JP4872474B2 - Laser driving circuit and laser driving method - Google Patents

Description

  The present invention relates to a laser driving circuit and a laser driving method.

  The IL characteristics of the laser element vary among the individual elements, and change according to temperature changes and time passage. Therefore, in order to stabilize the average intensity and extinction ratio of the signal light output from the laser element, a laser drive circuit that controls the amount of drive current based on the photocurrent from the monitor light receiving element is used.

  For example, the circuit disclosed in Patent Document 1 sequentially supplies drive currents of different magnitudes set in advance to a laser diode, and measures each output light corresponding to each drive current with a monitor photodiode. The amount of bias current and the amount of modulation current are determined according to the IL characteristics of the laser diode estimated based on the measurement result.

  Further, the circuit disclosed in Patent Document 2 detects the increment of the emission intensity by increasing the modulation current by a predetermined amount, and decreases the modulation current when the increment of the emission intensity is larger than the predetermined value. When it is small, the modulation current is increased. The circuit disclosed in Patent Document 2 decreases the bias current amount when the average light emission intensity of the laser diode is larger than a predetermined value, and increases the bias current amount when the average light emission intensity is smaller than the predetermined value.

Japanese Patent Laying-Open No. 2005-064132 JP 2001-094202 A

  However, it is difficult to perform the operation by the circuit described in Patent Document 1 while signal light is output from the laser diode. That is, the estimation of the IL characteristic by the circuit described in Patent Document 1 must be performed only at a specific timing, for example, immediately after power-on, because it is necessary to supply a test drive current to the laser diode. Therefore, there is a problem that it cannot cope with a temperature change or a change with time during the output operation of the signal light.

  Further, the circuit described in Patent Document 2 measures an increase in emission intensity (that is, slope efficiency) when the modulation current is increased by a predetermined amount, and controls the modulation current based on the slope efficiency. This control method is based on the assumption that the slope efficiency in the region exceeding the threshold current value is constant regardless of the light emission intensity. However, depending on the temperature of the laser element, the slope efficiency may vary depending on the emission intensity. In such a case, with the circuit described in Patent Document 2, it is difficult to control the modulation current with high accuracy.

  The present invention has been made in view of the above problems, and provides a laser driving circuit and a laser driving method capable of accurately controlling a bias current and a modulation current corresponding to a change in temperature and a change with time during an output operation of signal light. The purpose is to do.

  In order to solve the above problems, a laser driving circuit according to the present invention is based on a photocurrent from a light receiving element that supplies a driving current including a bias current and a modulation current to a laser element to generate signal light and monitors the signal light. A drive current generator for supplying a drive current to the laser element, and a controller for controlling a bias current and a modulation current included in the drive current based on the photocurrent. The control unit obtains the change in the photocurrent with respect to the change in the bias current as the first change, obtains the change in the photocurrent with respect to the change in the modulation current as the second change, and sets the ratio of the first and second changes. The bias current and the modulation current are increased or decreased so that the first predetermined value is obtained.

  In the laser drive circuit, when the bias current is smaller than the threshold current value of the laser element, the ratio between the first and second changes is a certain value (theoretically 1). When the bias current is larger than the threshold current value of the laser element, the ratio between the first and second changes is a different value (theoretically 2). Therefore, if the bias current is close to the threshold current value of the laser element, the ratio of the first and second changes is a value between these values.

  Thus, in the laser drive circuit described above, the bias current and the modulation current are increased or decreased so that the ratio of the first and second changes approaches the first predetermined value (that is, a value between the above-described values). Therefore, even during normal light output operation, it is not necessary to set the current experimentally as in Patent Document 1, and the bias current can be accurately set near the threshold current value that varies depending on the temperature. Further, since the bias current is stabilized near the threshold current value, the modulation current can be controlled based on the photocurrent. Therefore, according to the above laser drive circuit, it is possible to control the bias current and the modulation current following the temperature change during the output operation and the time-dependent change of the laser element. Further, the ratio of the first and second changes is determined by the magnitude relationship between the bias current and the threshold current value, and is hardly affected by the change in slope efficiency due to the light intensity. Therefore, according to the above laser drive circuit, the modulation current can be accurately controlled even when the slope efficiency varies according to the emission intensity.

  In the laser driving circuit, the first predetermined value is preferably larger than 1 and smaller than 2. As described above, when the bias current is smaller than the threshold current value of the laser element, the ratio of the first and second changes is theoretically 1. When the bias current is larger than the threshold current value of the laser element, the ratio between the first and second changes is theoretically 2. Therefore, the bias current can be set near the threshold current value by setting the first predetermined value to a value greater than 1 and less than 2.

  The laser driving circuit operates such that the control unit sets the ratio of the first change and the second change as the first predetermined value when the difference between the photocurrent and the predetermined target value is within a predetermined range. When the difference is outside the predetermined range, when the absolute value of the difference between the ratio between the first and second changes and the first predetermined value is smaller than the second predetermined value, the modulation current is increased or decreased. The bias current may be increased or decreased when the absolute value of the difference between the ratio between the first and second changes and the first predetermined value is larger than the second predetermined value.

  By setting the ratio of the first and second changes to the first predetermined value while the average light intensity is stable near the target light intensity, the bias current can be stably set near the threshold current value. it can. When the difference between the photocurrent and the target value is outside the predetermined range, it is preferable to first bring the photocurrent close to the predetermined target value (that is, make the output light intensity close to the target light intensity). In such a case, if the absolute value of the difference between the ratio between the first and second changes and the first predetermined value is relatively small (that is, if the bias current is near the threshold current value), the bias current is Since it is stable, the modulation current is increased or decreased. Conversely, if the absolute value of the difference between the ratio between the first and second changes and the first predetermined value is relatively large (that is, if the bias current is not near the threshold current value), the bias current is increased or decreased. As described above, the bias current and the modulation current are individually controlled in accordance with the ratio between the first and second changes, so that the output light intensity of the laser element can be controlled while the bias current is controlled in the vicinity of the threshold current value. It can be close to the light intensity.

