CALIBRATION NETWORK FOR A FEEDBACK LIGHHT DETECTOR AND A METHOD OF CALIBRATING FEEDBACK OUTPUT THEREFROM

Abstract: A calibration network for a feedback light detector (18) of a laser diode (12) comprises a first resistor (R₂) which is connected between the feedback light detector (18) and ground and which is adapted to produce a predetermined voltage (V₀) at an output terminal (28) when the feedback light detector (18) detects that the laser diode (12) is operating at a predetermined light power output. A second resister (R₁) is connected between the feedback light detec­ tor (18) and the output terminal (28) and is adapted to produce a predetermined impedence (C) at the output when the laser diode is operating at the predetermined light power output.

Description: This invention relates to calibration networks for feedback light detectors and methods of calibrating feedback outputs from feedback light detectors. Laser diodes have many applications. One of these is in optical disk drives. An optical disk drive comprises, among other things, a feedback light detector circuit for a laser light pen 10 having a power control circuit consisting of a control loop 22 and a laser driver 23 as shown in Figure 1. The laser light pen 10 comprises a laser diode 12, and a feedback light detector 18 for the purpose of monitoring the light power output of the laser diode. The current developed by the feedback light detector 18 varies from laser diode to laser diode due to various factors, among which are the large light beam dispersion angle from a rear facet 14 of the laser diode and mechanical tolerances in the optics of the light pen. Conventional power control circuits compensate for these variations by providing calibration potentiometers 24, 26. As the laser diode may be operated at two power levels, i.e., a read level and a write level, two corresponding potentiometers 24, 26 are required to provide a read set point and a write set point respectively. When, as is sometimes the case, a laser diode wears out or otherwise malfunctions, the light pen is replaced as a unit. This necessitates a technician recalibrating the set points on the two potentiometers 24, 26 and this may be a time consuming task. The procedure must be repeated each time the light pen is replaced. Elimination of the recalibration step would not only save valuable time when a light pen must be replaced, but could also eliminate the need for a technician altogether. The present invention seeks to provide a calibration network for a feedback light detector of a laser diode where calibration can be effected during manufacture to permit replacement of the combination of the laser diode and feedback light detector without recalibration of potentiometers used to set the read and write levels. According to one aspect of the present invention there is provided a calibration network for a feedback light detector of a laser diode comprising: a first resistor which is connected between the feedback light detector and ground and which is adapted to produce a predetermined voltage at an output terminal when the feedback light detector detects that the laser diode is operating at a predetermined light power output; and a second resistor which is connected between said feedback light detector and said output terminal, and which is adapted to produce a predetermined impedence at said output terminal when said laser diode is operating at said predetermined light power output. Said resistors may be thick film resistors. According to another aspect of the present invention there is provided a method of calibrating feedback output from the feedback light detector of a laser diode comprising: connecting a first resistor between the feedback light detector and ground; connecting a second resistor between the feedback light detector and a output terminal; operating the laser diode to produce a predetermined light power output; adjusting the resistance of the first resistor to produce a predetermined voltage at the output terminal; and adjusting the resistance of the second resistor to produce a predetermined impedence at the output terminal. Said resistors preferably are thick film resistors whose resistances are adjusted by trimming. The invention is illustrated, merely by way of example, in the accompanying drawings, in which:- Figure 1 is a schematic diagram of a conventional feedback light detector circuit and power control circuit for a laser light pen; Figure 2 shows part of a calibration network according to the present invention for the feedback light detector and power control circuit of Figure 1; Figure 3 is an equivalent circuit diagram of Figure 2; and Figure 4 is another equivalent circuit diagram of Figure 2. Throughout the drawings like parts have been designated by the same reference numerals. Figure 2 shows a calibration network according to the present invention for a feedback light detector 18 to which two thick film resistors R₁, R₂ are connected during manufacture. The resistor R₂ is connected between the feedback light detector 18 and ground and the resistor R₁ is connected between the feedback light detector and an output terminal 28. The output terminal 28 is connected to an output line 20 connected to the control loop 22 of the power control circuit (Fig. 1) when the light pen 12 is mounted in an optical disk drive (not shown). After the light pen 12 has been nearly fully assembled, the resistors R₁, R₂ are calibrated by trimming. Trimming may be performed using conventional techniques such as air abrasion or laser scribing. Figure 3 is an equivalent circuit diagram of the calibration network of Figure 2. The feedback light detector 18 is represented as a current source 30 producing a current I. The voltage, as measured at the output terminal 28, is equivalent to a voltage V developed across the resistor R₁: V = IR₁. The equivalent circuit diagram of Figure 3 can be modelled using a Thevenin voltage equivalent circuit shown in Figure 4. There the output impedence at the output terminal 28 can be seen to be the sum of the resistance values of the resistors R₁, R₂. The calibration network of Figure 2 is calibrated as follows: first, power is applied to the laser diode 12 to produce a predetermined light power output from a front facet 16, which may be measured using calibrated light detecting apparatus (not shown). The resulting current from the feedback light detector 18 flows through the resistor R₁ to ground. The voltage at the output terminal 28 is the voltage at the junction of the resistors R₁, R₂. This voltage is measured and the resistor R₂ is then trimmed to produce a predetermined voltage V₀ at the output terminal 28. Next, the resistor R₁ is trimmed to produce a predetermined impedence output, C = R₁ + R₂. C may, for example, be 50, 75 or 300 ohms, the resistance of typical transmission lines such as the output line 20, a coaxial cable, a twisted pair cable, etc. This latter step is necessary due to the high frequency at which the laser diode 12 is typically modulated in an optical disk drive. Unless the impedence at the output terminal 28 is controlled, the current of the output terminal can be expected to vary greatly between the unmodulated read level and the modulated write level.

Claims: A calibration network for a feedback light detector (18) of a laser diode (12) comprising: a first resistor (R₂) which is connected between the feedback light detector (18) and ground and which is adapted to produce a predetermined voltage (V₀) at an output terminal (28) when the feedback light detector (18) detects that the laser diode (12) is operating at a predetermined light power output; and a second resistor (R₁) which is connected between said feedback light detector (18) and said output terminal (28), and which is adapted to produce a predetermined impedence (C) at said output terminal when said laser diode is operating at said predetermined light power output. A calibration network as claimed in claim 1 characterised in that said resistors (R₁,R₂) are thick film resistors. A calibration network as claimed in claim 1 or 2 characterised by being in combination with the laser diode (12). A method of calibrating feedback output from the feedback light detector of a laser diode comprising: connecting a first resistor (R₂) between the feedback light detector (18) and ground; connecting a second resistor (R₁) between the feedback light detector (18) and an output terminal (28); operating the laser diode (12) to produce a predetermined light power output; adjusting the resistance of the first resistor (R₂) to produce a predetermined voltage (V₀) at the output terminal (28); and adjusting the resistance of the second resistor (R₁) to produce a predetermined impedence (C) at the output terminal. A method as claimed in claim 3 in which said resistors (R₁,R₂) are thick film resistors whose resistances are adjusted by trimming.

Inventors: HOWARD, GUY P.
Applicants: LASER MAGNETIC STORAGE INTERNATIONAL COMPANY
Application year: 1984
Main classification: H01S3
IPC codes: H01S5, G11B7