TW496047B - Sub-threshold bias control for burst mode optical transmitters - Google Patents

Abstract

The present invention is a method and apparatus for driving a laser in an optical transmitter. A sub-threshold circuit provides a sub-threshold current to adjust a bias current from a bias current source during an adjustment mode. A modulating circuit is coupled to the sub-threshold circuit to generate a first modulating current during an operational mode. The first modulating current includes a second modulating current and the sub-threshold current. The first modulating current and the bias current form a drive current to drive the laser.

Description

496047 V. Description of the invention (1) Background of the invention 1. Scope of the invention The present invention relates to an optical transmitter, and in particular, the present invention relates to a driving circuit for an optical transmitter. 2. Description of related techniques: The existing laser driver control circuit maintains a bias current that is slightly higher than the laser threshold current to optimize laser high-speed characteristics or has a near-zero bias current to maximize optics. Signal absorbance.

Conventional techniques are used to drive the performance of optical transmitters including optical transmitters. Optical transmitters operate at high frequencies with low absorbance or low frequencies with high absorbance. In addition, when the laser bias current is close to zero, the laser interface capacitor must be fully charged or discharged, during which the optical amplitude is adjusted. Therefore, there is a need for this technology to provide an effective method and device to control the bias current of the optical transmitter. Summary of invention

The present invention is a method and apparatus for driving a laser of an optical transmitter. During an adjustment mode, a threshold circuit provides a threshold current to adjust a bias current from a bias current source. During an operation mode, a modulation current is connected to the subthreshold circuit to generate a first modulation current. The first modulation current includes a second modulation current and a second threshold current. The first modulation current and the bias current form a driving current to drive the laser. In a preferred embodiment, the secondary threshold circuit includes a primary threshold current source and a secondary threshold switch, and the modulation current includes a modulation current source and a modulation switch. The secondary threshold current source generates a secondary threshold current at a threshold below the threshold level.

496047 V. Description of the invention (2) Positioning accuracy. During the adjustment period, the secondary threshold open relationship is connected to the secondary threshold current source to connect the secondary threshold current source to the bias current source. Based on an adjustment control number, the modulation current source generates a second modulation current. During the operation mode, the modulation on relationship is connected to the modulation current source and the subthreshold switch to connect the modulation current source and the subthreshold current source to generate a first modulation current. BRIEF DESCRIPTION OF THE DRAWINGS The characteristics and advantages of the present invention can be obtained from the following detailed description of the present invention, in which:

Fig. 1 illustrates an optical transmitter system using a driving circuit according to an embodiment of the present invention. FIG. 2 is a driving circuit diagram shown in FIG. 1 according to an embodiment of the present invention. FIG. 3A is a graph showing the photoelectric current characteristics of an embodiment of the present invention during the bias current adjustment period. Fig. 3B is a graph showing the photoelectric current characteristics of an embodiment of the present invention after the bias current is adjusted. FIG. 4 is a control circuit diagram illustrating an embodiment of the present invention in FIG. 1. FIG. Detailed description of the invention

The invention is a method and a device for driving a laser of an optical transmitter. During the adjustment mode, a threshold circuit provides a threshold current to adjust a bias current from a bias current source. During the operation mode, a modulation circuit is connected to the subthreshold circuit to generate a first modulation current. The first modulation current includes a second modulation current and a second threshold current. The first modulation current and the bias current form a driving current to drive the laser. In a preferred embodiment, the secondary threshold circuit includes a primary threshold current source and

Page 6 496047 V. Description of the invention (3)-Subthreshold switch, the modulation current includes-a modulation current source and a modulation switch, the subthreshold current source generates a subthreshold current at-a predetermined position lower than the threshold level quasi. During the adjustment period, the sub-threshold opening relationship is connected to the sub-threshold power source to connect the sub-threshold current source to the bias current source. Based on an amount of modulation control, the modulation current source generates a second modulation current. In the operation mode, the modulation on relationship is connected to the modulation current source and the subthreshold switch to connect the current source and the subthreshold current source to generate the first modulation power. Private: The advantages of the invention include a cluster mode optical transmitter Improved performance, your bias current is below the constant value of the laser threshold. This technique's frequency characteristics and maintain a very high absorbance. This is achieved by the use of bias current] field ... Below the critical current to obtain the optical transmitter system shown in Figure 1 is 100 ′, which uses a driving current diagram as in this example. The optical transmitter system includes a package circuit and a driving circuit 14.... Radiation 105, control electric laser 105 is a semiconductor laser diode that emits light and radiates two driving currents idr. The driving current Idr is produced by the driving circuit 140. The control circuit 107 generates a control amount to control the driving circuit. 14〇. 107 includes the induction circuit u0, the error circuit 12o, the update band operation: the road signal 150 and the threshold control circuit 16o. The induction current u = the sequence of the strong intensity of the incident, and generates a proportional to the radiation pen flow. Error Circuit! 20 produces an error voltage Error Di - a pole limb embodiment the monitoring photo-optical diode current of the diode and Tun Soil:;. A difference than the update circuit & you between the n + r the reference voltage circuit 13G receives ^ voltage and generating the number of control to update