  In the laser drive circuit, it is preferable that the control unit has at least one of a function of calculating a threshold current of the laser element from the bias current and a function of calculating the slope efficiency of the laser element from the modulation current.

  The laser driving method according to the present invention generates a signal light by supplying a driving current including a bias current and a modulation current to the laser element, and drives based on the photocurrent from the light receiving element that monitors the intensity of the signal light. A laser driving method for controlling current, wherein a change in signal light intensity with respect to a change in bias current is obtained as a first change, a change in signal light intensity with respect to a change in modulation current is obtained as a second change, and The bias current and the modulation current are increased or decreased so that the ratio of the second change approaches the first predetermined value.

  According to this laser driving method, similarly to the laser driving circuit described above, the bias current and the modulation current can be controlled following the temperature change during the signal light output operation and the time-dependent change of the laser element. Also, according to this laser driving method, the modulation current can be accurately controlled even when the slope efficiency varies due to the emission intensity.

  According to the laser driving circuit or the laser driving method of the present invention, it is possible to accurately control the bias current and the modulation current corresponding to the temperature change during the output operation of the signal light and the change over time of the laser element.

  Hereinafter, preferred embodiments of a laser drive circuit and a laser drive method according to the present invention will be described in detail with reference to the drawings. In the description of the drawings, the same or corresponding parts are denoted by the same reference numerals, and redundant description is omitted.

  FIG. 1 is a block diagram showing a configuration of a laser drive circuit 1a as an embodiment of a laser drive circuit according to the present invention. A laser driving circuit 1a shown in FIG. 1 is a circuit for supplying a driving current Id including a bias current Ibias and a modulation current Imod to a laser element (laser diode, hereinafter referred to as LD) 18 to generate a signal light Pa. . The laser drive circuit 1a generates a drive current Id by superimposing a modulation current Imod corresponding to a modulation signal (transmission signal) Smod from the outside and a steady bias current Ibias. The laser driving circuit 1a controls the bias current Ibias and the modulation current Imod based on the photocurrent Imon from a light receiving element such as a photodiode (hereinafter referred to as PD) 19 that monitors the intensity of the signal light Pa.

Here, FIG. 2A is a diagram illustrating an example of output characteristics (IL characteristics) of the LD 18. In FIG. 2A, the horizontal axis represents the forward current (drive current Id) flowing through the LD 18, and the vertical axis represents the light emission intensity of the LD 18. G1 in the figure indicates the IL characteristic when the temperature of the LD 18 is a certain value T ₁ [° C.], and G 2 is when the temperature of the LD 18 is T ₂ (> T ₁ ) [° C.]. The IL characteristic is shown. Ith ₁ in G1 and Ith _{2 in} G2 indicate threshold current values of the LD 18 at temperatures T ₁ and T ₂ , respectively.

FIG. 2B shows an example of the waveform of the signal light Pa output from the LD 18 and shows a waveform in which the intensity PL and the intensity PH are alternately repeated. FIG. 2C shows a waveform of the drive current Id necessary for outputting the signal light Pa shown in FIG. In FIG. 2C, G3 is an example of the drive current Id required at the temperature T ₁ [° C.], and G4 is an example of the drive current Id required at the temperature T ₂ [° C.]. Ibias in FIG. 2C indicates a bias current value. As the drive current Id, the modulation current Imod is superimposed on the bias current Ibias.

  As shown in FIG. 2A, the IL characteristic of the LD 18 varies greatly with temperature. That is, as the temperature of the LD 18 increases, the threshold current value increases and the slope efficiency decreases. Therefore, in order to keep the signal light intensity and extinction ratio of the LD 18 constant regardless of the temperature, it is necessary to change the bias current Ibias and the modulation current Imod as indicated by G3 and G4 in FIG. In the above example, it has been explained that the IL characteristic fluctuates depending on the temperature. However, the IL characteristic also changes with time, and there are individual differences of laser elements. Therefore, when the laser drive circuit 1a supplies the drive current Id modulated according to the modulation signal Smod to the LD 18, the drive current Id (that is, the bias current Ibias and the modulation current Imod) is based on the photocurrent Imon from the PD 19. To control.

  Refer to FIG. 1 again. The laser drive circuit 1a of this embodiment includes a drive current generation unit 2, a current-voltage conversion unit 3, a data change detection unit 4, a control unit 5, D / A conversion units 15a and 15b, and an A / D conversion unit 16a. .

  The drive current generator 2 is a circuit that supplies the drive current Id modulated according to the modulation signal Smod to the LD 18. The drive current generator 2 includes a current source 21, a modulation circuit 22, and a bias current source 23. The current source 21 and the modulation circuit 22 constitute a modulation current source in the present embodiment. That is, the current source 21 and the modulation circuit 22 generate a modulation current Imod that is modulated according to the modulation signal Smod.

  Specifically, the current source 21 is connected to the control unit 5 via the D / A conversion unit 15 a, and its output is connected to the modulation circuit 22. The current source 21 generates a constant current Im corresponding to a modulation current amount control signal Sm (described later) provided from the control unit 5 via the D / A conversion unit 15 a and supplies the constant current Im to the modulation circuit 22.

  A modulation signal Smod including transmission data is provided to the modulation circuit 22 from the outside of the laser drive circuit 1a, while a constant current Im is supplied from the current source 21. The modulation circuit 22 modulates the current Im with the modulation signal Smod to generate a modulation current Imod, and supplies this to the LD 18.

