CONTROL CIRCUIT FOR A RADIATION SOURCE AND METHOD FOR CONTROLLING A RADIATION SOURCE

Abstract: The invention pertains to a control circuit (4) for a radiation source (2). The circuit comprises means (43) for generating an error signal (Perr) which is indicative for a difference between a measured average value of the output power of the radiation source (2) and a desired average value (Pset) of the output power of the radiation source. The circuit further comprises combining means (44, 45, 46) for generating a control signal (Sout) for the radiation source (2) in response to said error signal (Perr) and to an information signal (Sin) for modulating the radiation source. The combining means comprise first means (44) for modifying the information signal by a multiplicative factor ( which is dependent on the error signal, and second means (45, 46) for modifying the information signal by an additive factor sigma which is dependent on the error signal.

Description: CONTROL CIRCUIT FOR A RADIATION SOURCE AND METHOD FOR CONTROLLING A RADIATION SOURCE The invention pertains to a control circuit for a radiation source compπsing means for generating an error signal which is indicative for a difference between a measured average value of the output power of the radiation source and a desired average value of the output power of the radiation source, combining means for generating a control signal for the radiation source in response to said error signal and to an information signal for modulating the radiation source The invention also pertains to an apparatus for reading and/or wπting a record earner compnsmg the control circuit. The invention further pertains to an optical transmitter compnsmg the control circuit. The invention further pertains to a method for controlling a radiation source From US 4 796 267 a laser controller is known which compnses a negatne feedback loop to control an average light quantity level despite vanations in temperature The feedback loop compnses a sensor for generating an output signal which is a measure for the power of radiation of the radiation source, a low-pass filter for low pass filtenng the output signal. The feedback loop further compnses a subtractor for generating a difference signal which is a difference between said low-pass filtered signal and a setpoint signal representing the desired average light quantity level. The known circuit maintains the average light quantity level when the temperature changes. However if the temperature changes also the proportionality factor between the control current and the light quantity level of the semiconductor radiation source changes This implies that the difference between the output levels of the radiation source changes with a changing temperature. Also control circuits are known which include feedback means which monitor the respective power of the radiation for each modulation level and adapt the control current accordingly This has the disadvantage that the feedback means require a large bandwidth It is a purpose of the invention to provide a control circuit for a radiation source \\ hich is capable of maintaining a plurality of intensity levels at a predetermined value, using a feedback loop with a relatively low bandwidth It is a further purpose to provide an apparatus for reading and/or wntmg a record earner compnsing such a control circuit, and an optical transmitter compnsing such a control circuit It is also a purpose to provide a method for controlling a radiation source having this capability According to the invention the control means compnse first means for modifying the information signal by a multiplicative factor γ which is dependent on the enor signal, and - second means for modifying the information signal by an additive factor σ which is dependent on the enor signal The output power of a laser diode is dependent on temperature (T) The output power (P) may be approximated by P = 0 for I < I t (la) P = η(T)* (I - I t (T) ) for I >= I, (lb) wherein I t is a threshold current required for activating the laserdiode and ηis a slope (relative efficiency) Both the threshold current I, and the slope η are dependent on the temperature In the control circuit of the invention the information signal is modified by a multiplicative factor y and an additive factor σ and both factors are dependent on the error signal This enables the control circuit to perform each of the different functions such as reading wnting, erasing and preheating at the appropnate pow er levels despite v anations in temperature It has been found that for most semiconductor light sources the following relationship exists between the threshold cunent and the slope ^- = (l - α) + α-^- (2) /,„„ Therein η, η υ are the slopes at a temperature and a reference temperature respectively, while I th , I th o are the threshold currents at those temperatures and a is a dimensionless constant which is charactenstic tor each semiconductor light source The value of this constant usually is within the range of 0 5 to 1 5 Hence in order to compensate at the same time for both the influence of the temperature on the slope and on the required threshold current, in a practical embodiment the multiplicative factor γ and an additive factor σ are related according to the function γ = kσ + b , (3) w herein k = a^- ∑md b = γ 0 (4) Therein γo and σo respectively are the values of γ and σ at a reference temperature. A control circuit according to the invention can be implemented in different ways. In a first prefened embodiment the additive factor σ is linearly dependent on the enor signal P err , while the multiplicative factor γ is computed from σ according to the relation 3 above. Such an embodiment of a control circuit according to the invention is charactenzed in that the first means compnse modification means which calculate a modified error signal from the enor signal and multiplication means for generating an intermediate signal which is indicative for the product of the modified enor