GB1569794A

GB1569794A - Miniaturised laser arrangement

Info

Publication number: GB1569794A
Application number: GB6181/77A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1977-02-15
Filing date: 1977-02-15
Publication date: 1980-06-18
Also published as: DE2803558A1

Description

(54) MINIATURISED LASER ARRANGEMENT (71) We, STANDARD TELE PHONES AND CABLES LIMITED, a British Company, of 190 Strand, London, W.C.2., England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to miniaturized pulsed lasers and electronic drive circuits According to the present invention there is provided a miniature laser module comprising: a header; a heat sink, formed either by an element secured to the header or by part of the header itself; a pad of electrically insulating material metallised on at least one surface and fixed to the heat sink so that the metallised surface is not in contact with it; a transistor chip fixed to the metallised surface; a laser diode chip fixed to the heat sink directly or to the metallsed surface; a capacitor chip fixed to the header with one of its electrodes electrically connected to it; the transistor and laser diode chips being connected in series, the capacitor being connected across the series connection of the laser diode and transistor chips and to two pins of the header, and a third pin of the header being electrically connected with a third electrode of the transistor chip.

Embodiments of the invention are described below with reference to the accompanying drawings which are not all to the same scale, of which figure 1 is a plan view of the miniaturized pulsed laser module; figure 2 is a side elevation of the miniaturized pulsed laser module of figure 1, to a different scale; figure 3 is a base view of the "header" carrying the miniaturized pulsed laser module assembly of figures 1 and 2; figure 4 is a circuit diagram of a miniaturized pulsed laser using a N.P.N. transistor; figure 5 shows an external triggering input circuit for a N.P.N. transistor; figure 6 shows a self triggering input circuit; figure 7 shows the side elevation of an alternative physical arrangement of the components of the circuit of figure 4, and figure 8 shows a modification of figure 7.

In figure 1 there are shown, in plan, the several components which go to make up a miniaturized pulsed laser module, which may be accommodated on a transistor header, such as a TOS header.

A pad of beryllia 1 is fixed to the header la, acting as a heat sink, between pins A and B. The pad has been metallised over its top and bottom surfaces (relative to the header).

A transistor chip 2 (shown here as npn) is soldered to the pad so that its collector is in electrical connection with the top metallized surface of the pad. A connecting lead 3 joins the base electrode of the transistor to pin A, and a connecting lead 4 joins the emitter electrode of the transistor to pin B. A- semiconductor laser diode chip 5 is also soldered to the beryllia pad so that its cathode is electrically connected to the top metallized surface, and so, of course, to the collector of transistor 2. A connecting lead 6 joins the anode of the laser diode to the top surface of the header, and so to a pin C. The laser diode is so arranged that it emits, when working, its radiation away from the header in a beam substantially parallel to the plane of the header surface.

A capacitor 7, here shown as made up of three ceramic chip capacitors connected in parallel, is soldered to the header so that one electrode of the capacitor is electrically connected to the top surface of the header, and so to the anode of the laser diode and to pin C. A connecting lead 8 joins the other electrode of capacitor 7 to the emitter of the transistor 2 and so to pin B.

Figure 2 shows, in side elevation, the spatial arrangement of the various components mentioned above on the header.

Figure 3 indicates the arrangement of the pins A, B, and C as seen from the underside of the header.

Figure 4 is a circuit diagram showing the interconnection of the various components. A direct current supply is connected across pins B and C, pin C being positive with respect to pin B. If now a triggering pulse be applied to pin A, that is to the base electrode of the transistor, the transistor which is operated in avalanche mode, conducts and causes the capacitor 7, charged from the d.c. supply, to discharge through the laser diode, thus causing it to radiate a pulse of radiation. When the discharge is complete the transistor turns off, the triggering pulse having already ceased.

Figure 5 shows a triggering circuit whereby trigger pulses from an external generator can be applied to pins A and B, and hence across the base-emitter circuit of the transistor to turn it on. The triggering circuit requires a series resistor 9 to be connected between the positive pole of the d.c. supply and pin C, and a pulse shaping circuit comprising a capacitor 10, in series between pin A and an input terminal 11 (for connection to a source of rectangular pulses, not shown), and a diode 12 in parallel with a resistor 13 connected across pins A and B. Pin B is connected to the negative pole.