Page 7 496047 V. Description of the invention (4) Drive circuit. The driving circuit 140 generates a driving current according to the number of controls. The input sfL number 1 50 provides timing and pulse signals to sense the circuit 1 1 0, the update circuit 1 3 0 and the drive circuit 1 40. The threshold control circuit 160 provides a control signal to drive the circuit 140. FIG. 2 is a driving circuit diagram of an embodiment of the present invention shown in FIG. 1. The driving circuit 140 includes a subthreshold circuit 21o, a modulation circuit 22o, and a bias current source 2300. The subthreshold circuit 210 provides a temporary Current limit '. The secondary threshold circuit 21 0 includes a primary threshold current source 2 1 5 and a primary threshold switch s 丨 2 丨 7. The sub-threshold current source 2 15 generates a sub-threshold current ist at a predetermined current level. In one embodiment, the predetermined level is an added value, which is below a threshold intended for operation. Sub-threshold switch S1 217 is a three-terminal switch with two positions such as two modes: one adjustment mode and operation mode. In the adjustment mode, the bias control circuit is adjusted to provide a desired light logic low level (for example, logic 0). In the operation mode, the driving circuit 140 normally operates to drive the laser 105. 2 = Intact mode. Secondary threshold switch S1 217 connects secondary threshold current source 215 to secondary current source 23. 'For the second threshold current' t is from the bias current source-plus if current 1 allows the optical low level to be large enough to accurately detect and control. In fact, the optical low level is measured from the monitoring circuit 110 Detected by the photodiode current.

The limit switch S1 217 is connected to a secondary threshold current source 21 5 to π y........... ▲ Modulation circuit 22 0 provides a first modulation current dagger. The modulation circuit 22o includes a modulation current source 225 and a modulation switch S2 227. The modulation current source 225 produces a

Page 8 ^ 6047 V. Description of the invention (5) A modulation current ImQd2. In one embodiment, the modulation current source 2 2 5 is a current mode number = analog converter (DAC). The current dac receives digital data and converts this data into a considerable current. The control digital data used to modulate the current source 2 2 5 comes from the update circuit 1 3 0 shown in FIG. 1. The switch S 2 2 2 7 controls the adjustment of the driving current. During the adjustment mode, the switches Sj 2 2 7 are turned on as discussed above to allow the secondary threshold current [M plus f bias ink current Ib. During operation mode, switch S2 227 is turned on or off under the control of input signal j1 50. The switch S 2 22 7 is connected to the second threshold: the switch s 丨 2 1 7 in the circuit 2 10. In the operating mode, the switches s2, 2 and 7 are connected to the subthreshold current source 21 5 to the modulation current source 2 2 5 to generate a first modulation current. The first modulation current imQdl is the sum of the second modulation current and the second threshold current 1st. In the operation mode, when the switch S2 2 2 7 is turned on, the modulation current source 22 5 and the L limit pen ", the L source 2 1 5 are all separated from the bias current source 2 3 0. The driving current Idr 'is therefore equal to the current provided by the bias current source 23 only, in other words, the bias current Ib. This driving current B is equivalent to a low output level. The adjustment of the bias current ib is performed with the secondary threshold current Ist, and the low level during the adjustment period is large enough to accurately detect and control. When the switch S 2 2 2 7 is closed, both the modulation current source 2 2 5 and the subthreshold current source 2 1 5 are connected to the bias current source 230. The driving current idr is therefore equal to the sum of the currents provided by the bias current Ib, the first modulation current Ijnc) d2, and the subthreshold current Ist, in other words, Ib + = Ib + 1_2 + ist. This sum current is equivalent to the high light level. When the switch S2 22 7 is controlled by the input signal 150, the switch of the switch S2 2 2 7 is synchronized with the input signal 150 low and high levels to generate a suitable

