GB2218566A

GB2218566A - Light emitting package

Info

Publication number: GB2218566A
Application number: GB8811428A
Authority: GB
Inventors: Andrew Stephen Turner
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1988-05-13
Filing date: 1988-05-13
Publication date: 1989-11-15
Anticipated expiration: 2008-05-13
Also published as: GB8811428D0; GB2218566B

Abstract

A package has a laser chip (1) and a thermistor (4), in place of the conventional photodiode, to monitor the beam strength from the rear facet (3) of the chip. A second thermistor (4A) monitors ambient temperature and compensates in controlling the drive current of the chip to maintain beam strength constant. The laser chip may be replaced by a light emitting diode. <IMAGE>

Description

OPTICAL TRANSMISSION PACKAGE This invention relates to semiconductor lasers.

Conventionally the light output of a semiconductor laser chip is monitored by a monitor photodiode which controls the laser drive current in such a manner as to tend to maintain the laser beam power constant despite e.g. ageing of the laser chip.

The use of a photodiode to sense a portion of the light output for this purpose is well known, but a serious disadvantage is that such arrangements make the cost of the laser package relatively high.

It is an object of the present invention to provide a semiconductor laser control arrangement in a more cost effective way than hitherto.

According to the present invention there is provided a semiconductor optical transmission package, such as a laser, comprising an optical transmission chip, and a monitor thermistor so arranged as to monitor the beam strength of the chip, so that changes in beam strength will cause corresponding changes in thermistor resistance, and means whereby the changes in resistance can be used in a feedback circuit to control the chip beam strength.

In one embodiment said means comprises external connection terminals coupled to the thermistor whereby the thermistor output is applied to external circuitry to control the laser drive current.

In a preferred embodiment there are two thermistors, one monitoring the laser beam by absorption of infra red radiation and the other monitoring ambient temperature within the package, and a change in the relationship of the resistances of the two thermistors is used to adjust drive current as necessary.

In order that the invention can be clearly understood, reference will now be made to the accompanying drawing in which: Fig. 1 shows a plan, somewhat schematic view of a laser package embodying the invention, Fig. 2 shows a side, somewhat schematic view of the package of Fig. 1 and Fig. 3 shows a circuit diagram of the laser package of Figs. 1 and 2.

Referring to Figs. 1 and 2 of the drawings, which are not to scale, the laser package comprises a base 6 on which is supported a pedestal 2. A semiconductor laser chip 1 is mounted on the pedestal 2 and radiators at 1.3um and the direction of the arrow A.

A thermistor 4 is placed in the rearward ex-facet beam A, to receive rearward radiation from the rear facet laser 1. Absorption of infra-red radiation by the thermistor 4 continues to saturation at which point heat energy in equals heat energy out due to re-radiation and conduction. At this point the thermistor has a resistance value which can be measured. A second thermistor 4A is placed in a configuration with the first thermistor 4 (but not in the rear facet beam A) so as to monitor ambient temperature.

The thermistors have a radial lead configuration and are suspended by their leads 4' and 4A', respectively, from the pins 5 and 5A.

Each thermistor is supported in a suspended fashion from pins 5 and 5A, respectively, mounted in the base 6 which in this embodiment is electrically insulating. The pins 5 and 5A form external connection terminals for connection of the thermistor to external circuits.

The second thermistor 4A has a resistance value different to the first thermistor 4 and is dependent on ambient temperature. The external circuits monitor change in the first thermistor resistance and the relationship of the first to the second as ambient temperature changes, and adjusts the drive current as necessary.

Analytical fibre is shown in broken line, coupled to the front output facet 3A of the laser chip.

A further form of monitoring can be by Wheatstone bridge. If the base 6 is non electrically conducting (as in this embodiment, two further thermistors could be provided in thick film printed form to form four sides of such a bridge).

Alternatively, two fixed resistors could be used and connected externally.

As examples of thermistors which could be used in this embodiment are those known as themoflakes and manufactured by Thermometrics of Edison, New Jersey, USA. Each thermistor is a thick film thermistor which has no substrate backing. Their lead wires are fired directly into their electrodes. The high surface-to-mass ratio results in low heat capacity and fast response time, and are suited for infrared detection. Since the thermistor materials used are good absorbers of IR energy, satisfactory results may be obtained without the use of special absorption coatings. Such coatings do, however, provide more uniform absorption over a specified spectral wave length band.

Preferably a filter coating is applied having a spectral window centred on the wavelength of the laser, e.g. 1.3um. The thermistor should be able to dissipate heat so that the operating temperature is not higher than lOS0C. The resistance range is 50 Kohms to 2 Megohms and the temperature coefficient lies in the range -3.9%/ C and -4.4%/ C.

An example of a central circuit for the embodiment of Figs. 1 and 2 is shown in Fig. 3.

Referring to Fig. 3, the output from the rear facet 3 of the laser 1 heats up the thermistor 4 which is connected into a Wheatstone Bridge with thermistor 4A and two other thermistors 4B and 4C which may be printed on the base 1 as shown in Fig. 1.

A voltage is applied across the bridge between points a and c and an out-of-balance signal will occur across points b and d.

This signal is applied to an amplifier 10 with local feedback (not shown) before fading into a comparator 11 for comparison with a reference voltage V corresponding to a preset peak optical power (digital) or r.m.s. optical power (analogue). The comparator output is fed to a current generating network 12 for modifying DC bias current 13 or modulation current 14, respectively, to the laser 1.

For analogue systems, the output of comparator 11 can also be fed to an automatic gain control unit 15. The desired level of operation of the laser 1 and the corresponding thermistor bridge output are determined, and the gain control 15 is then set to apply negative feedback to the current generation 12 proportional to excursions of the bridge output from that operating level.

The use of a thermistor makes the laser package much more economical to manufacture because it eliminates the need for a photodetector, relying instead on detecting changes in resistance in response to changes in beam output of the laser rather than using the photovoltaic effect in a photodiode. Should it be found that the change in thermistor resistance does not exactly correspond to the change in beam output of the laser, then the control circulates and/or the thermistor temperature coefficient can be chosen to compensate accordingly. As the laser gets older it gets less efficient and so generates more heat, but this is compensated by the thermistor 4A sensing ambient temperature.

The insulation is applicable also to light emitting diodes.

It should be understood that the control circuitry shown in Fig. 3 could be incorporated within the package if desired.

Claims

CLAIMS:

1. A semiconductor optical transmission package, such as a laser, comprising an optical transmission chip, and a monitor thermistor so arranged as to monitor the beam strength of the chip, so that changes in beam strength will cause corresponding changes in thermistor resistance, and means whereby the changes in resistance can be used in a feedback circuit to control the chip beam strength.

2. A transmission package as claimed in claim 1, comprising a second thermistor arranged to sense ambient temperature within the package, and means for connecting said second thermistor is a circuit so as to compensate resistance changes in the monitor thermistor caused by ambient temperature changes.

3. A package as claimed in claim 1 or claim 2, wherein the chip is a laser chip having a front facet coupled to a fibre and a rear facet, wherein the monitor thermistor is placed in the beam from the rear facet of the chip.

4. A package as claimed in any preceding claim in combination with a feedback control circuit coupled to the or each thermistor and to the chip and arranged to control the chip drive current so as to maintain constant the output beam strength.

5. An optical transmission package substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.