DE3153710C2 - Light output monitor from semiconductor laser - Google Patents

Abstract

Light or emission output control is from a semiconductor laser of the type used for telecommunications or similar equipment where a stable median value output is required. The semiconductor laser (1) may emit a forward (L) light beam for operational applications and a reverse beam (BB) which is used for control purposes. A photodetector (11) measures the output and feeds a signal into a comparator (14), a second input is fed from a reference source (13).The comparator output is fed to a timing circuit (16) which responds to start commands (T1) so that the control signal is fed through to the output device control (19) in the correct time slot. The time slot is one in which no transmission signal is passing through the system and control for it is generated by the oscillator (15), counter (16), D/A converter (17) and current amplifier (18) stages

Description

The invention relates to a radiation quantity control device according to the preamble of claim 1 as they is known from DE 28 13 513 A1.

DE 28 13 513 A1 describes a radiation quantity control direction known in which a part of the semiconductor laser emitted beam through a semi-transparent mirror is coupled out and directed onto a photodiode. Their Output signal is through a comparator with a reference signal compared, the comparator output signal di rectifies the modulation current driver stage, which is the half lead controls, acts. To rapid modulation-related Suppress fluctuations in the comparator output signal, is an additional between the photodiode and the comparator Detector connected which detects the average level of the photodiode Output signal forms and this signal instead of quickly fluctuating photodiode output signal to the comparator created. However, the average level of the photodiode Output signal directly from the information-dependent fluctuating Duty cycle of the modulation signal dependent, so that the modulation current driver stage controlled by the comparator fen-control from the actually desired setpoint more or  may differ less. In addition, the formation of the medium level of the photodiode output signal also to one greatly slowed response of the control loop, so that, for. B. short-term overloads of the semi-laser that last too long liable damage, not reliably excluded can be sen.

The control device known from DE 29 23 683 A1 works tet in a manner similar to that described above Control device, d. H. continuously monitors the light intensity of the laser using a photodiode, however an integrator is connected downstream. At the integrator input there is an additional control signal that depends on the duty cycle nis of the modulation signal is dependent.

Furthermore, a laser device is known from GB 1 540 064, which for material removal from workpieces, d. H. to surfaces profiling serves. With this device the laser beam focus on the workpiece to be machined using optics siert. Between the workpiece end area facing the op tic and at a short distance from this end area The grounded workpiece then ionizes the area between these two parts if the laser energy is sufficient for material removal. Around to capture this effect is between the end of the Optics and the workpiece a voltage that Ioni sation of the route in between a current flow and thus a voltage drop across a resistor calls. This voltage drop across the resistor is measured using a Schmitt triggers captured.

Finally, in the IBM publication (McMullin; IBM Tech nical Disclosure Bulletin, Vol 9. No. 11, April 1977, pp. 4474-4476) described a radiation power control device ben, at certain times, especially when lines return, a mirror in the optical path of the laser beam swung in and the latter thereby to a calibrated Sensor is redirected. The output signal of the calibrated Sensor is compared in a comparator with a reference level  same, with a counting process being started at the same time. The respective meter reading is done via a digital / analog conversion zer into a corresponding analog driver signal for the La water facility implemented, when a comparison occurs switchover signal of the count reached via an intermediate is stored caching member and its off output signal then to the digital / analog converter to follow Control of the laser device is applied. This Radiation setting can be successive for several side by side the arranged laser devices are performed.

In the known control devices is due to the rela tiv slow response a fast and highly accurate Regulation not possible or only with a high degree of construction to reach the wall.

The invention is therefore based on the object of a beam lungs crowd control device of the type mentioned Art to design that with an inexpensive and simple Chen structure a fast and precise regulation of the radiation is possible.

This object is achieved with the in the characterizing Part of claim 1 specified features solved.

In this way, the radiation quantity control device, the z. B. is suitable for recording devices, with several Light rays work and also photodetectors with ver have relatively low light response speed.

In the subclaims 2 to 4 are advantageous Ausgestal tion of the invention.

The invention is described below with reference to exemplary embodiments len explained with reference to the drawing.

