Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
  • H01S5/068—Stabilisation of laser output parameters
  • H01S5/0683—Stabilisation of laser output parameters by monitoring the optical output parameters
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
  • H01S5/068—Stabilisation of laser output parameters
  • H01S5/06825—Protecting the laser, e.g. during switch-on/off, detection of malfunctioning or degradation

Definitions

• the present invention relates to methods of controlling the biasing and modulation current of a laser diode used for emitting light carrying information, and to light emitting assemblies including laser diodes controlled according to the methods .
• the characteristics of a laser diode such as its threshold current I t h and its efficiency ⁇ , vary with temperature and the ageing of the diode in an unpredictable way (see t ⁇ he diagram in figure 1) .
• the optical power from the rear facet of the laser diode can be measured by a monitor diode, and the measured power value can then be used to adjust the laser current.
• a laser diode is biased slightly above the threshold current by a DC current.
• Laser diodes used in communication networks for transmitting information are then modulated with a signal, generally a bit sequence or bit-stream transmitted with a bit-rate, above this point as in the circuit shown in figure 2.
• the bias current is too close to, or below, the threshold current, the switching time of the laser diode will increase considerably, which will cause serious system degradation at high bit-rates.
• the bias current is too high, the modulation current will be too low, and the on-off ratio of the laser will decrease, which will give a penalty on the receiver sensitivity.
• both a mean power control and an efficiency control must be used.
• a control of the current used for biasing is conventionally made by a monitor diode sensing some light emitted by the laser diode.
• This monitor diode and/or the control circuits connected thereto generally have a limited bandwidth and cannot, for the very high bit-rates typically used in communication over optical links, sense the instantaneous power in the normal low and high 5 levels in the emitted light corresponding to the bits modulating the laser.
• a high frequency detector which is connected to a peak-to-peak detector, can be used to control the modulation current together with a mean power detector controlling the bias current.
• a mean power detector controlling the bias current.
• the detector will work. However, as the low frequency components vary randomly, some margin has to be added to the bias current to avoid getting too close to the 25 threshold current.
• a temperature sensor is not useful if neither the threshold current nor the efficiency temperature dependence is predictable and ageing will not be compensated for.
• the output from the monitor diode is band-pass filtered, and the detected amplitude is compared to the modulated amplitude at the laser, based on which the efficiency is calculated.
• the drawbacks of such methods include that some amount of the signal 35 is needed for this detection, since a lower amplitude gives a lower accuracy, and several sources for inaccuracy, such as data signal modulation power, low frequency signal modulation power, low frequency tone detection and mean power detection, are included in the setting of optical output power.
• U.S. patent 5 502 298 a method of controlling the extinction ratio of a laser diode is disclosed.
• the control of the extinction ratio is achieved by adding training pulses comprising several successive "0"s followed by several successive "l"s in information frames, i.e. training pulses, L0 each having first a low and then a high level, are added periodically over time.
• training pulses comprising several successive "0"s followed by several successive "l”s in information frames, i.e. training pulses, L0 each having first a low and then a high level, are added periodically over time.
• training pulses L0 each having first a low and then a high level
• U.S. patent 4 698 817 is disclosed how the light output from a laser is monitored, including two feedback loops for compensatory changes in the drive current, based on a low frequency pseudo random pattern which is filtered out of the laser frequency spectrum and measured.
• the method includes
• the method requires a pseudo random signal having a low frequency component which is constant over time.
• a deterministic signal could lack power in this frequency range, and under this condition the loops would not work.
• a so called pilot tone is included that is added to the data signal .
• a harmonic in the signal from the monitor is detected. It would seem that this method requires a non-linearity in the I/P curve of the laser to generate the harmonics of the pilot tone. This method thus cannot be used in a high bit-rate system, because the laser is then biased far above the threshold current to avoid turn-on delays, and the method does not seem to work in the linear region of the laser.
• the optical power emitted by the laser chip is measured by a monitor diode, e.g. as conventional by measuring the light emitted from the rear facet of the laser.
• the measured value represented by a suitable electrical signal, is used to adjust the bias and modulation currents.
• extra predetermined bits are used which are inserted by an earlier stage, such as in a standard protocol, in the input signal at regular intervals to provide a low frequency component.
• This component is detected from the signal output from the monitor diode, preferably using in the detecting the input signal.
• the detected signal is used to determine the optical amplitude, which can also be combined with the use of the integrated mean power from the monitor diode, which can have a limited bandwidth.
• an amplitude detector is used that can have a limited bandwidth and is arranged to detect or measure the values obtained from the monitor diode and representing the emitted optical power only when sufficiently long sequences of all "0"s or all “l”s are detected before the laser driver, combined with the use of the integrated mean power from the monitor diode. This also allows the use of a monitor diode having a limited bandwidth.