signal and the information signal and in that the second means compnse addition means for generating an output signal w hich is indicative for the sum of the intermediate signal and the enor signal, the output signal being the control signal. A vanant of this preferred embodiment is charactenzed in that the first means compnse modification means which calculate a modified enor signal from the enor signal and multiplication means for generating an intermediate signal which is indicative for the product of the modified enor signal and the information signal and in that the second means compnse further multiplication means for generating a further intermediate signal which is indicative for a product of the enor signal and a further information signal and addition means for generating an output signal which is indicative for the sum of the intermediate signal and the further intermediate signal, the output signal being the control signal This embodiment provides for an additional degree of freedom for controlling the radiation source. Likewise a second preferred embodiment is possible in which the multiplicative factor γ is linearly dependent on the enor signal. In that case the additive factor σ can be computed from the multiplicative factor γ by the inverse of relation 3 Hence Such a second prefened control circuit according to the invention is therefore charactenzed in that the first means compnse multiplication means for generating an intermediate signal which is indicativ e tor the product ot the enor signal and the information signal and in that the second means compnse modification means which calculate a modified enor signal from the enor signal and addition means for generating an output signal which is indicative for the sum of the intermediate signal and the modified error signal, the output signal being the control signal Vanous ways of feedback control are possible such that the multiplicative and the additive component comply with the mutual relation descnbed above However, in the first and the second preferred embodiment descnbed above one of these two components is directly dependent on the error signal, so that the control means can be implemented in a relativ ely simple way, e g with a relatively small amount of hardware A prefened embodiment of the first embodiment of the control circuit according to the invention is further charactenzed by a node for receiving a signal which is indicative for a measured reflection coefficient of an object which is radiated by the radiation source, by subtraction means for generating a difference signal which is indicative for a difference between a desired reflection coefficient and the measured reflection coefficient, by multiplication means for generating an intermediate signal which is indicative for a product of the difference signal and a signal indicative for a desired power, and addition means for generating a signal which is indicative for the sum of the intermediate signal and a further signal indicative for a desired power In practice dust, fingerpnnts etc. may absorb a part of the radiation from the radiation source, and therewith influence its effect on the record earner In this prefened embodiment these influences are compensated for, while at the same time still a good compensation for temperature influences is obtained Preferably this embodiment is charactenzed by multiplication means for generating a product signal which is indicative of a product of the difference signal and the enor signal and means for generating an output signal which is indicative for the sum of the product signal and the difference signal In this way a short response time is obtained The controller according to the invention is in particular suitable for application in a recording/playback device using a radiation source In accordance with the invention therefore a device for recording and/or playback of information on a record earner includes a controller according to the invention, - a radiation source which is controllable by the controller, a sensor for generating a signal which is indicative of an intensity of radiation emitted by the radiation source and which is coupled to an input of the controller, means for mapping the radiation beam at a spot at the information earner, means for causing a relative displacement between the spot and the record earner The control circuit according to the invention is also very suitable for application in an optical transmitter An optical transmitter according to the invention therefore compnses: input means for receiving an input signal, a control circuit according to one of the claims 1 to 7 coupled to said input means, a radiation source controlled by the control circuit - measunng means for measunng an amount of radiation generated by the radiation source having an output coupled to the control circuit, means to couple the output of the radiation source into a light conductor The invention also pertains to a method for controlling a radiation source compnsing - determining an enor signal which is indicative for a difference between a measured average value of the output power of the radiation source and a desired average value of the output power of the radiation source, generating a control signal for the radiation source by combining said enor signal and an information signal representative of information to be wntten to the radiation source, charactenzed in that, the step of generating the control signal compnses modifying the information signal by a multiplicative factor γ which is dependent on the error signal, and modifying the information signal by an additive factor σ which is dependent on the enor signal. These and other aspects of the invention are descnbed in more detail with reference to the figures Therein Figure 1 schematically shows a first embodiment of an apparatus according to the invention for reading and/or wnting an optical or opto-magnetic record earner, Figure 2 shows a portion of the apparatus of Figure 1 in more