Show it:

Fig. 1 is a schematic block diagram of a recording apparatus in which the control means applies,

Fig. 2 is a schematic block diagram of a first embodiment of the directional Steuerein,

Fig. 3 shows a modification of the gezeig th in Fig. 2 embodiment,

Fig. 4 is a schematic block diagram of a second embodiment of the Steuerein direction, which is suitable for successively controlling a plurality of beams with respect to the amount or intensity of the radiation, and

Fig. 5 is a schematic representation of recording surfaces on a photosensitive drum of a recording device.

Fig. 1 shows a radiation recording apparatus in which the control device is used. A semiconductor laser 1 emits a laser beam L, which is directed after passing through a collimator 2 on a rotatable polygonal mirror 3 . The polygonal mirror 3 passes in the direction of an arrow F with uniform speed to and forms a deflection device. The laser beam deflected by the polygonal mirror 3 is focused via a focusing lens 4 on a photosensitive drum 5 . The photosensitive drum 5 forms part of an electrostatic recording device. While the polygon mirror rotates in the above-mentioned manner, 3, a focused spot moves in the direction of an arrow P. may Therefore, with rotation of the drum 5 in a certain direction at a uniform VELOCITY ness all the surface of the drum 5 during the rotation of the polygon mirror 3 high speed can be scanned with the laser beam L.

6 with a radiation detector is designated, which is mounted outside of half the information recording surface of the drum 5 . The radiation detector 6 detects the arrival of the laser beam L in order to form a timing signal which is in turn applied to a processor or a central unit 7 . Under the timing controlled by the time control signal, the central unit applies a recording signal to the semiconductor laser 1 . The semiconductor laser 1 therefore emits a laser beam L modulated with the recording signal. This control process of the laser beam L is known, for example, from US Pat. No. 4,059,833 and is therefore not described further.

The photosensitive drum 5 , which is subjected to irradiation with a laser beam L modulated with the recording signal in the manner described above, was previously uniformly electrically charged over its entire surface by means of a charger (not shown). Therefore, when the modulated laser beam L is exposed to the drum surface in accordance with the irradiation on the previously charged drum 5, an electrostatic charge image is generated. The charge image is then visualized using a developing device (not shown), after which the developed image is transferred to an image-receiving material such as a sheet of paper. The image transferred to the image receiving sheet is fixed by means of a fixing device (not shown). In this way, a recording sheet having an image generated in accordance with the applied recording signal is obtained.

The semiconductor laser 1 emits laser beams not only to the front as the front beam L, but also to the rear as the rear beam BB. The rear beam BB is received by means of a photodetector 11 , which forms a detector signal corresponding to the intensity of the received beam. The detector signal is applied to a control circuit 12 which controls the intensity of the beams emitted by the semiconductor laser 1 .

In the first exemplary embodiment shown in FIG. 2, a bidirectional counter is used to shorten the time required for counting.

The control device for light intensity control according to the exemplary embodiment shown in FIG. 2 is used for a radiation recording device according to FIG. 1; the embodiment will be described below with reference to this beam recording apparatus in which a semiconductor laser 1-2 emits a single recording beam.

In Fig. 2, 11-2 denotes a photodetector that receives a rear beam bl from the laser 1-2 and outputs a detector output signal corresponding to the intensity of the received light. 30 is an upper limit reference signal generator that generates a reference signal indicating the upper limit of the detector signal. The detector output signal and the upper or upper limit reference signal are compared with one another in an upper limit comparator 31 . 32 is a lower limit reference signal generator that generates a reference signal indicative of the lower limit of the detector output signal. The detector output signal and the lower or lower limit reference signal are compared with one another in a lower limit comparator 33 . In this exemplary embodiment, the light intensity is not adjusted as long as the detector output signal remains within the range between the lower limit and the upper limit. The radiation intensity is only controlled when the detector output signal comes out of this area.

34 is a bidirectional counter. If the detector output signal is higher than the upper limit reference signal, a direction signal U is emitted by the upper limit comparator 31 for information about this. In this state, the bidirectional counter 34 is switched back by one level by a command signal applied to a signal line SL1. In contrast, when the detector output signal is lower than the lower limit reference signal, the lower limit comparator 33 outputs a direction signal L for information thereof. In this state, the bidirectional counter 34 is incremented by one level by a command signal applied to the signal line SL1.

