Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/50—Transmitters
  • H04B10/564—Power control
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/50—Transmitters
  • H04B10/501—Structural aspects
  • H04B10/503—Laser transmitters
  • H04B10/504—Laser transmitters using direct modulation
• • 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
  • H01S5/06832—Stabilising during amplitude modulation

Definitions

• the invention relates to a method of controlling the optical output level of a laser diode transmitter, comprising detecting a portion of an optical output signal of the laser diode transmitter, generating a monitor signal representing said optical output signal and controlling the mean level of the optical output of the laser diode transmitter on the basis of said monitor signal and a data signal to be transmitted.
• the inven ⁇ tion relates further to a control circuit embodying the method.
• a typical problem encountered in laser diodes is that the optical output power of the laser diode varies with temperature and time. More specifically, the effect of aging and increasing temperature on the laser diode is to increase the threshold current and decrease the slope efficiency thereof. Conventionally, this problem has been solved to a certain extent by the use of a Peltier cooler to eliminate temperature changes and a mean level feedback loop to compensate for the aging.
• the object of the invention is to obtain a new method for controlling the optical output level of a laser diode transmitter, to be used in a telecommunica ⁇ tions network where a lifetime of 20 years is expected.
• Extinction ratio is defined by the ratio of the optical "1" level to the "0" level.
• the "0" level is very small in comparison to the "1" level. Therefore, even a slight disturbance or offset in the control loop can cause a considerable change in the "0" level and consequently in the extinction ratio.
• the control loop must be insensitive to temperature changes, power supply changes, input signal changes and component tolerances. This means that when for example the temperature changes, the loop should act to compensate for the va ⁇ riation in laser efficiency, and not for the offsets and drift in the loop itself.
• control signal for the control loop is derived from a balanced modulator or mixer.
• the output of the balanced modulator or mixer depends directly upon the amplitude of the input sig ⁇ nals.
• data signal itself or the data signal after some logical operation is used as the re ⁇ ference signal.
• the data signal is said to be a digital signal, hence derived from a logic gate. It is well known that the output levels from any logic gate vary in amplitude with tem-perature changes. Hence the re- ference signal to the balanced modulator or mixer will change with tempera- ture. This will cause an erronious change in the output of the multiplier which in turn affects the extinction ratio.
• the balanced modulator or mixer itself is composed of transistors or diodes whose base-emitter voltages changes with temperature. Again this causes the output to be unnecessarily adjusted.
• control signal is derived from a com ⁇ bination of a positive peak detected signal and the average signal level. Again internal temperature compen ⁇ sation is not included and no provision is made for the signal duty cycle. In fact if transmission is interrup ⁇ ted during operation, the loop will act to increase the modulation current infinitely, and so damage or destroy the laser.
• the objects of the present invention are achieved by a method according to the invention, in which the peak level of the optical output of the laser diode transmitter is controlled on the basis of the difference between the peak levels of said monitor signal and the data signal to be transmitted.
• the method according to the invention is based on the principle that the monitor signal and the data sig ⁇ nal are generated by the same mechanism. Hence any changes which occur in the system will appear symmet- rically in both signals so that the output remains the same. Due to this principle, the circuit is internally temperature compensated and immune to power supply and input signal variations.
• both the peak level and the mean level of the optical output of the laser diode are si ⁇ multaneously monitored and controlled by using the data signal to be transmitted as a reference signal. Due to the use of the data signal as a reference signal, even during a long sequence of data bits without transitions, the operation will not be disrupted or the laser power falsely adjusted.
• the invention relates further to a control circuit for controlling an optical output level of a laser diode transmitter, comprising a monitoring photo diode means for detecting a portion of an optical output signal of the laser diode transmitter, a preamplifier generating a monitor signal representing said optical output sig ⁇ nal, and first control means for controlling the mean level of the output of the laser diode transmitter on the basis of said monitor signal and the data signal to be transmitted.
• the control circuit comprises peak level detector means for de ⁇ tecting the peak level of the monitor signal and the peak level of a data signal to be transmitted; and sec ⁇ ond control means for controlling the peak level of the optical output of the laser diode transmitter on the basis of a difference between said peak levels detected by the peak level detector means.
• Figure 1 is a block diagram of a control circuit according to the invention.
