore, the both currents should be adjusted in various temperatures to keep the output power and the extinction ratio. Such control to keep the output power and the extinction ratio is called as an Auto-Power-Control (hereinafter denoted by APC) operation.

[0005] In general, the optical output power and the extinction ratio of the laser diode are defined by the specification, for example, the output power is −3 dBm and the extinction ratio is 7 dB, respectively. Therefore, it is required that the laser diode must be operated by controlling the bias current and the modulation current so as to keep the output power and the extinction ratio.

[0006] In a conventional APC operation, both currents are measured at various temperatures and the values thereof are stored in storage in advance of a practical use in the field. Once the bias current and the modulation current of the laser diode are fixed, the output power and the extinction ratio are automatically determined. Therefore, when the optical output power of the laser diode is discrepant to a target power, a new combination of the bias current and the modulation current are read out from the storage and provided to the laser diode so as to compensate the discrepancy of the power and the extinction ratio.

[0007] However, the conventional method requires a large storage because various combinations of the bias and the modulation current must be stored as a reference table. Moreover, to keep the output power and the extinction ratio of the laser diode in precise must prepare the reference table with a larger size, which results in the large and complicated optical module.

[0008] One feature of the present invention relates to a method of controlling the semiconductor laser that emits light with a predetermined output power and an extinction ratio. The laser is installed in a light transmitting module that has a data storage, a means for monitoring a portion of the light emitted from the laser diode and a processing unit. The control method comprises the steps of: (a) storing a plurality of parameters into the data storage in the transmitting module, (b) monitoring a photo current generated by the monitoring means, the photo current corresponding to the optical output power of the laser diode, (c) comparing the photo current with a target photo current in the processing unit, (d) changing one of a bias current and a modulation current by a prescribed amplitude, (e) calculating the other current based on the specific function characterized by the plurality of parameters stored in the data storage, (f) supplying thus defined bias current and the modulation current to the laser diode, and (g) iterating the steps from (b) to (e) until the optical output power and the extinction ratio of the laser diode become the predetermined value.

[0009] The data stored in the data storage is only the parameters that relate one of the bias current and the modulation current to the other current. Therefore, the size of the data storage can be reduced, and one of the currents is calculated based on the other current and the specific function characterized in the parameters. The accuracy of the control can be enhanced by precisely adjusting the other current. Even in such precise control, since the data should be stored in the storage are only parameters that characterizes the specific function, it is not necessary to increase the size of the storage.

[0010] In the present invention, the control of the bias current and the modulation current completes when a result of comparing the photo current with target photo current is smaller than the first threshold, and the control thereof starts when the results is greater than the second threshold. The first threshold may be greater than a half of a change of the photo current when one of the bias current and the modulation current is changed by the prescribed magnitude, and the second threshold may be smaller than a twice of the change of the photo current.

[0011] The change of the photo current may be calculated based on the change of the photo current of a reference diode, namely, the change of the photo current may be a product of the change of the photo current of the reference diode multiplied by a ratio of the target photo current of the laser diode to that of the reference laser diode. This relation that the first threshold depends on the change of the photo current can escape the control from an unstable operation caused by the rigidly set threshold to laser diodes each having inherent characteristics.

[0012] The prescribed magnitude for changing one of bias current and the modulation current may be varied depending on a result of comparing the photo current with the target photo current. When the difference of the photo current to the target photo current is large, the prescribed magnitude may be expanded. On the other hand, the prescribed magnitude may be reduced when the difference is small. This adjustable setting for the prescribed magnitude can accelerate the control.

[0013] Another feature of the present invention relates to a control system for a laser diode. The system comprises a laser module and a control apparatus. The laser module includes a semiconductor laser diode and a means for monitoring light emitted from the laser diode. The laser diode emits light by supplying a bias current and a modulation current. The control apparatus has storage and a processing unit. The data storage stores a plurality of parameters to define a specific function that relates one of the bias current and the modulation current to the other of the bias and the modulation current. The processing unit compares a photo current generated by the monitoring means with a target photo current, in which the laser diode emits light with a predetermined optical power and an extinction ratio. The unit changes one of the bias current and the modulation current by a predetermined magnitude and calculates the other of the bias current and the modulation current based on the specific function and the changed one of the bias current and the modulation current, and finally both of the adjusted bias current and the adjusted modulation current are supplied to the laser diode.