  The bias current source 23 is a circuit that supplies the bias current Ibias to the LD 18. Specifically, the controller 5 and one end (anode or cathode) of the LD 18 are connected via the D / A converter 15b. The bias current source 23 generates a bias current Ibias that is a constant current according to a bias current amount control signal Sb (described later) provided from the control unit 5 via the D / A conversion unit 15b, and supplies this to the LD 18 To do. The bias current Ibias is supplied to the LD 18 as the drive current Id together with the modulation current Imod.

  The current-voltage converter 3 is a circuit that converts the photocurrent Imon output from the PD 19 that monitors the magnitude of the signal light Pa into a voltage signal Smon. The current-voltage conversion unit 3 receives the photocurrent Imon, outputs an analog signal Smon corresponding to the photocurrent Imon to the A / D conversion unit 16a, and the A / D conversion unit 16a converts this into a light amount signal Dmon. To the control unit 5.

  The data change detection unit 4 detects the mark rate of the modulation signal Smod. The data change detection unit 4 detects the mark ratio of the modulation signal Smod and provides the control unit 5 with a signal Mark corresponding to the detection result.

  The control unit 5 controls the magnitudes of the bias current Ibias and the modulation current Imod based on the light amount signal Smon. The control unit 5 of the present embodiment is configured by, for example, a central processing unit (CPU) that operates according to a predetermined program, and receives the light amount signal Dmon to control the bias current Ibias and the modulation current Imod.

  Specifically, the control unit 5 inputs the light amount signal Dmon from the A / D conversion unit 16a, and the signal Change corresponding to the mark rate of the modulation signal Smod from the data change detection unit 4. Then, the modulation current amount control signal Dm is output to the current source 21 through the D / A conversion unit 15a, and the bias current amount control signal Db is output to the bias current source 23 through the D / A conversion unit 15b. The internal operation of the control unit 5 will be described in detail later.

  The laser drive circuit 1a of the present embodiment further includes a power-on detection unit 11, a temperature detection unit 12, a storage unit 13, and an external communication circuit 14 in addition to the above configuration. The power-on detection unit 11 is a circuit that notifies the control unit 5 of the activation timing of the laser drive circuit 1a, and provides the control unit 5 with a signal Sp indicating the activation timing of the laser drive circuit 1a. The temperature detection unit 12 is a circuit that detects the temperature near the LD 18, and provides the control unit 5 with a temperature signal St corresponding to the temperature near the LD 18. The storage unit 13 stores various parameters used in the control unit 5. Further, the calculation result in the control unit 5 is also recorded in the storage unit 13. The external communication circuit 14 is a circuit for inputting a parameter to be stored in the storage unit 13 from the outside of the laser drive circuit 1a or for extracting a calculation result recorded in the storage unit 13 to the outside.

  Next, the operation of the laser driving circuit 1a and the laser driving method according to the present embodiment will be described focusing on the internal operation of the control unit 5.

FIG. 3A is a graph schematically showing the IL characteristic of the LD 18. FIG. 3B and FIG. 3C show examples of the drive current Id at which the signal light intensity of the LD 18 becomes a certain value P ₀ , respectively. FIG. 3B shows a case where the bias current Ibias is smaller than the threshold current value Ith of the LD 18, and FIG. 3C shows a case where the bias current Ibias is larger than the threshold current value Ith. In FIG. 3B and FIG. 3C, the mark ratio Smark of the modulation signal is shown as 1/2 for easy understanding.

但し、式（１）において、ηはＬＤ１８のスロープ効率である。また、Ｐ_ｏｆｆは、図３（ａ）に示すようにＬＤ１８のＩ−Ｌ特性における比例部分のｙ切片である。 Here, the signal light intensity P is given by the following formula (1).

However, in Formula (1), η is the slope efficiency of the LD 18. Further, P _off is a y-intercept of a proportional portion in the IL characteristic of the LD 18 as shown in FIG.

となるときである。従って、数式（１）及び（２）より、バイアス電流Ｉｂｉａｓが閾値電流値Ｉｔｈよりも大きい場合の光強度Ｐ_０は、

と表現できる。 When the bias current Ibias is larger than the threshold current value Ith (FIG. 3C), the signal light intensity P becomes the light intensity P ₀ (see FIG. 3A) because the drive current Id is

It is time to become. Therefore, from the equations (1) and (2), the light intensity P ₀ when the bias current Ibias is larger than the threshold current value Ith is

Can be expressed as

となるときである。従って、数式（１）及び（４）より、バイアス電流Ｉｂｉａｓが閾値電流値Ｉｔｈよりも小さい場合の平均光強度Ｐ_０は、

と表現できる。 When the bias current Ibias is smaller than the threshold current value Ith (FIG. 3B), the signal light intensity P becomes the light intensity P ₀ (see FIG. 3A) because the drive current Id

It is time to become. Accordingly, from the formulas (1) and (4), the average light intensity P ₀ when the bias current Ibias is smaller than the threshold current value Ith is

Can be expressed as

となる。同様に、変調電流Ｉｍｏｄの変化に対する光強度Ｐ_０の変化（第２の変化）は、数式（３）より

となる。従って、第１の変化と第２の変化との比Ｒａｔｉｏは、

となる。 Here, when the bias current Ibias is larger than the threshold current value Ith (FIG. 3C), the change (first change) in the light intensity P ₀ with respect to the change in the bias current Ibias is expressed by the equation (3).

It becomes. Similarly, the change (second change) in the light intensity P ₀ with respect to the change in the modulation current Imod is expressed by the equation (3)

It becomes. Therefore, the ratio Ratio between the first change and the second change is

It becomes.