Page 9 496047 V. Description of the invention (6) — Driving current. ak at low output level :; the second signal 150 is low: switch S2 2 27 is turned on 'equivalent to the high output level signal 150 south' switch S2 227 is closed, the phase-electricity TΛ-diagram shows during the adjustment of the bias current such as For the light radiation in the embodiments of the present invention, the horizontal axis / straight axis indicates the intensity of light emitted by the laser or the optical table does not drive the laser current. In accordance with the light level of the horizontal axis. Light: The characteristic is a straight line 31 defined by two points A and 6. By. Line #A 气 is again horizontal at point C. The point A is equivalent to the optical low level Ll and the phase Lh. During the bias adjustment period, the current L is equivalent to light: the low level [equal to the voltage from the bias current source 23, which is shown in FIG. 2: Reporting the lightning current I, 4rr 丄 b A-human I port W ^ > St with. As the secondary threshold current U is increased during the adjustment, some of them are still below the ideal low level. This figure shows the current characteristic diagram of the generator after the bias current is adjusted. 1 j t first ~ The photo-current characteristics are now represented by the two-line segments 叱 and ⑶, and the optical low level and point B are equivalent to the optical high level. Point B is roughly as shown in Figure 3A. During the operation mode period, or after adjustment, the flow rate is the same as the low level, which is only equal to the current from the bias current source 23 (in Figure L 2.5 optical flow Ib. Because this bias during the adjustment The current Ib is less than L! Piezoelectric sum, and the synthetic Ll is less than ll. So the low level in the adjustment mode ^ gate 'is now lower than the low level. The optical low level is generated during the period 3, and it is optically detectable. It is large enough to detect and control T is too low 3 A, but then adjusted to a very small level as shown in Figure 3b.

Page 10 496047 V. Description of the invention (7) A ---- rate 0 Figure ^ is a control circuit diagram of an embodiment of the present invention. The control circuit is 14 times / then laser 1 1 〇 The intensity of the radiated light and the resulting control digits are expected to adjust and bias the current sources 22 5 and 23. The control current 107 includes the induction circuit 11o, the error circuit 12o, and the update circuit 2 shown in FIG. 1. The control circuit 107 operates using a synchronous switching method. The induction circuit includes a monitoring photodiode 410, a capacitor c 42 5, a switch S3 43 〇, a biased G current-shedding source 435, and a reference modulation current source 440. The error circuit 12 includes a conversion impedance amplifier 450, a feedback resistor R 445, and an update circuit 1 3 of an analog comparator, and includes a bias counter 470 and a modulation counter 480. The monitoring photodiode 410 induces the intensity of the light radiated by the laser, and the photodiode current is proportional to the radiation intensity. Capacitance C 42 5 indicates the interface capacitance with the same light pole body 4 10. The conversion impedance amplifier 450 and the feedback resistor R 445 form a current-voltage amplifier to generate a voltage% equivalent to the filtered photodiode current. The voltage Vp is therefore also proportional to the intensity of light such as laser radiation. The voltage Vp is then applied to a comparator 460. The analog comparison ^ 460 compares the voltage vP with the reference voltage Ref and generates an error voltage Ve. Bias counter 470 is a counter which can count up (or increase) and count down (or decrease) by 4. The bias counter 470 has an up / down (u / d #) control turn-in and a pulse (CLK) input. When the U / D # input is low, the bias counter 470 counts down, in other words, decreases. When u / d #input system = high level, the bias counter 470 counts up, in other words, increases. The partial double counter 470 is timed by the forward pulse signal. In other words, the count occurs when the pulse signal is converted in the forward direction. Bias counter 470 generates a bias digitizer