[0014] In the configuration of the present invention, since the data stored in the data storage is only the parameters that relate one of the bias current and the modulation current to the other current, the size of the data storage can be reduced. Moreover, since the other current is calculated from one of the current and the specific function, the accuracy of the control can be enhanced by precisely adjusting the other current. Even in such precise control, since the data should be stored in the storage are only parameters that characterizes the specific function, it is not necessary to increase the size of the storage.

[0022] The preferred embodiments of the present invention will be described in detail as referring to the drawings.

[0023] FIG. 1 is a schematic diagram of the control system 10 and an optical transmitting module 14. The system 10 comprises a laser module 12 and a controller 14. The laser module 12 includes a laser diode 16 (hereinafter denoted by LD), a driver 18 for driving the LD 16, and a means 20 for monitoring a light output from the LD 16.

[0024] The LD 16 emits light by supplying a bias current and a modulation current from the driver 18. The portion of the light emitted from the LD 16 is monitored by the monitoring means 20, which outputs a photo current I_PD corresponding to the optical output of the LD 16. The monitoring means 20 is preferably a photo diode.

[0025] FIG. 2 shows a relation between the supply current I_SP to the LD 16 and the optical output power P of the LD 16, which is called I-L characteristic of the LD 16. The figure also shows a temperature dependence of the I-L characteristic. In FIG. 2, the LD 16 emits light when the supply current I_SP exceeds a threshold current I_TH. At the optical output P₂, the bias current I_B which involves some margin in addition to the threshold current I_TH is supplied to the LD 16. While at the optical output P₁, a current that is a sum of the bias current and a modulation current is supplied. The extinction ratio is defined by 10* log(ΔP/P₂), where ΔP is a difference of the respective optical outputs P₁-P₂. The behavior of the I-L characteristic at various temperatures are also shown in FIG. 2, where T_L corresponds to the I-L characteristic at relatively lower temperature while T_H is that at relatively high temperature.

[0026] As shown in FIG. 2, both the bias current I_B and the modulation current I_M should be adjusted to maintain the optical output power and the extinction ratio to the predetermined values P₁ and P₂. This control of the LD 16, to keep both the optical output power and the extinction ratio to be constant even under the conditions that the temperature varies, is called APC operation.

[0027] Next, the operation of the controller 14 will be described.

[0028] The controller 14 performs the APC operation, and comprises an analog-to-digital converter (A/D-C) 22, a first storage 24, a second storage 26, a processing unit 28, and a digital-to-analog converter (D/A-C) 30. The A/D-C 22 outputs a signal in a digital form that corresponds to the photo current I_PD output from the monitoring means 20. The first storage 24 temporarily stores the digital signal of the photo current I_PD.

[0029] The second storage 26 stores parameters by which a specific function relating the bias current I_B to the modulation current I_M is defined. The relation means that the output power P₁ and the extinction ratio are kept constant under the combination of the bias current and the modulation current in various temperatures, which is denoted by I_M=ƒ(I_B). Exemplary, the specific function is an n-th order homogeneous function, the parameters of which is denoted by X₀, X₁, . . . X_n. FIG. 3 shows an example of the relation of the modulation current to the bias current under various temperatures from −5° C. to 85° C. FIG. 3 also shows two example of the specific function in the second order and the third order, respectively. In the second order homogeneous function, the parameters of X₀=43.61, X₁=0.2209, and X₂=0.0071 are stored in the storage, while the case of the third order function, X₀=47.56, X₁=−0.2391, X₂=0.0215, and X₃=−0.0001 are stored. The second storage 26 is preferably a read-only-memory (ROM).

[0030] The processing unit 28 calculates the bias current I_B and the modulation current I_M to be supplied to the LD 16 based on the monitored photo current I_PD, which corresponds to the optical output power of the LD 16, and the specific function defined by the parameters X_i stored in the second storage. When an absolute difference D_IPD of the photo current I_PD to a target photo current I_PD(0) is greater than a predetermined upper threshold Th_U, the processing unit 28 starts the calculation for obtaining the modulation current I_M. While, the absolute difference D_IPD becomes smaller than a lower threshold Th_L, the processing unit 28 completes the calculation for the modulation current I_M. The D/A-C 30 converts the results of the bias current I_B and the modulation current I_M calculated by the processing unit 28 to analog forms. Both currents I_B and I_M are finally supplied to the LD 16 through the driver 18.

[0031] Thus, the data stored in the second storage is only the parameters that relates the bias current to the modulation current, the size of the second storage can be reduced. Further, the modulation current is calculated based on the bias current and the specific function characterized in the parameters, the accuracy of the controlling of the laser diode 16 can be enhanced by precisely adjusting the bias current. Even in such precise control, since the data should be stored in the second storage are only parameters that characterizes the specific function, it is not necessary to increase the size of the second storage.