となる。同様に、変調電流Ｉｍｏｄの変化に対する光強度Ｐ_０の変化（第２の変化）は、数式（５）より

となる。従って、第１及び第２の変化の比Ｒａｔｉｏは、

となる。 On the other hand, when the bias current Ibias is smaller than the threshold current value Ith (FIG. 3B), the change (first change) in the light intensity P ₀ with respect to the change in the bias current Ibias is expressed by the equation (5).

It becomes. Similarly, the change (second change) in the light intensity P ₀ with respect to the change in the modulation current Imod is expressed by the equation (5).

It becomes. Therefore, the ratio Ratio of the first and second changes is

It becomes.

  The above formulas (8) and (11) assume that the IL characteristic of the LD 18 is ideally bent at the threshold current value Ith as shown in FIG. The IL characteristic is smooth in the vicinity of the value Ith. Therefore, in this region, the ratio Ratio between the first and second changes is a value of 1 or more and 1 / Smark (≈2) or less. The mark ratio Smark is determined by the signal standard and is usually in the range of 1/2 ± several%. In the following description, the case where Mark is 1/2 is shown for easy understanding.

  Further, as will be described below, the control unit 5 of the present embodiment has a predetermined value (first predetermined value, for example, 1.5) in which the ratio Ratio of the first and second changes is within a range of 1 to 2. By increasing / decreasing the bias current Ibias and the modulation current Imod so as to approach, the bias current Ibias is controlled in the vicinity of the threshold current value Ith.

  4 and 5 are flowcharts showing the internal operation of the control unit 5 and show the laser driving method according to the present embodiment. First, as shown in FIG. 4, the first predetermined value refRatio is initially set to 1.5 (step S1). The first predetermined value refRatio is stored in advance in the storage unit 13 (FIG. 1), for example. The first predetermined value refRatio is preferably a value greater than 1 and less than 2 for the reasons described above.

Subsequently, the current average light intensity P ₀ (n) is detected (step S2). Actually, the average value of the light amount signal Dmon input to the control unit 5 is set as the light intensity P ₀ (n). Then, it is determined whether or not the difference between the light intensity P ₀ (n) and the target light intensity refP ₀ is within a predetermined range (step S3). Here, the target light intensity refP ₀ is a target value of the light intensity P ₀ , and the target light intensity refP ₀ is stored in the storage unit 13 (FIG. 1).

In step S3, an absolute value of a difference between the current light intensity P ₀ (n) and the target light intensity refP ₀ is calculated, and a predetermined ratio (for example, 1) between this value and the target light intensity refP ₀ (ie, a predetermined target level). %). When the calculated absolute value is larger than the predetermined ratio (1%), the light intensity P ₀ (n) is away from the target light intensity refP _0, so that the bias current Ibias and the modulation current Imod are almost the same. In order to perform proper optimization, the process proceeds to step S4.

  In step S4, the ratio Ratio of the first and second changes is compared with a first predetermined value refRatio (= 1.5). Specifically, the absolute value of the difference between the ratio Ratio of the first and second changes and the first predetermined value refRatio is calculated, and this value is compared with a second predetermined value (for example, 0.1). When the calculated absolute value is larger than the second predetermined value (0.1), the bias current Ibias is away from the threshold current value Ith, so that the bias current Ibias is roughly optimized. Then, the process proceeds to steps S5 to S9. Further, when the calculated absolute value is smaller than the second predetermined value (0.1), the bias current Ibias is in the vicinity of the threshold current value Ith, so that the modulation current Imod is generally optimized. Then, the process proceeds to steps S10 to S14.

  In the present embodiment, the ratio Ratio of the first and second changes is calculated in step S27 or S34 (described later) shown in FIG. 5, so that the first and second ratios immediately after the start of the operation of the laser driving circuit 1a. The ratio Ratio ratio is unknown. Therefore, immediately after the operation is started, an initial value (for example, 2.0) stored in advance may be used as the ratio Ratio of the first and second changes.

In steps S5 to S9, the bias current Ibias is largely controlled. First, the light intensity P ₀ (n) is compared with the target light intensity refP ₀ (step S5). When the light intensity P ₀ (n) is smaller than the target light intensity refP ₀ , since the bias current Ibias is smaller than the threshold current value Ith, the controller 5 causes the bias current to increase the bias current Ibias by a predetermined amount ΔIbias. A quantity control signal Sb is generated (step S6). Further, when the light intensity P ₀ (n) is larger than the target light intensity refP ₀ , the bias current Ibias is larger than the threshold current value Ith. Therefore, the control unit 5 decreases the bias current Ibias by a predetermined amount ΔIbias. A bias current amount control signal Sb is generated (step S7). However, as a unique situation, since the bias current Ibias is zero immediately after the start of the operation of the laser driving circuit 1a, the control unit 5 determines this case (step S8) and decreases the modulation current Imod by a predetermined amount ΔImod. Thus, the modulation current amount control signal Sm is generated (step S9).

In steps S10 to S14, the modulation current Imod is largely controlled. First, the light intensity P ₀ (n) is compared with the target light intensity refP ₀ (step S10). When the light intensity P ₀ (n) is smaller than the target light intensity refP ₀ , since the modulation current Imod is too small, the control unit 5 generates the modulation current amount control signal Sm so as to increase the modulation current Imod by a predetermined amount ΔImod. (Step S11). When the light intensity P ₀ (n) is larger than the target light intensity refP ₀ , the modulation current Imod is excessive, so that the control unit 5 controls the modulation current amount control signal Sm so as to decrease the modulation current Imod by a predetermined amount ΔImod. Is generated (step S12). However, as a unique situation, when the modulation current Imod is zero, only the bias current Ibias is supplied to the LD 18, so the control unit 5 determines this case (step S 13), and the bias current A bias current amount control signal Sb is generated so as to decrease Ibias by a predetermined amount ΔIbias (step S14).

