FI106677B - Regulation method and regulation circuit for a laser diode transmitter - Google Patents

Description

106677 Control Method and Control Circuit for Adjusting the Laser Diode Transmitter

The invention relates to a method for adjusting the optical output level of a laser diode transmitter, comprising detecting a portion of an optical output signal of a laser diode transmitter and generating a monitoring signal representing said optical output signal and adjusting the average optical output signal of said laser diode transmitter. The invention further relates to a control circuit implementing this method.

10 A typical problem encountered with laser diodes is that the optical output power of the laser diode changes with temperature and time. More specifically, the effect of aging and increasing temperature on the laser diode is that the laser diode threshold current increases and the slope efficiency decreases. Conventionally, this problem has been solved to some extent by using a Peltier cooler to eliminate temperature changes and a mean feedback loop to compensate for aging.

Several methods of adjusting the optical output level without a condenser have also been tried, but no fully satisfactory system is known. Most known methods are either too complicated or susceptible to interference 20 for use in dynamic operating conditions or for assembly in a production line.

It is an object of the invention to provide a novel method of adjusting the optical output level of a laser diode (>; i · transmitter for use in a telecommunications network with a life expectancy of 20 years. Specifically, the shutdown ratio performances are tight so that the control circuit The quench ratio 25 is defined as the ratio of the optical "Τ" level to the "0" level. For large quench rates: "· * ·., the" 0 "level is very small compared to the" Τ "level. ♦ * .. * A loop or offset in the control loop can cause a significant change in the "0" level • · · *** * and thereby the extinction ratio.

To prevent this from happening, the control loop must be non-sensitive to temperature changes, power supply changes, and component tolerances. This means that, for example, as the temperature changes, the loop should compensate for the variation in laser efficiency rather than offset and creep in the loop itself.

♦ · The system known from EP 218 449 does not: V 35 meet the above requirements for several reasons. In this system, the control loop control signal is derived from a balanced modulator or mixer.

2 106677

The output of a balanced modulator or mixer directly depends on the amplitude of the input signals. The system itself uses the data signal or the data signal after a few logical operations as a reference signal. The data signal is said to be a digital signal, so it is derived from a logical ve-5 interface. It is well known that the output levels of any logical gate vary with temperature changes. Therefore, the reference signal to the balanced modulator or mixer changes with temperature. This causes an incorrect change in the narrator output, which in turn affects the extinction ratio.

10 The balanced modulator or mixer itself consists of transistors or diodes whose base emitter voltages change with temperature. This again causes unnecessary adjustment of the output.

In this known system, the steps of the recovered signal and the reference signal need to be synchronized. This method is not suitable for high bit rates at which the pulse width is comparable to the phase shift caused by the parasitic capacitances within the circuit. This requirement for separate control of each circuit is a major drawback in mass production.

In one embodiment of European Patent Application 218,449, positive and negative peaks of a signal are detected. Because the negative peak equals a "0" level with a low value, even a small offset or creep causes a large error.

In the control circuit of U.S. Patent No. 4,277,846, the control signal is derived from a • negative peak detector and a fixed reference signal. This circuit is not internally temperature compensated and thus the peak detector base emitter voltage is strongly reflected as errors in the modulation current.

In the control circuit according to JP 55-83280, the control signal is derived from a combination of a positive peak signal level and an average signal level. Again, there is no internal temperature compensation and no charge has been made for the signal operating ratio. In fact, if the transmission is interrupted during operation, the loop will increase the modulation current indefinitely, thereby damaging or destroying the laser.

The objects of the invention are achieved by the method of claim 1 and the control circuit of claim 4.

The method according to the invention is based on the principle that the exact signal and the data signal are generated by the same mechanism. Thus, all changes in the system occur symmetrically in both sig- nals, so that the output remains the same. Because of this principle, the circuit is internally temperature compensated and immune to variations in the power supply and the input signal.