At the semiconductor laser 1-2 , a recording signal is applied via a terminal T1 and a switching element 35 (which is a digital signal). When the recording signal is applied to the switching element 35 , this causes a current I to flow from a signal line SL2 to the semiconductor laser 1-2 in accordance with the recording signal. However, if the recording signal has a low level, the switching element 35 blocks any current flow to the semiconductor laser 1-2 . That is, the current I is supplied to the semiconductor laser 1-2 only when the recording signal is at its high level. The recording signal is also supplied to a pulse duration monitoring circuit 36 to thereby determine whether the duration of the high level of the recording signal is longer than a certain time period t or shorter. If the time period is longer than t, the monitoring circuit 36 outputs the above-mentioned command signal to the counter 34 .

The count of the counter 34 is converted by means of a digital-to-analog or D / A converter 37 into an analog signal which is amplified by means of a current amplifier 38 . The amplified signal is then entered as current I in the signal line SL2.

If a "carry" or a "borrow" signal is generated by the bidirectional counter 34 on a signal line SL3 in order to thereby activate a safety circuit 39 , a reset signal for resetting the bidirectional counter 34 is generated on a signal line SL4 as well also give a warning signal at port T2.

The light quantity control described above direction works as follows:

For explanation, it is assumed here that the device is in an operating state in which the counter 34 stores a value Na, the semiconductor laser 1-2 is operated with a current Ia corresponding to the count value Na, the amount of light LEa emitted by the laser above the upper one The limit of a preselected range lies and therefore the upper limit comparator 31 applies a direction signal U to the bidirectional counter 34 (of course no direction signal L is generated).

If a recording signal is applied at this position, the high level of which has a duration which is longer than the time duration t, the pulse duration monitoring circuit 36 emits a command signal by which the bidirectional counter 34 of the value Na has the value "1 "subtracts. As a result, the content of the counter is reduced to N (a-1). With this reduction in the count value, the analog value converted by means of the D / A converter 37 is correspondingly reduced and the current Ia is therefore reduced to I (a-1). As a result, the amount of light emitted from the semiconductor laser 1-2 is reduced to LE (a-1).

If, after the first reduction in the emitted light, the comparator 31 still emits its direction signal U, repeats the above-described control process again upon the arrival of a recording signal whose duration is at a high level longer than the period t. In this way, the count of the bi-directional counter 34 is further reduced to N (a-2) and therefore the current is reduced to I (a-2).

Assuming that the emitted light quantity LE (a-2) is just in the range between the upper limit level and the lower limit level, neither the direction signal U nor the direction signal L is generated. Therefore, even when a further command signal is applied to the bidirectional counter 34, the count remains unchanged. The bidirectional counter 34 retains this value or stores it.

If, in the opposite state to the above-described state at the count value Na, the emitted light quantity LEa is below the lower limit of the selected range, the lower limit comparator 33 outputs a direction signal L to the bidirectional counter 34 so that it outputs it by means of a command signal the monitoring circuit 36 adds the value "1" to the count value Na. Therefore, the count is increased to N (a + 1) and, accordingly, the current Ia to I (a + 1). As a result, the amount of light emitted from the semiconductor laser 1-2 is increased in accordance with the current I (a + 1).

As stated above , the amount of light emitted by the semiconductor laser 1-2 is greatly influenced by the ambient temperature. The amount of light decreases with increasing ambient temperature; if the temperature is very high, there may be a situation where the desired value of light intensity is not reached even after the current I is considerably increased. In this case, the device continues to increase the current I until the amount of light reaches the desired value. In this process of increasing the current I, however, there is a possibility that the semiconductor laser 1-2 may be damaged by the excessively increased current. In order to prevent such a risk of damage, the exemplary embodiment described is designed such that a transmission signal is generated when the bidirectional counter 34 reaches a certain predetermined count value N2. When this carry signal is emitted, the bidirectional counter 34 is reset to a value NR by means of the safety circuit 39 . In this way, damage to the semiconductor laser 1-2 can be prevented. At the same time, the safety circuit 39 emits a warning signal at the connection T2 in order to warn the operator of the danger and to cause them to take the necessary measures. Of course, the value NR is smaller than the value N2 and the current I corresponding to the value NR can operate the semiconductor laser 1-2 without damage or shortening the service life. For example, the minimum count of the bidirectional counter 34 can be selected as the value NR.