The controller 5 repeats the above steps S2 to S14 until the difference between the light intensity P ₀ (n) and the target light intensity refP ₀ falls within a predetermined ratio (1%) of the target light intensity refP ₀ .

Next, when it is determined in step S3 that the difference between the light intensity P ₀ (n) and the target light intensity refP ₀ is smaller than a predetermined ratio (1%) of the target light intensity refP ₀ (predetermined target level). Will be described. In this case, since the light intensity P ₀ (n) is almost converged to the target light intensity refP ₀ , the process proceeds to steps S20 to S34 shown in FIG. 5 in order to control the extinction ratio with high accuracy.

  First, in step S20, the magnitude of the ratio Ratio of the first and second changes and the first predetermined value refRatio (= 1.5) are compared. When the ratio Ratio of the first and second changes is larger than the first predetermined value refRatio (= 1.5), the bias current Ibias is larger than the threshold current value Ith, and according to steps S21 to S27. Fine adjustment of the bias current Ibias and the modulation current Imod is performed.

That is, since the bias current Ibias is larger than the threshold current value Ith, first, the bias current amount control signal Sb is generated so as to decrease the bias current Ibias by ΔI (preferably smaller than ΔIbias described above) (step S21). ). Then, the light intensity P ₀ (n + 1) after the decrease of the bias current Ibias is detected (step S22), and the difference ΔP ₀ (n + 1) between the light intensity P ₀ (n + 1) and the previous light intensity P ₀ (n) is obtained. Calculate (step S23). Note that the difference ΔP ₀ (n + 1) is a numerical value obtained by reducing the bias current Ibias by ΔI. Therefore, to calculate the first change from the difference ΔP ₀ (n + 1), −1 is multiplied.

Next, the modulation current amount control signal Sm is generated so as to increase the modulation current Imod by 2ΔI (step S24). Here, the reason why the increase amount of the modulation current Imod is set to be twice as large as ΔI is that the bias current Ibias is larger than the threshold current value Ith. Therefore, the modulation current Imod is biased in consideration of the equations (6) and (7). This is because the current Ibias is changed with a width twice as large. Then, the light intensity P ₀ (n + 2) after the modulation current Imod is increased is detected (step S25), and the difference ΔP ₀ (n + 2) between the light intensity P ₀ (n + 2) and the previous light intensity P ₀ (n + 1) is obtained. Calculate (step S26). Note that the difference ΔP ₀ (n + 2) is a numerical value obtained by increasing the modulation current Imod by a predetermined amount 2ΔI. Therefore, to calculate the second change from the difference ΔP ₀ (n + 2), ½ is multiplied.

Subsequently, a ratio Ratio of the first and second changes is calculated by calculating the following formula (12) (step S27).

  On the other hand, if the ratio Ratio of the first and second changes is smaller than the first predetermined value refRatio (= 1.5) in step S20, the bias current Ibias is smaller than the threshold current value Ith. Then, the bias current Ibias and the modulation current Imod are finely adjusted according to steps S28 to S34.

That is, since the bias current Ibias is smaller than the threshold current value Ith, first, the bias current amount control signal Sb is generated so as to increase the bias current Ibias by ΔI (step S28). Then, the light intensity P ₀ (n + 1) after the increase of the bias current Ibias is detected (step S29), and the difference ΔP ₀ (n + 1) between the light intensity P ₀ (n + 1) and the previous light intensity P ₀ (n) is obtained. Calculate (step S30). The difference ΔP ₀ (n + 1) means the first change.

Next, the modulation current amount control signal Sm is generated so that the modulation current Imod is decreased by ΔI (step S31). Here, since the bias current Ibias is smaller than the threshold current value Ith, the modulation current Imod is changed at the same ratio with respect to the bias current Ibias in consideration of the equations (9) and (10). Then, the light intensity P ₀ (n + 2) after the decrease of the modulation current Imod is detected (step S32), and the difference ΔP ₀ (n + 2) between the light intensity P ₀ (n + 2) and the previous light intensity P ₀ (n + 1). Is calculated (step S33). Note that this difference ΔP ₀ (n + 2) is a numerical value obtained by reducing the modulation current Imod by ΔI. Therefore, to calculate the second change from this difference ΔP ₀ (n + 2), −1 is multiplied.

Subsequently, a ratio Ratio of the first and second changes is calculated by calculating the following formula (13) (step S34).

  Thereafter, the bias current Ibias and the modulation current Imod are controlled by repeating the operations after step S2 based on the change ratio Ratio calculated in step S27 or S34.

FIGS. 6A to 6C and FIGS. 7A and 7B are graphs showing the movement of each signal as an operation example of the laser driving circuit 1a according to the present embodiment. FIGS. 8 (a) to 8 (c) and FIGS. 9 (a) and 9 (b) are shown in FIGS. 6 (a) to 6 (c) and FIGS. 7 (a) and 7 (b), respectively. It is a graph which expands and shows from 0 unit time to 1000 unit time of the graph. 6A and FIG. 8A show the modulation current Imod, FIG. 6B and FIG. 8B show the bias current Ibias, and FIG. 6C and FIG. 8C show the extinction ratio (FIG. 2 of PH / PL), FIG. 7 (a), the FIG. 9 (a) average light intensity _{P 0,} FIG. 7 (b), the FIG. 9 (b) ratio ratio of the first and second change Each is shown. The unit time here is the time required for one control routine. In such a routine, the unit time is often determined by the conversion time of the A / D converter. For example, this conversion time (≈unit time) is set to 100 μm. In the case of the second interval, 1000 unit time is approximately 100 milliseconds.