Figure 6 shows a self triggering circuit. A fixed resistor 9a is in series with a variable resistor 9b between the positive pole of the d.c. supply and the pin C, while a resistor 13d is across pins A and B. This circuit depends on the transistor taking a time, dependent on the values of resistors 9a, 9b and 13a, to recover after an avalanche discharge, to a condition in which it becomes conductive again. The time is, of course, partially determined by the time constant of the resistorcapacitor circuit composed of capacitor 7 and the resistors 9a and 9b.

The arrangement of figures 1, 2, and 3 produces a radiation beam parallel to the plane of the header. A protective case may surround this arrangement being transparent in at least the vicinity of the laser diode so as to allow the beam to escape. Figure 7, however, shows another embodiment in which by spatially rearranging some of the components the radiation beam emerges axially from the top of the device.

Capacitor 7 again has one electrode directly connected to the header, but beryllia pad 1 now is fixed, not directly to the header, but to one side of an additional heat sink element or block 14 so that the plane of the "top" surface of the pad 1 (that surface carrying the transistor and the laser diode) is perpendicular to the plane of the header surface. Heat sink 14 is electrically conductive as well as heat conductive and is connected to the header surface, and so to pin C. The lead 6a from the anode of the laser diode 4 goes to a point on the surface of the heat sink 14, rather than down to the header itself, as did lead 6.

A cap 15 with a top window 16 transparent to the laser's radiation covers the arrangement on the header and can be hermetically sealed. By reversing the polarity of the laser diode it can be bonded directly to the heat sink of figure 7, as shown in figure 8. A similar modification can be made to the construction shown in figure 2.

Instead of an non transistor, a pnp transistor may be used, with appropriate changes in connection to supply voltages.

WHAT WE CLAIM IS: - 1. A miniature laser module comprising: a header; a heat sink, formed either by an ele ment secured to the header or by part of the header itself; a pad of electrically insulating material metallised on at least one surface and fixed to the heat sink so that the metallised surface is not in contact with it; a transistor chip fixed to the metallised sur face; a laser diode chip fixed to the heat sink directly or to the metallised surface; a capaci tor chip fixed to the header with one of its electrodes electrically connected to it; the transistor and laser diode chips being connected in series, the capacitor being connected across the series connection of the laser diode and transistor chips and to two pins of the header, and a third pin of the header being electrically connected with a third electrode of the transistor chip.

2. A laser module as claimed in claim 1 in which the cathode of the laser diode is bonded to the metallised surface of the insulating pad, and its anode is connected to the header by a lead.

3. A laser module as claimed in claim 1 in which the anode of the laser diode is bonded to the heat sink and its cathode is connected to the metallised surface of the pad by a lead.

4. A laser module as claimed in claims 1, 2 or 3 in which the insulating material of the pad is beryllia.

5. A laser module as claimed in claim 1, 2 or 3, in which the insulating pad is attached directly to the header and radiation from the laser diode, when working, is in a beam substantially parallel to the plane of the header.

6. A laser module as claimed in claim 1, 2 or 3, in which the insulating pad is attached to one side of a heat sink block of electrically conductive material, which is secured to the header, and radiation from the laser diode, when working, is in a beam perpendicular to the plane of the header.

7. A laser module as claimed in claim 1, 2, 3, 4, 5 or 6 in which the header is a transistor header.

8. A laser module as claimed in claim 1, 2, 3, 4, 5, 6 or 7 in which the module is enclosed by a cover of protective material which, at least in the vicinity of the laser diode, is transparent to emission from the laser.

9. A miniature pulsed laser module substantially as described with reference to figures 1, 2, 3, and 4 or figures 6 and 7 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

direct current supply is connected across pins B and C, pin C being positive with respect to pin B. If now a triggering pulse be applied to pin A, that is to the base electrode of the transistor, the transistor which is operated in avalanche mode, conducts and causes the capacitor 7, charged from the d.c. supply, to discharge through the laser diode, thus causing it to radiate a pulse of radiation. When the discharge is complete the transistor turns off, the triggering pulse having already ceased.