• Figure 1 is a diagram of the power of light emitted from a laser diode for increasing temperature and ageing, the diagram also showing the efficiency of the laser diode,
• Figure 2 is a schematic diagram of a conventional circuit including a laser diode and circuits for controlling the bias and modulating currents of the laser diode,
• FIG. 3 is a detailed circuit diagram of a control circuit for the bias and modulating currents of a laser diode
• Figure 4 is a schematic diagram of a control circuit according to the invention for controlling the bias and modulating currents of a laser diode
• Figures 5A, 5B, 5C are schematic waveform diagrams of signals from a monitor diode, from an encoder and from a detector respectively occurring in the control circuit of figure 4
• Figure 6A is a schematic diagram of a control circuit according to an alternative method of controlling the bias and modulating currents of a laser diode
• Figure 6B is a schematic diagram of a control circuit according to another embodiment of the alternative method of controlling the bias and modulating currents of a laser diode including control of the duty cycle.
• a conventional circuit for driving and controlling a laser diode 1 transmitting light into an optical fiber 3 is shown, e.g. as used in a telecommunication system for transferring information.
• the laser diode 1 obtains its driving or biasing current lb from a first current source 5 or at least as controlled by said source.
• a modulating current I m obtained from or controlled by a second current source 7 that receives and is controlled by the data to be transmitted over the optical fiber 3 , i.e. the useful signal such as a serial digital signal or bit-stream, from some signal source, not shown.
• the biasing source 5 is controlled by a control signal obtained from an integrating amplifier or comparator 9 receiving on one of its input terminals a reference voltage V re fb and on its other input terminal a voltage obtained from the positive electrode of a photosensitive monitor diode 11 arranged to receive in some suitable way, e.g. from the rear facet of the laser diode 1, some light emitted therefrom.
• the negative electrode of the monitor diode 11 is connected to a constant positive supply voltage V cc and the positive electrode is connected to a constant negative supply voltage V ee through a resistor 13. Other polarities can be used.
• the voltage received by the integrator 9 represents the electrical current Id through the photodiode 11.
• the monitor diode 11 normally has a limited bandwidth and thus cannot sense the instantaneous optical power in the different levels of the light issued from the laser diode 1 but will sense an average of the power of the issued light.
• the substantially constant current lb provided by the biasing source 5 is controlled to adopt a value dependent on said control signal . That the biasing current is substantially constant means that it does not change appreciably during single periods of the input signal, i.e. during any time period having a length corresponding to e.g. the bit time of the input serial signal .
• a circuit diagram of the modulator and bias generator is shown in figure 3.
• the input serial data is here assumed to arrive at two different lines, a first line for a positive level and a second line for a negative level of the input signal .
• the first and second input lines are connected to the bases of npn- transistors in a differential amplifier comprised in a duty cycle controller circuit 14.
• the voltages on the collectors of the transistors are coupled to the bases of npn-transistors Ql, Q2 comprised in a limiting differential amplifier of the current modulator 7, one Q2 of the transistors having the laser diode 1 connected as the collector resistor thereof.
• the limiting amplifier can switch the modulation either through the laser diode 1 or through the collector resistor Rl of the other Ql of the two transistors.
• the peak value of the modulation current is set by a current mirror formed by two npn-transistors
• a modulation control voltage can be connected to the resistor R2 , a variation of the control voltage changing accordingly the peak value of the modulation current.
• the modulation control voltage can e.g. be connected to the positive supply voltage V cc and then the modulation current is constant.
• the bias generator 5 comprises an npn-transistor Q5 receiving on its base the bias control signal from the integrator 9.
• the duty cycle of the modulation is controlled by applying suitable voltages to the bases of two npn-transistors Q6, Q7 in the duty cycle controller 14, the collectors of these transistors being through collector resistors connected to the transistors of the limiting amplifier. The applied voltages will lower the voltages input to the limiting amplifier so that there will be an unbalance in the control thereof resulting in a change of the duty cycle of the output current through the laser diode 1.
• the bias current l will have a value differing more than acceptable from the intended value slightly above the actual threshold current It h of the laser diode.
• the electrical signal data are often processed in a parallel shape in low-speed electronic circuits, e.g. of type CMOS, to obtain a low power consumption.
• the input data arrive as electrical signals on parallel lines to an optical fiber end, the parallel signals being combined in a multiplexer to control a laser modulator injecting corresponding light signals into the optical fiber.
• the multiplexer and modulator are conventionally manufactured on SiGe or GaAs substrates and thus have a higher power consumption.
• the input data used for modulating the light issued from the laser diode 1 arrives in a parallel shape on a plurality of parallel lines 15, e.g. eight parallel lines as shown.
• the parallel lines are terminated in a parallel-to-serial converter or multiplexer 17 which converts the parallel bit-streams to a single or serial bit-stream used for controlling the modulator 7 and thereby the light issued by the laser diode 1.
• a band-pass filter 25 is connected to receive a signal representing the current I through the photodiode 11, e.g.