detail, Figure 3 schematically shows a second embodiment of an apparatus according to the invention, Figure 4 shows a portion of the apparatus of Figure 3 in more detail, Figure 5 shows an embodiment of a control circuit according to the invention, Figure 6 shows a further embodiment of a control circuit according the invention. Figure 7 shows an optical transmitter according to the invention Figure 8 schematically shows a method for controlling a radiation source Figure 1 schematically shows an embodiment of an apparatus for reading and/or wnting an optical or opto-magnetic information earner 1. In the embodiment shown the apparatus is both suitable for reading and wnting. The apparatus compnses a radiation source 2, for generating radiation The apparatus also compnses an optical s stem for mapping the radiation at a scanning spot 3 at an information earner and displacing means for displacing the record earner 1 and the scanning spot 3 with respect to each other. As the optical system and the displacing means are not relevant to the present invention they are not shown, nor descnbed here in detail. The optical system could be of any type known to the person skilled in the art of optical recording and compnse for example one or more optical components such as lenses, minors, gnds, and holograms. Likewise the displacing means may be implemented in different ways known to the skilled person. For example an apparatus for a disc shaped information earner may compnse a spindle motor for rotating the record earner, and displacing means for displacing the radiation spot. The latter may compnse a single stage, in the form of a sledge dnven by a linear motor or a rotatable arm dnven by a voice coil motor. Preferably the radiation spot displacing means compnse another stage for performing small movements, for example in the form of an actuator for controlling a lens in the optical system. In the case of an optical card, the apparatus may compnse displacing means for longitudinally displacing the card, for example a linear motor or a rotating motor in combination with a transmission system to convert the rotating movement in a linear movement. In a further embodiment the information earner is an optical tape In that embodiment the apparatus preferably has a first displacing means for displacing the tape in its longitudinal direction and second displacing means for moving the scanning spot in a direction transverse thereto. The apparatus shown is provided with a control circuit 4 for the radiation source 3 which compnses means 41. 42, 43 for generating an enor signal P erτ which is indicative for a difference between a measured average value P mcas of the output power of the radiation source and a desired average value P set of the output power of the radiation source 3. In the embodiment shown said means compnse input means 41, e.g an input terminal or a connection for receiving a signal P meas from a light sensitive diode 5, which signal P meas is indicative for a measured output power of the radiation source 2. The means for generating the enor signal also compnse setting means 42 for setting a desired average value P set of the output power of the radiation source, and combination means 43, in this case a subtractor for subtracting the signal P meas from the signal P set . The light sensitive diode 5 receives a portion of the radiation of the radiation source 2, for example via a partly reflecting mirror in the optical path from the light source 2 to the information earner 1. In the embodiment shown the light sensitive diode 5 has a relatively small bandwidth, so that it is not necessary to use a separate low pass filter to filter the output signal of the light sensitive diode 5 The apparatus shown is further provided by detection means 6, for detecting radiation which is reflected by the record earner 1. The detection means 6 may be used for generating a read signal which is representative of information stored at the record earner 1. The detection means 6 may compnse a plurality of detectors, which enables a signal processing system known as such to generate a radial error and a focus enor signal. The control circuit further compnses combining means 44, 45, 46 for generating a control signal S out for the radiation source 2 in response to the enor signal P er r and to an information signal S ιn representative of information in accordance with which the radiation source 2 is to be modulated. The combining means compnse first means 44 for modifying the information signal S,„ by a multiplicative factor γ which is dependent on the enor signal P err The information signal S ιn may be further modified by a wnte strategy puls generator 47 which generates one or more pulses for each information symbol in the information signal. The combining means further compnse second means 45 for modifying the information signal S m by an additive factor σ which is dependent on the enor signal P err . In the embodiment shown the setting means 42 determine the desired average value P se t of the power from the information signal S, n If the desired average value P set of the power is constant, however, the setting means 42 may be in the form of a register containing said constant value The multiplicative factor γ and the additive factoi σ are related according to the function γ - kσ + b In the embodiment shown the first means 44 compnse a multiplier for generating an intermediate signal γS m which is indicative for the product of the enor signal P en - and the information signal S ιn and in that the second means 45. 