Figure 5 shows a triggering circuit whereby trigger pulses from an external generator can be applied to pins A and B, and hence across the base-emitter circuit of the transistor to turn it on. The triggering circuit requires a series resistor 9 to be connected between the positive pole of the d.c. supply and pin C, and a pulse shaping circuit comprising a capacitor 10, in series between pin A and an input terminal 11 (for connection to a source of rectangular pulses, not shown), and a diode 12 in parallel with a resistor 13 connected across pins A and B. Pin B is connected to the negative pole.

Figure 6 shows a self triggering circuit. A fixed resistor 9a is in series with a variable resistor 9b between the positive pole of the d.c. supply and the pin C, while a resistor 13d is across pins A and B. This circuit depends on the transistor taking a time, dependent on the values of resistors 9a, 9b and 13a, to recover after an avalanche discharge, to a condition in which it becomes conductive again. The time is, of course, partially determined by the time constant of the resistorcapacitor circuit composed of capacitor 7 and the resistors 9a and 9b.

The arrangement of figures 1, 2, and 3 produces a radiation beam parallel to the plane of the header. A protective case may surround this arrangement being transparent in at least the vicinity of the laser diode so as to allow the beam to escape. Figure 7, however, shows another embodiment in which by spatially rearranging some of the components the radiation beam emerges axially from the top of the device.

Capacitor 7 again has one electrode directly connected to the header, but beryllia pad 1 now is fixed, not directly to the header, but to one side of an additional heat sink element or block 14 so that the plane of the "top" surface of the pad 1 (that surface carrying the transistor and the laser diode) is perpendicular to the plane of the header surface. Heat sink

14 is electrically conductive as well as heat conductive and is connected to the header surface, and so to pin C. The lead 6a from the anode of the laser diode 4 goes to a point on the surface of the heat sink 14, rather than down to the header itself, as did lead 6.

A cap 15 with a top window 16 transparent to the laser's radiation covers the arrangement on the header and can be hermetically sealed. By reversing the polarity of the laser diode it can be bonded directly to the heat sink of figure 7, as shown in figure 8. A similar modification can be made to the construction shown in figure 2.

Instead of an non transistor, a pnp transistor may be used, with appropriate changes in connection to supply voltages.

WHAT WE CLAIM IS: - 1. A miniature laser module comprising: a header; a heat sink, formed either by an ele ment secured to the header or by part of the header itself; a pad of electrically insulating material metallised on at least one surface and fixed to the heat sink so that the metallised surface is not in contact with it; a transistor chip fixed to the metallised sur face; a laser diode chip fixed to the heat sink directly or to the metallised surface; a capaci tor chip fixed to the header with one of its electrodes electrically connected to it; the transistor and laser diode chips being connected in series, the capacitor being connected across the series connection of the laser diode and transistor chips and to two pins of the header, and a third pin of the header being electrically connected with a third electrode of the transistor chip.
2. A laser module as claimed in claim 1 in which the cathode of the laser diode is bonded to the metallised surface of the insulating pad, and its anode is connected to the header by a lead.
3. A laser module as claimed in claim 1 in which the anode of the laser diode is bonded to the heat sink and its cathode is connected to the metallised surface of the pad by a lead.
4. A laser module as claimed in claims 1, 2 or 3 in which the insulating material of the pad is beryllia.
5. A laser module as claimed in claim 1, 2 or 3, in which the insulating pad is attached directly to the header and radiation from the laser diode, when working, is in a beam substantially parallel to the plane of the header.
6. A laser module as claimed in claim 1, 2 or 3, in which the insulating pad is attached to one side of a heat sink block of electrically conductive material, which is secured to the header, and radiation from the laser diode, when working, is in a beam perpendicular to the plane of the header.
7. A laser module as claimed in claim 1, 2, 3, 4, 5 or 6 in which the header is a transistor header.
8. A laser module as claimed in claim 1, 2, 3, 4, 5, 6 or 7 in which the module is enclosed by a cover of protective material which, at least in the vicinity of the laser diode, is transparent to emission from the laser.
9. A miniature pulsed laser module substantially as described with reference to figures 1, 2, 3, and 4 or figures 6 and 7 of the accompanying drawings.