[0032] Next, a method for controlling the optical output of the LD 16 accompanied with the method to decide the upper threshold Th_U and the lower threshold Th_L will be described as showing an algorithm thereof in FIG. 4. In the explanation, an APC flag that denotes whether the APC is under the operation or not is set to be “0” at the beginning.

[0033] First, some bias current I_B and some modulation current I_M are provided to the LD 16 through the driver 18 at sequence S104. The monitoring means 20 monitors the optical output power of the LD 16 and outputs a photo current I_PD at sequence S106. The photo current I_PD output from the monitoring means is converted to the digital form by the A/D-C 22 and stored in the first storage 24 at sequence S108.

[0034] Next, the processing unit 28 reads out the magnitude of the photo current I_PD stored in the first storage 24 at sequence S110 and calculates adjusted bias current I_B′ and the modulation current I_M′ based on the photo current I_PD and the specific function ƒ characterized by parameters X_i. The following will be described the algorithm of the calculation at the processing unit 28.

[0035] An absolute difference D_IPD of the monitored photo current I_PD to the target photo current I_PD(0) is calculated at sequence S112. The absolute difference D_IPD is compared to the predetermined upper threshold Th_U that decides whether the APC operation starts or not at sequence S114. When the absolute difference D_IPD is greater than the upper threshold Th_U, the processing unit 28 set the APC flag to “1” at sequence S116, and further compares the photo current I_PD to the target photo current I_PD(0) at sequence S118. When the photo current I_PD is greater than the target photo current I_PD(0), which corresponds to the case “Y” at sequence S118, the bias current I_B is reduced by a prescribed magnitude ΔI_B at sequence S120. On the other hand, the photo current I_PD is smaller than the target photo current I_PD(0), the bias current to be supplied to the LD 16 is increased by the prescribed magnitude ΔI_B at sequence S122. The adjusted modulation current I_M to be provided to the LD 16 is calculated at sequence 124 based on the specific function ƒ characterized by the parameters Xi and the bias current I_B thus defined at sequence S120 or S122. The adjusted bias current I_B and the calculated modulation current I_M are converted to analog forms at D/A-C 30 and supplied to the LD 16 through the driver 18 at sequence 126.

[0036] Next, another case will be described. When the absolute difference D_IPD of the photo current I_PD to the target photo current I_PD(0) is smaller than the upper threshold Th_U, which corresponds to the case “N” in sequence S114.

[0037] First, whether the APC flag is set or not is judged at sequence S128. When the APC flag is set to “1”, which corresponds “Y” at sequence S128, a further judgement whether the absolute difference D_IPD is greater that the lower threshold Th_L or not is performed at sequence S130. The lower threshold Th_L is the threshold for completing the APC operation. When the absolute difference D_IPD is greater than the lower threshold Th_L, which is “Y” in sequence S130, the operation moves to sequence S118. On the other hand, the absolute difference D_IPD is smaller than the lower threshold Th_L, which is “N” in sequence S130, then the APC flag is set to “0” at sequence S132 and the operation goes to sequence S126. In the latter case, the unchanged bias current I_B and the modulation current I_M are supplied to the LD 16.

[0038] Thus described above, the APC operation for adjusting the bias current I_B and the modulation current I_M of the LD 16 starts when the absolute difference D_IPD exceeds the upper threshold Th_U. The sequence above described will be iterated until the absolute difference D_IPD becomes smaller than the lower threshold Th_L and the bias current I_B and the modulation current I_M supplied to the LD 16 are stabilized, in which the LD 16 emits light with the predetermined output power and the predetermined extinction ratio. The APC operation will complete when the difference D_IPD is smaller than the lower threshold Th_L.

[0039] Next, the second embodiment of the present invention will be described. In the case that two thresholds are rigidly fixed for various laser diodes, characteristics of which are varied in respective lasers, it is considered that the absolute difference of the photo current I_PD to the target photo current I_IPD(0) magnified due to the increase/decrease of the bias current by the predetermined magnitude, which causes an instability of the operation of the laser diode. This instability occurs when the change of the photo current due to the APC operation exceeds twice of the lower threshold Th_L.

[0040] Therefore, in the second embodiment, the processing unit 28 performs a function to decide the upper threshold Th_U and the lower threshold Th_L based on the following relations:
ΔI_PD/2<Th_L<Th_U<2·ΔI_PD.  (1)
Where, ΔI_PD corresponds to a change of the photo current I_PD when the bias current I_B is varied by a prescribed magnitude ΔI_B.