The invention simultaneously monitors and controls the data signal transmitted using the peak optical level and the average level of the optical output of the laser diode 5 as a reference signal. Due to the use of the data signal as a reference signal, even during a long data bit sequence with no changes, operation is not interrupted or laser power is incorrectly adjusted. Therefore, the invention allows the level and quench rate to be kept constant despite the variable ratio.

The invention further relates to a control circuit for controlling the optical output level of a laser diode transmitter, comprising a monitoring light diode means for detecting a portion of the optical output signal of a laser diode transmitter, generating a by. The control circuit according to the invention comprises a peak level detecting means for detecting a peak level of a monitoring signal and a peak of a data signal to be transmitted, a second adjusting means for adjusting the peak level of the optical output of the laser diode-20 transmitter.

The control circuit according to the invention is characterized by what is said *: * '*' in the characterizing part of claim 4.

·: ··: In the following, the invention will be described in more detail by means of 25 exemplary embodiments with reference to the accompanying drawings, in which: Fig. 1 is a block diagram of a control circuit according to the invention.

• ·

Now, some technical terms used herein will be described.

Slope efficiency is related to the ability of a laser to convert electric current into light, i.e. a laser with a good efficiency needs a low current - '.V.' 30 · · · · · · · · · · · · · · · · · · ·: · · · · · ·: · · ··· * while the inefficient laser requires a larger current change for the same change ·: ···: light output.

•: · ·: Slope variations are related to changes in the efficiency of the laser with time and temperature. Typically, as the laser ages and: * '35 operating temperature rises, the efficiency drops, requiring increased control current.

4, 106677

Figure 1 illustrates an optical transmitter comprising a laser diode LD, a laser diode driver 1, a modulation current source 2, and a biasing current source 3. The laser diode LD is biased in conventional manner with a bias current 1Bias from the biasing current source 3. The modulation current source 2 generates a modulo 5 generation current IMod based on an incoming data signal which is supplied as input to source 2. The modulation current IMod is supplied to the laser diode controller 1 to modulate the laser diode LD so that an optical signal corresponding to this input data

10 A portion of the optical output power of the laser diode LD is applied to a monitoring LED FD and is detected therewith for monitoring the output level of the laser diode LD. The photodiode FD is coupled to a high bandwidth preamplifier 4 which outputs an electrical signal 4a representing the optical output of the laser diode LD. The preamplifier 4 is implemented as a broadband transimpe-15 amplifier so that its output voltage amplitude is independent of temperature changes.

The output signal 4a is supplied to a peak level detector 6 which determines the peak value of the signal 4a and outputs the output signal 6a. The incoming data to be transmitted is supplied to a limiter amplifier 12 which amplifies and limits the data signal so that its output level 1 remains constant at all temperatures. The preamplifier 4 and the limiter amplifier 12 both have the same .. · i operating voltage Vcc, and any variations in Vcc caused, for example, by temperature changes, affect both signals in the same way. From the limiter amplifier 12, the signal is supplied to a second peak level detector 7, which determined · 25 to peak the incoming data and outputs the output signal 7a.

· * · *: The peak constants 6 and 7 should have the same time constants. Top Level Expressions- • ♦. ·: ·. The simulators are preferably implemented using transistors or other circuit devices integrated on the same silicon chip, so that they are continuously at the same temperature, and there is no error in comparing the temperature between their outputs. The output signals from the detectors 6 and 7 are supplied as control inputs to a comparator means 9 such as a differential amplifier which ·: **: generates an asymmetric difference signal 9a representing the difference between the signals 6a and 7a ·: ···. Fluency level detectors 6 and 7 have the same reference voltage V1, *, so that variations in the voltage caused by, for example, temperature fluctuations affect the output signals of both detectors in the same way. The difference signal 9a is supplied to the modulation current source 2 as a negative feedback signal 106677 5 to maintain a zero error signal in the closed loop by adjusting the modulation current 1Mod and thereby the peak level of the optical output signal of the laser diode LD.