• Slope efficiency refers to the ability of a laser to convert electrical current to light, i.e. a laser with good efficiency will need little current variation to produce a large deviation in the output light level, whereas an inefficient laser will require more current variation for the same change in light output.
• Slope variations refers to changes in the effi ⁇ ciency on the laser over time and temperature. Typical ⁇ ly, as the laser ages and the operating temperature rises, the efficiency decreases, hence requiring increased drive current.
• Figure 1 shows an optical transmitter comprising a laser diode LD, a laser diode driver 1, a modulation current source 2 and a bias current source 3.
• the laser diode LD is forward biased by a bias current I Bia ⁇ l from the bias current source 3 in a conventional manner.
• the modulation current source 2 generates a modulation cur ⁇ rent I Hod on the basis of an incoming data signal in ⁇ putted to the source 2.
• the modulation current l Hod is fed to the laser diode driver 1 to modulate the laser diode LD so that an optical signal which corresponds to the incoming data is transmitted by the laser diode LD to an optical transmission medium, such as an optical fiber 11.
• a portion of the optical output power of the laser diode LD is led to and detected by a monitoring photo- diode PD in order to monitor the output level of the laser diode LD.
• the photodiode PD is connected 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 realized for example as a wideband transimpedance preamplifier so that the amplitude of its output voltage is independent of temperature changes.
• the output signal 4a is applied to a peak level detector 6, which determines the peak value of the signal 4a and outputs the result as an output signal 6a.
• Incoming data which is to be transmitted, is applied to a limiting amplifier 12 which amplifies and limits the data signal so that the 1-level of its output re ⁇ mains constant at all temperatures.
• the preamplifier 4 and the limiting amplifier 12 have both the same supply voltage V cc , and so variations in V cc , caused by e.g. temperature changes, affect both signals in the same way.
• the signal from the limiting amplifier 12 is ap ⁇ plied to another peak level detector 7, which determines the peak value of the incoming data and outputs the result as an output signal 7a.
• the time constants of the peak level detectors 6 and 7 should be the same.
• the peak level detectors are preferably realized by using transistors or other circuit means integrated on the same chip so that they are constantly at the same tem ⁇ perature with respect to each other, and no error caused by a temperature difference occurs at their outputs as compared with each other.
• the output signals from the detectors 6 and 7 are inputted to a comparator means 9, such as a differential amplifier, which forms a single- ended difference signal 9a, representing the difference between the signals 6a and 7a.
• the peak level detectors 6 and 7 have both the same reference voltage V ⁇ so that variations in V ⁇ , caused by e.g. temperature changes, affect the output signals of both said detectors in the same manner.
• the difference signal 9a is applied to the modulation current source 2 as a negative feedback signal to maintain zero error signal within the closed loop by adjusting the modulation current I ⁇ and thereby the peak level of the optical output signal of the laser diode LD.
• the output signal 4a is also applied to a mean level detector 5, which determines the mean level of the signal 4a and outputs the result as an output signal 5a.
• Incoming data which is to be transmitted, is applied to another mean level detector 8, which determines the mean level of the incoming data and outputs the result as an output signal 8a.
• the time constants of the mean level detectors 5 and 8 should be the same.
• the output signals from the detectors 5 and 8 are inputted to a comparator means 10, such as a differential amplifier, which forms a single-ended difference signal 10a repre ⁇ senting the difference between the signals 5a and 8a.
• the differential amplifier 10 is preferably an integrat- ing amplifier in order that the dependence on the phase of the detector output signals 5a and 8a should be eliminated.
• the difference signal 10a is applied to the bias current source 3 as a negative feedback signal to maintain zero error signal within the closed loop by ad- justing the bias current I Bia8 and thereby the mean level of the optical output signal of the laser diode
• the bias control loop should preferably be faster than the peak control loop.

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• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Optics & Photonics (AREA)
• Semiconductor Lasers (AREA)
• Optical Communication System (AREA)

Priority Applications (4)

Applications Claiming Priority (2)

Publications (1)

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Family Applications (1)

Country Status (4)

Cited By (2)

Citations (2)

• 1991
  • 1991-03-21 FI FI911382A patent/FI911382A0/fi unknown
• 1992
  • 1992-03-20 GB GB9319225A patent/GB2269932B/en not_active Expired - Fee Related
  • 1992-03-20 WO PCT/FI1992/000079 patent/WO1992017007A1/en active IP Right Grant
• 1993
  • 1993-09-21 SE SE9303077A patent/SE516099C2/sv not_active IP Right Cessation

Patent Citations (2)

Non-Patent Citations (1)

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