Page 11 496047 V. Description of the invention (8) Number (CNTB) output. The CNTB output is a bias control quantity, which is applied to the digital input of the current source 230 to control the generation of the bias current ib. The modulation counter 48 0 is substantially the same as the bias counter 47 0, except that the modulation counter 48 0 is triggered by a negative edge. In other words, the counting occurs in the negative direction of the pulse signal. The modulation counter 4 8 0 generates an adjusted digital count (c n τ μ) output. The CNTM output is an adjustment control quantity, which is added to the digital input ′ of the modulation current source 225 to control the generation of the modulation current. The number of bits of the counters 0 and 4 80 is predetermined and controlled based on the granularity of the laser drive current. In one embodiment, the counters 470 and 48 0 are 8-bit up / down counters that provide from 0 to 2 5 5 of 2 5 6 count value. The switch S 3 4 3 0 is controlled by the input signal 150. In other words, the input signal 1 50 now drives the switch s 3 4 3 0, the bias counter 4 7 0, the modulation counter 4 8 0, and the switch S 2 2 2 7 (in Fig. 2). This composition is therefore confident of the operation of the entire system. In the bias mode, in other words, when the switches s 2 2 2 7 and S 3 430 are turned on, the reference bias current source 435 is provided to provide a fixed current source 1 to the switching impedance amplifier 450. In the adjustment mode, in other words, when both the switches s 2 27 and S3 43 0 are turned off, the reference modulation current source 44 is connected to the switch S3 430 to provide a fixed modulation current Ipm. The analog comparator 46 compares the Vp voltage with a fixed reference voltage to generate an error voltage%. Error% is the sign of a digital signal that indicates an error. The counting direction is determined by%. Two counters 470 and 480 use this error voltage ^ to update their counts at different pulse edges. The output of these counters is used for control, as shown in sources 2 2 5 and 2 3 0 in Figure 2. & The pulse signals of the two counters 470 and 480 are connected together and connected to

Page 12 V. Description of the invention (9) ί ϊ ΐ 5〇 # ^ ^ ^ ^ ^ ^ ^ > . . 7〇 and 4δ〇, and the control switch S2 227 drive error at the output ^ power to the control circuit 107 synchronously change on__ are all connected to the same, difference Ί Therefore, although the counter 470 and 48 〇 ν Γ voltage, The mother's counter is updated with different errors-M ^ σσ 幵 guan S 3 4 3 0 and two reference current sources can be assisted by simple hardware. The comparator 45 0 and the analog comparator ". Form the error voltage _. Use = to update the number-such as-every digital digit counting wheel out When the input signal 150 is at a low level, switches S2 2 27 and 33 43 〇 open. Drive current Idr is equal to the bias current ^. On the input side, the input current conversion impedance amplifier 45 0 is equal to the sum of the photodiode current and the reference bias L electric kiPb. Does the input current Ita convert the VP electricity through the conversion impedance amplifier 45 °. Because the reference bias current L is fixed, the electrical material system is still proportional to the photodiode current, which sequentially proportionally radiates the light emitted by the laser 105. The hum ratio comparator 460 compares the fixed voltage to the% voltage and generates Control voltage, counting 1§ 470 updated error voltage%. When the input signal 15 is changed from a low level to a high level, the bias counter 47 is updated (in other words, increased or decreased) the value of WVE. The bias current Source 2 30 is controlled. When the input signal 150 is at a high level, the two switches S2 227 and S3 43 0 are turned off. The driving current 1 ^ is equal to the sum of Ib + I_2 + Ist. Reference modulation current source 4 4 0 Connected to switching impedance amplifier 4 5 〇Input. Switching impedance amplifier

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496047 V. Description of the invention (11) Although the present invention has been described with reference to the illustrated embodiments, this description should not be inferred in a limited sense. Various modifications of the illustrated embodiment, as well as other embodiments of the present invention, are understood by those skilled in the art to which the present invention pertains and are deemed to conform to the spirit and scope of the present invention.

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Claims (1)