46 compnse modification means 46 which calculate a modified error signal σ from the enor signal P err and addition means 45 for generating an output signal S out which is indicative for the sum of the intermediate signal γS m and the modified enor signal σ. The output signal is the control signal S out for the radiation source The modification means 46 are shown in more detail in Figure 2. The modification means compnse a multiplier 461, an adder 462, a first register 463 and a second register 464.The first and the second register respectively contain the values: 1 σ 0 , a - 1 and σ 0 a γ 0 a Hence the output signal σ of the modification means 46 complies with 1 σ 0 a - 1 . ,. σ = °-γ + σ ϋ (6) a γ 0 a Figure 3 shows a second embodiment of a control circuit 7 according to the invention. In Figure 3 parts having the same reference numerals as in Figure 1 conespond to each other. Furthermore, the parts 71, 72, 73 conespond to parts 41, 42, 43 in Figure 1. The optional part 77 conesponds to the optional part 47. In the embodiment shown in Figure 3 the first means 74, 76 compnse modification means 76 which calculate a modified enor signal γ from the enor signal P eπ . The first means further compnse multiplication means 74 for generating an intermediate signal γS ιn which is indicative for the product of the modified enor signal γ and the information signal S ιn . The second means 75 compnse an adder for generating an output signal S out which is indicative for the sum of the intermediate signal γS ιn and the enor signal P eπ , the output signal S out being the control signal. The modification means 76 are shown in more detail in Figure 4. The modification means 76 compnse a first and a second multiplier 761, 762, an adder 763, and a first and a second register 764, 765. The first register 764 contains the value — and the second register contains the value γ 0 . The modified enor signal γ complies with = 0 + — σ (7) A modification of the embodiment of Figure 3 is shown in Figure 5. Mutuallv conesponding parts in those Figures have the same reference numerals In Figure 5 the second means 75, 78 compnse further multiplication means 78 for generating a further intermediate signal σS, n which is indicative for a product of the enor signal P erτ and a further information signal. Another modification of that embodiment of the control circuit 7 according to the invention is shown in Figure 6. Said modification is charactenzed by a node 83 for receiving a signal R meas which is indicative for a measured reflection coefficient of an object which is radiated by the radiation source 2 The modified embodiment further compnses subtraction means 80 for generating a difference signal R , ff which is indicative for a difference between a desired reflection coefficient R set and the measured reflection coefficient R meas - The control circuit 7 has multiplication means 81 for generating an intermediate signal which is indicative for a product of the difference signal R dlf , and a signal indicative for a desired power P set ι. The control circuit also has addition means 82 for generating a signal P set which is indicative for the sum of the intermediate signal and a further signal P set2 indicative for a desired power The signal Pset2 is a measure for the minimum supply cunent required to activate the radiation source. The signal Psetl is a measure for an additional current required to obtain the desired power. In an implementation of the embodiments of Figures 5 and 6 the modification means 76 are the modification means shown in Figure 4 and the value of the register 765 is equal to the signal R d , ff . In that embodiment the multiplication means 762 and 761 generate a product signal which is indicative of a product of the difference signal and the enor signal The adder 763 serves as means for generating an output signal which is indicative for the sum of the product signal and the difference signal Rdiff. Figure 7 schematically shows an optical transmitter The transmitter compnses input means for receiving an input signal S ιn . The transmitter further compnses a control circuit 4 according to the invention, for example as shown in Figure 1 which is coupled to said input means The transmitter also compnses a radiation source 2 controlled by the control circuit 4 and measunng means 5, e g. in the form of a light sensitive diode for measunng an amount of radiation generated by the radiation source 2 The light sensitive diode 5 has an output coupled to the control circuit 4 The optical transmitter further has means 8 to couple the output of the radiation source 2 into a light conductor 9 Figure 8 schematically sho s a method for controlling a radiation source 2 The method compnses steps SI to S3 for determining an enor signal P eπ First a desired average v alue P set of the output power of the radiation source 2 is calculated in step S 1 Subsequently a measured average value P meaS of the output power of the radiation source 2 is determined in step S2 Then the difference between those values is determined in step S3 Then in steps S4 to S7 a control signal S out is generated for the radiation source 2 by combining said enor signal P err and an information signal S ιn representative of information to be wntten to the radiation source 2 In step S4 a multiplicative factor γ is determined from the error signal Pen Subsequently in step S5 the information signal S ιπ is modified by the multiplicative factor γ In step S6 an additi e factor σ is calculated from the error signal In step S7 the information signal S ιn is modified by the additive factor σ The method may for example be implemented as a computer program on a general purpose processor It is remarked that the scope of protection of the invention is not restncted to the embodiments descnbed herein Neither is the scope of protection restncted by the reference numerals included in the claims The word 'compnsing' does not exclude other parts than those mentioned in a claim The word 'a(n)' preceding an element does not exclude a plurality of those elements The invention resides in each new feature or combination of features