In these graphs, the initial values of the bias current Ibias and the modulation current Imod are set to 0 [mA], and the operation until reaching a constant target light intensity is shown up to 800 unit time, and thereafter up to 2400 unit time. In order to show the following characteristics when the target light intensity changes, the target light intensity refP ₀ is changed every 400 unit hours, and after 2400 unit hours, the target light intensity refP ₀ is changed every 200 unit hours. Yes. And, FIG. 6 (a) ~ FIG 6 (c) and FIG. 7 (a), (b) , following the change of such target light intensity REFP _0, the modulation current Imod, the bias current Ibias, quenching A state in which the ratio PH / PL, the average light intensity P ₀ , and the ratio Ratio of the first and second changes converge is shown.

Referring to FIGS. 8 (a) to 8 (c) and FIGS. 9 (a) and 9 (b), which are enlarged from the start of operation to 1000 unit time, in the laser drive circuit 1a according to the present embodiment, FIG. First, as shown in FIG. 4, the bias current Ibias is increased (corresponding to step S6 in FIG. 4). As a result, as shown in FIG. 9A, the average light intensity P ₀ reaches the target light intensity refP ₀ (about 1.2 mW in this example) by 50 unit times. Thereafter, up to 500 unit time, while maintaining the light intensity P ₀ (FIG. 9A) substantially constant (as after step S20 in FIG. 5), the ratio Ratio of the first and second changes (as shown in FIG. 5). 9 (b)) is controlled so that bias current Ibias and modulation current Imod approach 1.5 (corresponding to steps S21 to S34 in FIG. 5), the extinction ratio PH / PL (FIG. 8 (c)) is substantially reduced. Kept constant.

In the laser drive circuit 1a of this embodiment, the accuracy of the difference ΔP ₀ (n) between the light intensity P ₀ and the target light intensity refP ₀ and the ratio Ratio of the first and second changes is important. In order to obtain these with high accuracy, for example, the conversion accuracy of the A / D converter 16a (FIG. 1) may be increased. For example, the number of conversion bits is increased, an averaging process is performed, After stabilization, it is desirable to take a technique such as increasing the interval at which the current light intensity P ₀ (n) is detected. Further, in the operation example described above, the amount of change ΔIbias (see steps S6 and S7 in FIG. 4) and ΔImod (see steps S11 and S12 in FIG. 4) for controlling the bias current Ibias in the vicinity of the threshold current value Ith. Fixed value. These change amounts ΔIbias and ΔImod may be changed according to the situation. For example, when the difference between the ratio Ratio of the first and second changes and the first predetermined value refRatio is large, the change amounts ΔIbias and ΔImod. Can be made larger to increase the convergence speed of the bias current Ibias and the modulation current Imod. In the above operation example, ΔIbias and ΔImod are set to 500 [μA], respectively. In addition, ΔI (see steps S21, S24, S28, and S31 in FIG. 5) when determining the ratio Ratio of the first and second changes was set to 50 [μA].

  According to the laser driving circuit 1a and the laser driving method according to the present embodiment, when the bias current Ibias is smaller than the threshold current value Ith of the LD 18, as shown in the above-described equation (11), the first and second The ratio Ratio of changes to a certain value (theoretically 1). When the bias current Ibias is larger than the threshold current value Ith of the LD 18, the ratio Ratio of the first and second changes is a certain other value (theoretically, as shown in the above equation (8)). 2). Therefore, if the bias current Ibias is close to the threshold current value Ith of the LD 18, the ratio Ratio of the first and second changes is a value between them.

  Thus, in the laser drive circuit 1a and the laser drive method of the present embodiment, the control unit 5 obtains the ratio Ratio of the first and second changes, and this ratio Ratio is the first predetermined value (that is, the above-described ratio). Since the bias current Ibias and the modulation current Imod are increased or decreased so as to approach each other), even during normal light output operation, the threshold current value fluctuated depending on the temperature without setting the current experimentally. A bias current Ibias can be set in the vicinity of Ith. Further, since the bias current Ibias is stabilized in the vicinity of the threshold current value Ith, the modulation current Imod can be controlled based on the photocurrent Imon. Further, the ratio Ratio of the first and second changes is determined by the magnitude relationship between the bias current I bias and the threshold current value Ith, and is hardly affected by fluctuations of the slope efficiency η depending on the light intensity. Even when the slope efficiency η varies according to the emission intensity, the modulation current Imod can be accurately controlled.

Further, the control unit 5 performs control (steps S20 to S34 in FIG. 5) to increase or decrease the bias current Ibias and the modulation current Imod so that the ratio Ratio of the first and second changes approaches the first predetermined value refRatio. This is preferably performed when the difference between the current light intensity P ₀ (n) and the target light intensity refP ₀ is within a predetermined range (N branch in step S3 in FIG. 4). As described above, by controlling the ratio Ratio to be close to the first predetermined value refRatio while the light intensity P ₀ (n) is stable in the vicinity of the target light intensity refP ₀ , the bias current Ibias is set to the threshold current value Ith. It is possible to approach the vicinity stably.

Further, when the difference between the average light intensity P ₀ and the target light intensity refP ₀ is outside the predetermined range (Y branch in step S3 in FIG. 4), the control unit 5 sets the ratio Ratio and the first predetermined value refRatio to Is smaller than a second predetermined value (for example, 0.1) (N branch in step S4 in FIG. 4), the modulation current Imod is controlled, and when the difference is larger than the second predetermined value (in FIG. 4). It is preferable to control the bias current Ibias for the Y branch in step S4.