In contrast, if the ambient temperature is very low, there is a possibility that even if the count value Na is decreased over several steps, the amount of light cannot reach the desired value within the selected range. In this case, the bidirectional counter 34 emits a "borrow" signal after reaching the minimum count and then returns to its maximum count. To eliminate such a fault, the safety circuit 39 is also actuated in the exemplary embodiment described if a signal "borrow" is emitted by the bidirectional counter 34 . The safety circuit 39 resets the bidirectional counter 34 to the above-mentioned value NR and also emits a warning signal at the terminal T2.

In the embodiment described above, it is illustrated and described that the bidirectional counter 34 is only operated when the photodetector output signal is not in the range between the preselected upper and the preselected lower limit, four circuits being used, namely the reference signal Generators and the comparators for the upper and lower limits.

Fig. 3 shows a modification of the embodiment described above, shown in Fig. 2. In the modification shown in Fig. 3, the above four circuits 30 to 33 are omitted and a reference signal generator 40 and a comparator 41 are used instead. The comparator 41 is switched so that it compares the detector signal with the reference signal from the generator 40 . If the detector signal is higher than the reference signal, the comparator 41 outputs a signal "over" to a signal line SL5; on the other hand, if the former signal is lower than the latter, the comparator 41 outputs a signal "under" to a signal line SL6. While the "About" signal is present on the signal line SL5, the application of the above-mentioned command signal causes the two-way counter 34 to be switched back by one stage. In contrast, when the "under" signal is present, the application of this command signal causes the bidirectional counter 34 to be held up by one step.

The control device shown in FIG. 3 is composed of fewer circuit components and can be used in particular to control an amount of light whose change range is relatively narrow.

In the embodiment described above The amount of light is controlled during a game Recording process when a recording arrives voltage signal with a duration longer than the previous agreed period of time t is. However, the control can also during the time when no recording is taking place, be carried out in such a way that at terminal T1 a signal is applied that the aforementioned record voltage signal with the duration over the duration t is equivalent.

Fig. 4 shows a further embodiment of the control device.

1-1 with a semiconductor laser array or series is referred to, the plurality of front beams L1 to Ln and rear beams BL1 to Bln emits. The laser beams BL1 to BLn fall on a photo receiver or photodetector 11-3 . The upper limit for the intensity of the laser beams BL1-BLn can be set as desired using an upper limit reference level setting circuit 30-1 , while the lower limit can be set arbitrarily using a lower limit reference level setting circuit 32-1 . From any selected backward laser beam BL1 to BLn (which is selected by means of a selector circuit 44 , as will be described later), which is detected by the photodetector 11-3 , a signal is obtained about the intensity of the detected beam, which is applied to an upper limit comparator 31-1 is created.

The upper limit comparator 31-1 compares the intensity signal with the upper limit reference signal from the upper limit reference level setting circuit 30-1 . If the former signal is higher than the latter, the comparator 31-1 outputs a digital signal "1" to a signal line SL1. If the former signal is lower than the latter, the comparator 31-1 outputs a signal "0" to the signal line.

Similarly, the signal on the intensity of the selected beam BL is applied to a lower limit comparator 33-1 to be compared with the lower limit reference signal from the lower limit reference level setting circuit 32-1 . If the determined beam intensity is lower than the lower reference value, the lower limit comparator 33-1 outputs a digital signal "1" to a signal line SL2. When the intensity is higher than the reference value, the lower limit comparator 33-1 outputs a signal "0" to the signal line.