Claims: CLAIMS 1. A control circuit (7) for a radiation source (5) compnsing means (73) for generating an error signal (P err ) which is indicative for a difference between a measured average value (P meas ) of the output power of the radiation source and a desired average value (P set ) of the output power of the radiation source, - combining means (74, 75, 76) for generating a control signal (S out ) for the radiation source (5) in response to said error signal (P err ) and to an information signal (S ιn ) for modulating the radiation source, charactenzed in that, the combining means compnse first means (74, 76) for modifying the information signal (S ιn ) by a multiplicative factor γ which is dependent on the enor signal (S err ), and second means (75) for modifying the information signal (S m ) by an additive factor σ which is dependent on the enor signal (S err ) 2. A control circuit according to claim 1, charactenzed in that the multiplicative factor γ and the additive factor σ are related according to the following function γ = kσ + b 3. A control circuit according to claim 1 or 2, charactenzed in that the first means (74, 76) compnse modification means (76) which calculate a modified error signal (γ) from the enor signal (P err ) and multiplication means (74) for generating an intermediate signal (γS m ) which is indicative for the product of the modified enor signal (γ) and the information signal (S ln ) and in that the second means (75) compnse addition means for generating an output signal (S out ) which is indicative for the sum of the intermediate signal (γS ιn ) and the enor signal (P er r), the output signal being the control signal. 4. A control circuit according to claim 1 or 2, charactenzed in that the first means (74, 76) compnse modification means (76) which calculate a modified error signal (γ) from the enor signal (P err ) and multiplication means (74) for generating an intermediate signal (ySm) which is indicative tor the product of the modified enor signal (γ) and the information signal (S ιn ) and in that the second means (75, 78) compnse further multiplication means (78) for generating a further intermediate signal (σS ιn ) which is indicative for a product of the enor signal (P eπ -) and a further information signal (S ιn ) and addition means (75) for generating an output signal (S out ) which is indicative for the sum of the intermediate signal (γS ιn ) and the further intermediate sιgnal(σSm), the output signal being the control signal 5 A control circuit according to claim 1 or 2, charactenzed in that the first means compnse multiplication means (44) for generating an intermediate signal (γS ιn ) which is indicative for the product of the enor signal (P eιτ ) and the information signal (S, n ) and in that the second means compnse modification means (46) which calculate a modified enor signal (σ) from the enor signal (P err ) and addition means (45) for generating an output signal (S ou t) which is indicative for the sum of the intermediate signal (γS ιn ) and the modified enor signal (σ), the output signal being the control signal 6. A control circuit according to claim 3, further charactenzed by a node (83) for receiving a signal (R meas ) which is indicative for a measured reflection coefficient of an object which is radiated by the radiation source, by subtraction means (80) for generating a difference signal (R d i ff ) which is indicative for a difference between a desired reflection coefficient and the measured reflection coefficient, by multiplication means (81) for generating an intermediate signal which is indicative for a product of the difference signal (R d iff) and a signal (P set i) indicative for a desired power, and addition means (82) for generating a signal which is indicative for the sum of the intermediate signal and a further signal (P set2 ) indicative for a desired power. 7. A control circuit according to claim 6, charactenzed by multiplication means (761, 762) for generating a product signal which is indicative of a product of the difference signal (R d if f ) and the enor signal (P e r r ) and means (763) for generating an output signal (γ) which is indicative for the sum of the product signal and the difference signal 8. A device for recording and/or playback of information on a record earner, the device including a controller (4) as claimed in any of the claims 1 to 7 a radiation source (2) which is controllable bv the controller (4), a sensor (5) for generating a signal (P meas ) which is indicative of an intensity of radiation emitted by the radiation source (2) and which is coupled to an input of the controller, means for mapping radiation emitted by the radiation source (2) at a spot (3) at the information earner (1), means for causing a relative displacement between the spot (3) and the record earner (1) 9 Optical transmitter compnsing - input means for receiving an input signal (S ιn ), a control circuit (4) according to one of the claims 1 to 7 coupled to said input means, a radiation source (2) controlled by the control circuit (4), measunng means (5) for measunng an amount of radiation generated by the radiation source (2) having an output coupled to the control circuit (4), means (8) to couple radiation of the radiation source (2) into a light conductor (9). 10. Method for controlling a radiation source (2) compnsing - determining an enor signal (P err ) which is indicative for a difference between a measured average value (P m eas) of the output power of the radiation source (2) and a desired average value (P set ) of the output power of the radiation source, generating a control signal (S ow ) for the radiation source (2) by combining said enor signal (P err ) and an information signal (S ιn ) representative of information to be wntten to the radiation source (2), charactenzed m that, the step of generating the control signal compnses modifying the information signal (S ιn ) by a multiplicative factor γ which is dependent on the enor signal (S eπ ), and modifying the information signal (S ιn ) by an additive factor σ which is dependent on the enor signal (S err )

Inventors: MCCORMACK JAMES J A, MCCORMACK, JAMES, J., A.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2001
Main classification: H01S5
IPC codes: G01J1, G11B7, H01L31, H01S5, H04B10