When the difference between the light intensity P ₀ (n) of the LD 18 and the target light intensity refP ₀ is outside the predetermined range, first, it is preferable to make the light intensity P ₀ (n) close to the target light intensity refP ₀ . In such a case, if the difference between the ratio Ratio and the first predetermined value refRatio is relatively small (that is, if the bias current Ibias is a value near the threshold current value Ith), the bias current Ibias has already been set. In this state, the modulation current Imod is increased or decreased. Conversely, if the difference between the ratio Ratio and the first predetermined value refRatio is relatively large (that is, if the bias current Ibias is out of the vicinity of the threshold current value Ith), the bias current Ibias is increased or decreased. Thus, by individually controlling the bias current Ibias and the modulation current Imod according to the ratio Ratio value, the light intensity P ₀ (n) of the LD 18 is controlled while controlling the bias current Ibias in the vicinity of the threshold current value Ith. Can be suitably approximated to the target light intensity refP ₀ .

  In the laser driving circuit 1a and the laser driving method described above, the mark rate of the modulation signal Smod is described as 1/2. However, even when the mark rate changes from 1/2, the laser driving method described above is effective. is there. For example, when the mark rate is greater than ½, the light intensity increases according to the mark rate, but the first predetermined value RefRatio is expressed by the equation (8) based on the mark rate detected by the data change detection unit 4. ), And it is good to correct | amend so that it may become the value between Formula (11).

  In addition, since the bias current Ibias is maintained with high accuracy in the very vicinity of the threshold current value Ith, the change in the threshold current value Ith of the LD 18 can be observed by observing the change in the bias current Ibias with time. In the laser element, the threshold current value Ith changes according to the operating time (generally, the threshold current value Ith increases and the light emission efficiency deteriorates). The threshold current value Ith of the LD 18 also depends on the temperature. Therefore, by recording the bias current Ibias, the operation time, and the temperature in the storage unit 13 or taking them out of the laser drive circuit 1a via the external communication circuit 14, long-term fluctuations in the threshold current value Ith are observed. It becomes possible.

Further, the slope efficiency η of the LD 18 can be easily observed. That is, the step S26 and S33 shown in FIG. 5, the change amount [Delta] P ₀ of the light intensity _P 0 (n) with respect to a change in a predetermined amount ΔI of the modulation current Imod (or _{2ΔI) (= P 0 (n} + 1) -P 0 ( n)) is calculated. Since the bias current Ibias is controlled with high accuracy in the vicinity of the threshold current value Ith, the amount of change ΔP ₀ is exactly a numerical value indicating the slope efficiency η of the LD 18. Therefore, by obtaining the change amount ΔP ₀ together with the operating time and the temperature of the LD 18, it is possible to observe the temperature characteristic of the slope efficiency η and the change with time.

  Furthermore, the life of the LD can be estimated by judging the deterioration of the laser element from the change over time of the threshold current value Ith and the slope efficiency η, and issuing an alarm, and the module using the LD can be replaced. .

The bias current Ibias and the modulation current Imod are set to 0 mA as initial values for feedback control. As described above, even if the initial value is 0 mA, the light intensity P ₀ converges to the target light intensity refP ₀ . Therefore, it is not necessary to store in advance the initial values of the bias current Ibias and the modulation current Imod corresponding to the laser element temperature as in the conventional laser drive circuit. Even if the temperature of the laser element changes during the output of the signal light Pa, the bias current Ibias converges in the vicinity of the threshold current value Ith at the changed temperature, and a predetermined target light intensity refP is generated under the bias current Ibias. _The modulation current Imod that realizes ₀ is automatically determined. Therefore, in the laser driving circuit 1a, the initial setting generally performed for the conventional laser driving circuit, that is, the temperature characteristics of the threshold current value Ith and the slope efficiency η of the laser element are measured in advance. Initial setting for storing the temperature characteristics in the memory becomes unnecessary. In the laser drive circuit 1a, only some parameters such as the target light intensity refP ₀ and the first predetermined value refRatio need to be stored, and these parameters are for stabilizing the control loop. It does not depend on temperature characteristics. In other words, the optical module can be manufactured with very simple adjustment by this driving method.

  On the other hand, the threshold current Ith and the slope efficiency η can be calculated. By storing these data together with the temperature and creating a lookup table, it is possible to calculate the control initial value of the bias current value Ibias and the modulation current value Imod from the temperature at the time of power-on, and to converge the control loop at a higher speed.

(Modification)
FIG. 10 is a block diagram showing a configuration of a laser drive circuit 1b according to a modification of the embodiment. In FIG. 10, portions overlapping with the configuration of the above embodiment (FIG. 1) are omitted as appropriate. The difference between this modification and the above embodiment is that a part of the function of the control unit 5 is separated and configured as an analog circuit.

  That is, the laser drive circuit 1 b of the present modification further includes a second control unit 6 connected between the current-voltage conversion unit 3 and the control unit 5. The second control unit 6 is an analog circuit for calculating a signal Sdt indicating the time change and a signal Sdiff indicating a difference from the target level based on the light amount signal Smon from the current-voltage conversion unit 3.

  Specifically, the second control unit 6 includes an integrator 61, a sample / hold (S / H) circuit 62, and amplifiers 63 and 64. The integrator 61 generates an average value Save of the light amount signal Smon. The integrator 61 of this modification can include, for example, an amplifier, and a capacitive element and a resistive element connected in parallel to the amplifier. The S / H circuit 62 is a circuit for holding the output Save of the integrator 61 for a predetermined time using the timing signal from the control unit 5 as a trigger.

  The amplifier 63 generates a time change amount Sdt of the signal Save. The amplifier 63 compares the difference between the current light amount signal Save and the signal Save held in the S / H circuit 62 in synchronization with the previous trigger signal (that is, the change in the average light amount signal Save over time). Minutes) is generated, converted into a digital signal Ddt via the A / D converter 16b, and then provided to the controller 5.