[0041] The processing unit 28 preferably has a function to reduce the change of the photo current ΔI_PD by the following relation to decide the upper threshold Th_U and the lower threshold Th_L:
ΔI_PD=ΔI_PD^(S)·I_PD(0)/I_PD(0)^(S).  (2)
Where ΔI_PD^(S) is a change of the photo current I_PD of a reference laser diode different to the LD 16, which is measured by changing the bias current I_B of the reference diode by the prescribed magnitude ΔI_B, and I_PD(0)^(S) is a target photo current of the reference diode, both values are measured in advance to the APC operation of the present LD 16. The reason why the change of the photo current ΔI_PD can be calculated as described above is that the change thereof proportions to the target photo current I_PD(0).

[0042] FIG. 5 shows an algorithm according to the second embodiment. First, the processing unit 28 calculates the change of the photo current ΔI_PD from the change of the photo current ΔI_PD^(S), the target photo current I_PD(0)^(S), each depends on the reference diode, and the target photo current I_PD(0) of the LD 16 in accordance with Eq. 2 at sequence S100. Next at sequence S102, the unit 28 decides the upper threshold Th_U and the lower threshold Th_L based on the change of the photo current ΔI_PD calculated at sequence S100 so as to satisfy Eq. 1. After two thresholds are decided, the LD 16 are provided with the bias current I_B and the modulation current I_M. Subsequently sequences are same as those of the first embodiment.

[0043] FIG. 6 schematically shows a case that the APC operation falls into unstable, in which the change of the photo current ΔI_PD is fluctuated, namely the lower threshold Th_L is smaller than a half of the change ΔI_PD. In FIG. 6, the vertical axis denotes the difference D_IPD between the photo current I_PD and the target photo current I_PD(0), while the horizontal axis corresponds to the iteration of the APC operation. The absolute difference D_IPD is greater than the upper threshold Th_U in regions A₁ and A₂. In regions B₁ and B₂, the difference D_IPD is between the lower threshold Th_L and the upper threshold Th_U. The difference D_IPD is smaller than the lower threshold in regions C₁ and C₂.

[0044] As shown in FIG. 6, the absolute difference D_IPD moves from D₀ in the region A₁ to D₁ in the region A₁ after one cycle of the APC operation. The difference between D₀ and D₁ corresponds to the change of the photo current ΔI_PD in Eqs. 1 and 2. After another iteration of the APC operation, the difference of the photo current moves to D₂ in the region B₁. Since the APC operation does not complete in the region B₁, another iteration of the APC operation will be started. However, the difference of the photo current moves from D₂ in B₁ to D₃ in B₂ because the change of the photo current ΔI_PD is twice greater than the lower threshold Th_L. Subsequently to the second iteration of the APC operation, the change of the photo current moves between the region B₁ and the region B₂ even if the bias current I_B and modulation current I_M are adjusted. Thus, the case that the APC operation eternally continues will be considered.

[0045] FIG. 7 shows an example of the modified APC process according to the second embodiment. In the embodiment of FIG. 7, the change of the photo current ΔI_PD is calculated from the target photo current I_PD(0) of the LD 16, the change of the photo current ΔI_PD^(S) and the target photo current I_PD(0)^(S) of the reference diode as shown in Eq. 2. The lower threshold Th_L is equal to the change of the photo current ΔI_PD, and at the same time, the upper threshold Th_U is selected so as to satisfy Eq. 1.

[0046] As shown in FIG. 7, after one iteration of the APC operation, the difference of the photo current I_PD to the target photo current I_PD(0) moves from D₀ in the region A₁ to D₁ in the region B₁. Further iteration of the APC operation for the bias current I_B and the modulation current I_M moves the difference D₁ in the region B₁ to D₂ in the region C₁ because the lower threshold Th_L is greater than a half of the change of the photo current ΔI_PD. After moving to D₂ in the region C₁, the APC operation is completed, thereby the operation does not fall into instability.