These appear on the signal lines SL1 and SL2, the signals are input to a counter control signal shaping circuit 43 . This signal shaping circuit 43 forms a subtraction signal when the signals on lines SL1 and SL2 are "1" and "0", an addition signal when the signals are "0" and "1", a blocking signal when the signals are both " 0 "and an error signal if the signals are both" 1 ".

This addition signal, subtraction signal or blocking signal is applied via the selection circuit 44 to a selected one of bidirectional counters 34-1 to 34- n. The selector circuit 44 operates in response to a timing signal applied to a signal line SL4 and selects in sequence one of signal lines SWL1 to SWLn and a switching element 35-1 to 35- n corresponding to the selected signal line SWL.

37-1 to 37 -n are D / A converters for converting a value counted by means of the bidirectional counter 34 into a corresponding analog signal. The analog signal is applied to an assigned current amplifier 38-1 to 38- n in order to convert it into a corresponding value of a current. The current is input to the respective switching element 35-1 to 35- n for driving the semiconductor laser 1-1 .

Below is the operation of the above described described facility.

In the normal state of the recording, the control circuit or control device shown in FIG. 4 has at its switching elements 35-1 to 35- n currents available with values which are determined by the current amplifiers 38-1 to 38- n; the switching on and off or by switching or blocking the switching elements is controlled by digital levels "0" or "1" of input signals S1 to Sn on signal lines iL1 to iLn. By switching the switching elements 35-1 to 35- n, signal lines ML1 to MLn are supplied with currents which are determined by the current amplifiers 38-1 to 38- n. In this way, the laser elements LD1 to LDn are operated in accordance with the strengths of the currents supplied in each case. The amount of light is controlled according to the procedure explained below during the period during which no data is recorded with the semiconductor laser arrangement 1-1 .

First, a timing signal is applied to the signal line SL4 so that the input signal S1 assumes its high level "1" with the aid of the selector circuit 44 . As a result, the switching element 35-1 is switched through while the other switching elements remain blocked. This also selects the signal line SWL1.

Therefore, a current corresponding to the count value of the bidirectional counter 34-1 is now input to the signal line ML1 in order to selectively operate only the laser element LD1 which emits the laser beams L1 and BL1. The laser beam BL1 is received by the photodetector 11-3 , which converts the intensity of the received light into an electrical signal (detector signal). The detector signal is compared in the comparators 31-1 and 33-1 with the upper limit reference signal and the lower limit reference signal. When the detector signal is higher than the upper limit reference signal, a subtraction signal for subtracting from the content of the bidirectional counter 34-1 is output. In contrast, if the detector signal is lower than the lower limit reference signal, an addition signal is used to add the content of the bidirectional counter 34-1 . In this way, the current supplied to the signal line ML1 is more or less reduced or increased. When the level of the detector signal falls in the range between the upper reference level and the lower reference level, an inhibit signal is issued so that the bidirectional counter 34-1 maintains its content.

After completion of the control of the intensity of the laser beams L1 and BL1 in the manner described above, a second timing control signal is applied to the signal line SL4 to bring the input signal 52 to the high level "1", so that the second switching element 35-2 is switched through while the other switching elements are blocked. At the same time, the signal line SWL2 is selected. The intensity of the laser beams L2 and BL2 is thus controlled in the manner described above. After completion of the control of the intensity of the laser beams L2 and BL2, a third timing signal is output to control the next laser beams L3 and BL3.

The control process explained above is repeated up to the last laser beams Ln and BLn. After completion of the control of the intensity of the last laser beams Ln and BLn, a final timing signal is applied, with which the selection of the switching elements 35 and the signal lines SWL, which is followed by the selector circuit 44 , is canceled. Therefore, the recording device is ready for a new initiation of data recording by applying input signals S1 to Sn to the signal lines iL1 to iLn.

Fig. 5 shows schematically and over time a recording paper sheet on which data is recorded by means of a radiation recording apparatus as described above.

The data is recorded on P1 and P2 recorded, of which P1 an area unit of a Page and P2 an area unit of the next page represents. As suggested by A and B, that can advance radiation intensity control described below Use of the period during which the laser beam L is the area outside the recording area P1 sweeps. In this case it can go ahead outgoing time control signal described with a return or frequency of one signal per scan using the Laser beam L are emitted.