The amplifier 64 calculates a signal Sdiff indicating a difference between the average value of the light amount signal Smon and a predetermined target level. The amplifier 64 generates a signal Sdiff indicating the difference between the current light amount signal Save and the target level signal Sref (that is, the difference between the current average light intensity P ₀ and the target light intensity refP ₀ ), and this is expressed as A / After being converted into the digital signal Ddiff via the D conversion unit 16c, it is provided to the control unit 5.

As in the present modification, the laser drive circuit may perform a part of the processing of the control unit (see FIGS. 4 and 5) using an analog circuit. That is, in the present modification, the second control unit 6 calculates the amount of change in the light amount signal Save (signal Sdt) corresponding to the passage of a predetermined time, so that the control unit 5 performs steps S23 and S26 shown in FIG. , S30, and S33 can be omitted. Similarly, since the second controller 6 calculates the difference (signal Sdiff) between the current light intensity P ₀ and the target light intensity refP ₀ , the controller 5 performs the calculation in step S 3 shown in FIG. Can be omitted. As a result, the difference signal is input to the A / D converter, and as a result, the number of bits necessary for the A / D conversion can be reduced, and the circuit scale of the A / D converter can be reduced.

  The laser driving circuit or the laser driving method according to the present invention is not limited to the above-described embodiments and modifications, and various modifications can be made. For example, in the above embodiment, the light amount signal, which is an analog signal from the current-voltage conversion unit, is A / D converted and input to the control unit as it is, but when the communication rate is fast, it is shown in FIG. An averaging circuit such as the integrator 61 may be connected between the current-voltage converter and the A / D converter.

FIG. 1 is a block diagram showing a configuration of an embodiment of a laser driving circuit according to the present invention. FIG. 2A is a diagram illustrating an example of output characteristics (IL characteristics) of an LD. FIG. 2B is a graph showing an example of a time waveform of signal light output from the LD. FIG. 2C is a graph showing an example of a time waveform of the drive current necessary for outputting the signal light shown in FIG. FIG. 3A is a graph schematically showing the IL characteristic of the LD. FIG. 3B is a graph showing an example of the waveform of the drive current when the average light intensity of the LD becomes a certain value P ₀ when the bias current is smaller than the threshold current value of the LD. FIG. 3C is a graph showing an example of the waveform of the drive current when the average light intensity of the LD becomes a certain value P ₀ when the bias current is larger than the threshold current value of the LD. FIG. 4 is a flowchart showing the internal operation of the controller, and shows the laser driving method according to the present embodiment. FIG. 5 is a flowchart showing the internal operation of the controller, and shows the laser driving method according to the present embodiment. FIGS. 6A to 6C are graphs showing the movement of each signal as an operation example of the laser driving circuit. FIGS. 7A and 7B are graphs showing the movement of each signal as an operation example of the laser driving circuit. FIG. 8A to FIG. 8C are graphs showing an enlargement from 0 unit time to 1000 unit time of the graphs shown in FIG. 6A to FIG. 6C, respectively. FIGS. 9A and 9B are graphs showing an enlarged view from 0 unit time to 1000 unit time of the graphs shown in FIGS. 7A and 7B, respectively. FIG. 10 is a block diagram illustrating a configuration of a laser driving circuit according to a modification.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1a, 1b ... Laser drive circuit, 2 ... Drive current generation part, 3 ... Current voltage conversion part, 4 ... Data change detection part, 5 ... Control part, 6 ... 2nd control part, 11 ... Power-on detection part, 12 DESCRIPTION OF SYMBOLS ... Temperature detection part, 13 ... Memory | storage part, 14 ... External communication circuit, 15a-15c ... D / A conversion part, 16a-16c ... A / D conversion part, 18 ... LD, 19 ... PD, 21 ... Current source, 22 ... modulation circuit, 23 ... bias current source, 61 ... integrator, 62 ... S / H circuit, 63, 64 ... amplifier, Ibias ... bias current, Id ... drive current, Imod ... modulation current, Imon ... photocurrent.

Claims (4)

A laser drive circuit that supplies a drive current including a bias current and a modulation current to a laser element to generate signal light, and controls the drive current based on a photocurrent from a light receiving element that monitors the signal light;
A drive current generator for supplying the drive current to the laser element;
A control unit that controls the bias current and the modulation current included in the drive current based on the photocurrent, and
The controller is
The change in the photocurrent with respect to the change in the bias current is obtained as a first change, the change in the photocurrent with respect to the change in the modulation current is obtained as a second change, and the ratio of the first and second changes is 1 A laser driving circuit, wherein the bias current and the modulation current are increased or decreased so as to be a first predetermined value that is larger and smaller than 2 . The controller performs an operation of setting a ratio between the first change and the second change as a first predetermined value when a difference between the photocurrent and a predetermined target value is within a predetermined range;
When the difference is out of a predetermined range, the modulation current is increased or decreased when the absolute value of the difference between the ratio between the first and second changes and the first predetermined value is smaller than a second predetermined value. , wherein the increase or decrease said bias current when the absolute value of the difference between the first and the ratio between the first predetermined value of the second change is greater than said second predetermined value, according to claim 1 The laser drive circuit according to 1. The control unit has at least one of a function of calculating a threshold current of the laser element from the bias current and a function of calculating a slope efficiency of the laser element from the modulation current. 3. The laser driving circuit according to 1 or 2 . A laser driving method in which a driving current including a bias current and a modulation current is supplied to a laser element to generate signal light, and the driving current is controlled based on a photocurrent from a light receiving element that monitors the intensity of the signal light. And
A change in the intensity of the signal light with respect to a change in the bias current is obtained as a first change, a change in the intensity of the signal light with respect to a change in the modulation current is obtained as a second change, and the first and second changes. The laser driving method is characterized in that the bias current and the modulation current are increased or decreased so as to approach a first predetermined value larger than 1 and smaller than 2 .

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