[0047] When the difference of the photo current I_PD to the target photo current I_PD(0) is in the region A₁, the bias current I_B and the modulation current I_M are modified by the APC operation. The upper threshold Th_U satisfies Eq. 1 and also the lower threshold Th_L is equal to the change of the photo current ΔI_PD, transitions of the difference of the photo current from the region A₁ to the regions A₂, B₂, or C₂ do not occur. The difference of the photo current moves from the region A₁, to one of regions A₁, B₁, and C₁. When the transition from the region A₁ to the region A₁, the iteration of the APC operation performs the difference of the photo current from the region A₁ to the region B₁, and finally subsequent iterations can falls the difference from the region B₁ into the region C₁. When the transition from the region A₁ to the region B₁, another adjustment of the bias current I_B and the modulation current I_M are performed because the APC operation does not complete in the region B₁. Since the lower threshold Th_L is equal to the change of the photo current ΔI_PD, the transition from the region B₁ to the region C₁ inevitably occur, thus completing the APC operation. The APC operation also completes when the directly transition from the region A₁ to the region C₁ is occurred.

[0048] Although the operations above described are the case when the difference of the photo current I_PD to the target photo current I_PD(0) is in the region A₁, an similar process can be operated when the difference of the photo current is in the region B₁. Namely, further adjustment of the bias and the modulation current according to the algorithm described in FIG. 4 moves the difference of the photo current in the region B₁ into the region C₁. Similarly, when the difference of the photo current is in the regions A₂ or B₂ at the starting, the iteration of the APC operation moves the difference to the region C₂, thereby completing the APC operation without any fluctuation.

[0049] Thus in the present embodiment, since the lower threshold Th_L is equal to the change of the photo current ΔI_PD and the magnitude of the upper threshold Th_U is smaller than twice of the change of the photo current ΔI_PD in the present embodiment, relatively fewer iteration of the APC sequence enables the difference of the photo current D_IPD smaller than the lower threshold Th_L, consequently results on the rapid stability of the optical output of the LD 16.

[0050] The processing unit 28 calculates the change of the photo current ΔI_PD based on both the target current I_PD(0) of the LD 16, that of the reference diode I_PD(0)^S and the change of the photo current of the reference diode ΔI_PD^(S), it is not necessary to measure the change of the photo current ΔI_PD for every laser diode.

[0051] From the invention thus described, the invention and its application may be varied in many ways. Although the magnitude of the change of the bias current preset for the calculation of the modulation current is fixed in both embodiment, it can depend on the absolute difference of the photo current I_PD to the target photo current I_PD(0). Namely, a relatively large magnitude is set when the absolute difference is large, while a relative small value is set when the difference is small. By varying the magnitude, the convergence of the APC operation can be accelerated.

[0052] The second storage 26 stores parameters Xi to define the specific function ƒ that relates the modulation current I_M to the bias current I_B to maintain the optical output power and the extinction ratio of the LD 16. The processing unit 28 reads out the parameters Xi from the second storage and decides the modulation current I_M and the bias current I_B to be supplied to the LD 16 based on the specific function. Therefore, the size of the second storage is required for storing only the parameters Xi.

[0053] When the pair of the bias current I_B and the modulation current I_M is stored as a table, the number of the data pair must be increased when the accuracy of the control to maintain the output power and the extinction ratio of the LD 16 is enhanced. In the present invention, because the modulation current I_M is calculated in the processing unit, the accuracy of the controlling can be enhanced only by shrinking the magnitude of the increase/decrease of the bias current ΔI_B, namely, the accuracy of the controlling can be increased without expanding the size of the data to be stored.

[0054] Although the modulation current I_M is calculated from the bias current I_B in both embodiments, the bias current I_B may be calculated from the modulation current I_M, namely the function of I_B=ƒ(I_M) is also applicable. In this case, the bias current I_B will be calculated from the increase/decrease of the modulation current Δ I_M and the characteristic function ƒ(I_m).

[0055] Moreover, although the lower threshold Th_L is equal to the change of the photo current ΔI_PD in the second embodiment, it is not restricted to the such relation and the lower threshold Th_L is only required to satisfy Eq. 1. When the lower threshold Th_L is not equal to the change of the photo current ΔI_PD, the transitions from the region A₁ to the region C₂ and from the region B₁ to the region C₂ will be considered in FIG. 6. However, in such transition may occur, the APC operation will complete when the difference of the present photo current and the target photo current falls into the region C₂. Therefore, even the lower threshold TH_L does not coincide with the change of the photo current ΔI_PD, the APC operation will reliably compete when the relation of Eq. 1 is satisfied.

[0056] Although the upper threshold Th_U is set to be twice of the change of the photo current ΔI_PD, it is not restricted to such relation. For example, the upper threshold Th_U may be the difference between the maximum applicable in the target photo current and the target photo current. Moreover, it may be applicable that the upper threshold Th_U and the lower threshold Th_L are coincided with each other.