The control of the radiation intensity can also done using an idle time C that the Time between the end of the recording of a page on the recording surface P1 and the beginning of the opening drawing of the next page on the recording surface P2 is.

Claims (4)

1. Radiation amount control device with

• - A semiconductor laser device ( 1-2 ; 1-1 ) for generating at least one laser beam modulated by a recording signal (iL1-iLn), the semiconductor laser device ( 1-2 ; 1-1 ) having at least one front laser beam (l, L1- Ln) and emits at least one rear laser beam (bl; BL1-BLn),
• a detector device ( 11-2 ; 11-3 ) for detecting one of the laser beams (bl; BL1-BLn) generated by the semiconductor laser device ( 1-2 ; 1-1 ),
• - a reference signal generator device ( 30 , 32 ; 30-1 , 32-1 ) for generating at least one reference signal,
• - A comparator device ( 31 , 33 ; 31-1 , 33-1 ) for comparing the detection output signal of the detector device ( 11-2 ; 11-3 ) with the at least one reference signal; wherein the at least one reference signal generated by the reference signal generator device ( 30 , 32 ; 30-1 , 32-1 ) represents an upper and a lower limit or a level and wherein the comparator device ( 31 , 33 ; 31-1 , 33-1 ) compares whether the detection output signal of the detector device ( 11-2 ; 11-3 ) is in the range between the upper and lower limits or not or whether it has reached the predetermined level or not, characterized in that
• - At least one bidirectional counter ( 34 , 34-1 to 34- n) is provided for incrementing or decrementing its count depending on the comparison result of the comparator device ( 31 , 33 ; 31-1 , 33-1 ),
• at least one digital / analog converter device ( 37 ; 37-1 to 37- n) for converting the count of the at least one bidirectional counter ( 34 ; 34-1 to 34 n) into at least one analog signal is present,
• - At least one device ( 35 , 38 ; 35-1 to 35- n, 38-1 to 38- n) for supplying at least one current to the semiconductor laser device ( 1-2 ; 1-1 ) as a function of that through the at least one digital / Analog conversion device ( 37 ; 37-1 to 37- n) provided at least one analog signal is provided,
• - The at least one bidirectional counter ( 34 ; 34-1 to 34 n) was the count for reducing the via a digital / analog conversion device ( 37 ; 37-1 to 37- n) to the semiconductor laser device ( 1-2 ; 1- 1 ) decrements applied current when the comparison result of the comparator ( 31 , 33 ; 31-1-33-1 ) indicates that the detection output of the detector means ( 11-2 , 11-3 ) is higher than the reference signal upper limit or than the predetermined level, and
• - The at least one bidirectional counter ( 34 ; 34-1 to 34- n) was the counter for increasing the via a digital / analog conversion device ( 37 ; 37-1 to 37- n) to the semiconductor laser device ( 1-2 ; 1-1 ) applied current is incremented when the comparison result of the comparator means ( 31 , 33 ; 31-1 , 33-1 ) indicates that the detection output of the detector means ( 11-2 ; 11-3 ) is lower than the reference signal lower limit or as the predetermined level, so that the detection output of the detector means ( 11-2 ; 11-3 ) is brought to the predetermined level in the range between the reference signal upper limit and the reference signal lower limit.

2. Radiation amount control device according to claim 1, characterized in that a time duration detector device ( 36 ) is present, via which it can be detected that the duty cycle of the recording signal is above a predetermined time interval, and that the bidirectional counter ( 34 ) is an incremental or decrementing counting process when the detection output signal of the time duration detector device ( 36 ) occurs. 3. Radiation quantity control device according to claim 1 or 2, characterized by a device ( 39 ) for generating a warning signal as a function of the count of the bidirectional counter ( 34 ). 4. Radiation amount control device according to one of the preceding claims, characterized in that by the modulated by the recording signal laser beam (l, L1-Ln) an image is recorded on a recording material ( 5 ) and the radiation quantity control takes place during an interval in which no image is recorded on the recording material ( 5 ).