The output signal 4a is also supplied to the average level indicator 5 which determines the average level of the signal 4a and outputs the output signal 5a. The incoming data to be transmitted is supplied to a second average level detector 8, which determines the average level of the incoming data and outputs the output signal 8a. The time constants of the average level detectors 5 and 8 should be the same. The output signals from the detectors 5 and 8 10 are input as inputs to a comparator means 10, such as a differential amplifier, which produces an asymmetric difference signal 10a representing the difference between the signals 5a and 8a. The differential amplifier 10 is preferably an integrating amplifier so that the dependence on the phase of the detector output signals 5a and 8a should be eliminated. The difference signal 10a is supplied to the bias current source 3 as a negative feedback signal 15 to maintain a zero error signal in the closed loop by adjusting the bias current 1Bias and thereby the average level of the optical output signal of the laser diode LD.

To control the operation and interaction of these two parallel feedback loops, the biasing control loop should preferably be 20 faster than the peak level control loop.

The figure and the description related thereto are only intended to illustrate the invention. The method of the invention and the control circuit in accordance with the invention may vary in detail within the scope of the appended claims.

a «« · · »» · · ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ * * * * · · • · * • • • • • • • • •

Claims (7)

A method of controlling the optical output level of a laser diode transmitter (LD), which method comprises detecting a portion of the optical output of the laser diode transmitter (LD), generating a monitoring signal representing said optical output signal, averaging the level of the optical output of the laser diode transmitter (LD) on the basis of said monitoring signal and a data signal to be transmitted; 15 2. A method according to claim 1, characterized in that the control of the peak level for the optical output power comprises controlling a modulation current in the laser diode transmitter (LD) so that the difference remains zero. 3. A method according to claim 1 or 2, characterized in that the average level of the optical output power comprises controlling a bias current in the laser diode transmitter (LD) on the basis of a difference between the average levels of the monitoring signal and the data signal to be transmitted, so that the difference remains zero. ; in 4. Control circuit for controlling the optical output of a laser diode transmitter; <<· Level, which control circuit comprises a monitoring photodiode means (FD) for the detection of a portion of: v. the optical output of the laser diode transmitter (LD), • a preamplifier (4) for generating a monitoring signal (4a) representing * said optical output, ... a first control means (3.10) for control • · * ·· ♦ * 30 the average output level of the laser diode transmitter (LD) on the basis of said monitoring signal and the data signal to be transmitted, a peak level detector means (6, 7) for detecting peak level of ....: the monitoring signal (4a) and the data signal to be transmitted, and a second control means ( 2, 9) for controlling the peak level of the laser: V diode (LD) optical output power based on the difference between the peak levels detected by the peak level detector means (6, 7), characterized in that the control circuit further comprises a constraint amplifier (12) coupled in front of the top level detector (7) of the data signal to make the T signal of the data signal independent of temperature, and the ring is faster than the top level control. 5. Regulatory circuit according to claim 4, characterized in that the preamplifier is a broadband type transimpedance preamplifier (4). Control circuit according to claim 4 or 5, characterized in that the top level detectors (6, 7) of the data signal and the monitoring signal are identical in design and function. Control circuit according to claim 4, characterized in that said second control means (2, 9) regulates a modulation current in the laser diode transmitter (LD), and said first control means (3, 10) regulates a bias current in the laser diode transmitter (LD) on the basis of a a difference between the average levels of the monitoring signal and the data signal to be transmitted, which is detected by an average level detector means (5, 8) which detects the average level of the monitoring signal (4a) and the average level of the data signal to be transmitted. c «I €« «11« · «· * ·· - ·« ··· ♦ ♦ · # · ♦ • · • · ··· ♦ · · · · ·· «4» «· ♦ ·· 1 • · • · «·· ♦« • · • · ♦ · · · · · · · · ·