6. The scope of the patent application includes i ·· 1 for driving light emission buckle Φ • The method of buckling the laser, this method includes, during the 17-week whole mode, the -4, and the shark comes from the bias current source I limit Circuit, providing a threshold current in a selective bias voltage) ¾ · g's during the operation, the-▲ heart, and /; IL, the first-modulation current includes ^-wither circuit produces a The first modulation electric-modulated current and bias current shape the current and the second threshold current. The second, such as the complicated range of patent application, the driving current to drive the laser. The method includes: the method of item, in which a second threshold current is provided at a predetermined level; and, Baer said that a voltage lower than the threshold is earlier than the adjustment mode during the bias current source. . ° Limit switch, connect the secondary threshold current 3. ππ by the priestess method: ...__ secondary threshold current source generates the standard; and ° limit current is raised slightly below the threshold level ^ — to the bias voltage Battery. ... interval, connecting the secondary threshold current. For example, the second item in the scope of patent application includes: Method, where the first modulation 1 is generated in _ Shitian m ...... The road includes: based on a modulation control quantity, New Zealand The current; and, two are generated by the modulation current source. During the operation mode, the modulation current source is modulated to generate the first $ OFF 'connection to the modulation current source and times. The third scope of application for patents. To the bias current source, including the method of 'connecting the threshold current off, in order to control the second threshold current source controlled by the control circuit during the adjustment mode, and during the operation mode, the current limiting source is Connect to bias current. σ weekly current source to provide the first modulation Page 16 496047 VI. Patent application scope 5. The method of patent application scope item 4, wherein the adjustment is performed as defined by a Full Service Access Network (FSAN) specification Time interval. 6 · The method according to item 5 of the patent application, wherein the threshold level is selected to separate the intensity of light from a laser into a light high level and an optical low level. 7 · The method according to item 6 of the patent application range, wherein the predetermined level is selected so that the optical low level is large enough to accurately detect and control. 8-A laser device for driving a light transmitter, the device comprising: during the adjustment mode, a secondary threshold circuit provides a secondary threshold current to adjust the bias current from a bias current source; and during operation During the mode, the modulation circuit is connected to the subthreshold circuit to generate a first modulation current. The first modulation current includes a second modulation current and a subthreshold current. The first modulation current and the bias current form a driving current to drive the laser. 9. For the device under the scope of patent application, the subthreshold circuit includes: A secondary threshold current source that generates a secondary threshold current at a predetermined level below the threshold level; and a primary threshold switch that is connected to the secondary threshold current during the adjustment mode to connect Subthreshold current source to bias current source. 10. The device according to item 9 of the patent application, wherein the modulation circuit includes: a second modulation current source that generates a modulation current based on the number of modulation controls. Page 17 496047 6. The scope of patent application Current source; and a modulation switch which is connected to the tutor cut-off 1 during the operation mode to connect and adjust the current source and the next two :: the second modulation current. The charter plane was originally designed to produce the 11th device such as the 10th in the scope of the patent application, where · ow; controlled by a threshold control circuit, in order to adjust the second limit of the adjustment mode, "the current rejection source is connected to a bias current source, Operate two current-limiting sources to provide the first modulation current. During the bifurcated mode, the modulation power 12. As the device in the scope of patent application No. 丨 1, 苴: via a full service access network (defined by F Qing specifications: Among them, the threshold level system is selected so that the optical = is set, and the predetermined level system is selected to select a sufficient amount of light to be able to accurately detect and receive ^ The field emission system responds to the driving current to emit light and emit a light and a driving circuit. The generation of the drive =, including ... the 'claw is used to move the laser, the drive circuit includes a threshold circuit, which is used to provide a secondary threshold current during the adjustment of the bias from the wheel source' to produce the first —: Electricity: Connected to the sub-threshold circuit during the ^: mode, the sub-threshold circuit, the first-modulation band, and the five-week current include the second modulation current and the port. The private machine and the bias current form a driving current to drive 6. Scope of patent application Dynamic laser. The road includes the light transmitter of Yuming Patent Qianwei No. 15, in which the secondary threshold power is preset to a wide range: the teaching source 'generates a secondary threshold current at a source below the threshold level' to connect :: limit J :: Connected to the second threshold current 17. Electricity / claw, primary to bias current source. Including: the light transmitter of item 16 of the patent scope, wherein the modulation circuit current; = the control current source, which is based on adjusting the control number to generate a second modulation electric threshold. About # connected to the modulation current source during the operation mode, and The "-modulation" is connected to adjust the current source and the secondary threshold current source to produce 1: as the light transmitter of the patent application scope item i 7, wherein the secondary threshold is controlled by the odd threshold control circuit, during the adjustment mode The secondary threshold current source is connected to the bias current source, and during the operation mode, the current source is modulated to provide the first modulation current. 19 · As the light emitter of the 18th scope of the patent application, the adjustment is performed at A time interval as defined by the Full Service Access Network (FSAN) specification. ''20. The light transmitter of item 19 of the patent application, wherein the threshold level is selected to separate from the laser The light intensity becomes the high level of light and the low level of light. Page 19, 496047 VI. Patent application scope 21 · For the light transmitter of the 20th scope of the patent application, the pre-selection level is selected as the optical low level Large enough to be accurately measured and controlled. Page 20