• the filtered-out frequency component is from the band-pass filter 25 provided to a detector 27, in which it is used together with known information arriving on an input line, such as a voltage V c , see Fig. 5B, from an encoder, not shown, indicating whether there is a "01" or "10" pattern, to detect the optical amplitude of the light issued by the laser diode 1.
• a control signal Vdet see Fig. 5C, is provided by the detector 27 to an input of an integrator - or possibly a comparator - 23, the integrator receiving on its other input a reference voltage V re f .
• the integrator 23 has its output connected to a control input terminal of the modulator 7, the output signal of the integrator thereby controlling the modulation current I m .
• the detector 27 can be a mixer, in which the filtered signal Vf representing the current Id through the monitor diode 11 is mixed with a 156.25 MHz signal from the encoder which is phase shifted 180° when a "10" pattern is sent and 0" when a "01" pattern is sent. Obviously the opposite phase shifts can instead be equally well used.
• This method can obviously be used for any signal that has a known frequency component .
• a detector 19 is connected to sense the input data, e.g. as illustrated it can be connected to the incoming parallel lines 15.
• the detector 19 gives an output signal when long sequences of only and/or mainly only "0"s or of only or mainly only "l”s are detected in the bit-stream arriving to modulate the laser diode. In the figure this is indicated as the detector 19 sensing when there are simultaneously all "0"s or "l”s in the parallel lines 15.
• the output signal of the detector 19 is provided as a control or clocking signal to a control circuit including a sample and hold circuit 21.
• the signal controls the sample and hold circuit which when receiving the signal samples the current Id in the line in which the monitor diode 11 is connected, i.e. actually gets a value representing the instantaneous power of the light emitted from the laser diode 1.
• the sampled current value is provided to one input of a first integrator or comparator 23, which on its other input receives a reference voltage V re fm.
• the minimum number of equal symbols to be detected after which the sample and hold circuit 21 is enabled by the signal from the detector 19, depends, in order to get an appropriate control of the laser diode 1, on the bandwidth of the monitoring diode 11, which as has already been mentioned can be considerably lower than that of the laser diode 1 and its driver circuits.
• the signal received by the sample and hold circuit 21 from the monitor diode 11, i.e. the detector diode current Id, should have reached an almost steady state before the value of the current through the detector diode 11 is sampled.
• the minimum number of detected symbols is also set depending on the bandwidth of the control circuit including the sample and hold circuit 21 and the first integrator 23.
• the control circuit comprising the sample and hold circuit 21 and the integrator 23 can be replaced by a circuit including an A/D-converter, a digital comparator and a D/A-converter, this case not being shown in the figure. Also, an envelope detector, not shown, can be used as the control circuit.
• the two signal patterns comprising mainly "0"s and “l”s can be detected and in addition thereto the mean power, e.g. as detected by a digital control circuit, not shown, or by the circuit illustrated in figure 6B.
• the value sampled for "0"s or the low electrical signal level or generally for the level having the smallest absolute value is then used for adjusting the bias current lb.
• the value sampled for "l"s or for the high electrical signal level is used for adjusting the levels or span of the modulation current I m and the determined mean power is used for adjusting the duty cycle of the modulation.
• the sample and hold circuit 21 receives a control or sampling signal from the detector 19 indicating a state of all "l"s, i.e. a state being a time period of substantially only high input signal levels.
• the signal sampled by the sample and hold circuit 21, thus representing the high signal level of the laser diode 1, is used for controlling the level of the modulating performed in the modulator 7.
• a second sample and hold circuit 21' is connected to sample the same voltage as the first sample and hold circuit and receives as sampling or clock signal a signal from the detector 19 indicating a state of all "0"s, i.e. a time period of substantially only low input signal levels.
• the output of the second sample and hold circuit 21 ' is connected to one input of a second integrator or possibly comparator 24, the second integrator receiving on its other input a reference voltage Vrefb-
• the output of the second integrator 24, thus representing the high signal level of the laser diode 1 is connected to the control input of the bias current source 5 thereby setting the bias current lb, instead of the integrator or comparator 9 used in the other embodiments .
• a third integrator 9 ' can be connected to receive input signals like the integrator 9 in the circuit of figure 6A, the third integrator receiving on one input a voltage representing the photocurrent I and on the other input a reference signal V re fdut-
• the output terminal of the third integrator 9 ' is connected to the modulator 7.
• the signal output from this integrator 9' represents an average of the photodiode current Id and is in the modulator 7 used for controlling the duty cycle of the modulated current.
• the integration time of the third integrator 9' has a value that is selected, considering the frequency of the input, modulating signal and the bandwidth of the monitor diode 11, to produce the desired signal representing the average photocurrent. If the bandwidth of the photodiode 11 is sufficiently small, the third integrator can be replaced by a comparator (not shown) .

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• Physics & Mathematics (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Optics & Photonics (AREA)
• Semiconductor Lasers (AREA)
• Optical Communication System (AREA)

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Cited By (3)

Citations (3)

• 2001
  • 2001-09-07 SE SE0102980A patent/SE523137C2/sv not_active IP Right Cessation
• 2002
  • 2002-09-06 WO PCT/SE2002/001603 patent/WO2003023917A1/en not_active Application Discontinuation

Patent Citations (3)

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