IES990865A2

IES990865A2 - Method and apparatus for determining the drive currents of a laser

Info

Publication number: IES990865A2
Authority: IE
Inventors: Ronan O'dowd
Original assignee: Ronan O Dowd
Priority date: 1999-10-15
Filing date: 1999-10-15
Publication date: 2001-11-14

Abstract

A novel method to characterise a multi-section semiconductor laser for use as a wavelength tunable transmitter. The invention exploits to full advantage a filter whose optical throughput or transmission varies in a know way with wavelength so that the output power is measured and this is used to compute wavelength in the described manner. Furthermore, a data input/output unit is used to increment currents, as required, to the laser and also to read back current from a photo-detector which is in turn a measure of the wavelength as modified by the filter in a known fashion. This means that a single data acquisition unit, or DAQ, which multiple input and output channels can be used to replace the host of electronic current sources and a wavelength meter or analyser, while avoiding the associated time-consuming delays. A microprocessor may be used to replace the DAQ.

Description

A novel method to characterise a multi-section semiconductor laser for use as a wavelength tunable transmitter.

Multi-section semiconductor lasers are used in telecommunications, sensors, spectroscopy and elsewhere to provide a light output that can be tuned in wavelength by the variation of the input electrical currents to the sections.

The conventional method is to input electric currents to the sections of the multi-section laser in a controlled manner and measure the resultant output wavelength with a wavemeter or optical spectrum analyser. This involves expensive apparatus and takes considerable time, even when fully automated under computer control due to instrument communication delays. As many as 1000,000 or more data points need to be measured. For example, to increment in 0.1 mA steps from 0 to 100 mA for just two sections alone requires 1000x1000 measurement points.

The invention exploits to full advantage a filter whose optical throughput or transmission varies in a known way with wavelength so that the output power is measured and this is used to compute wavelength in the described manner.

Furthermore, a data input/output unit is used to increment currents, as required, to the laser and also to read back current from a photo-detector which is in tum a measure of the wavelength as modified by the filter in a known fashion. This means that a single data acquisition unit, or DAQ, with multiple input and output channels can be used to replace the host of electric current sources and a wavelength meter or analyser, while avoiding the associated time-consuming delays. A microprocessor may be used to replace the DAQ. We have reduced a week long session to one hour in this way. A further benefit is that environmental changes that de-stabilise optical equipment have minimal time to act.

The filter can be of many types. For example a fibre optic coupler designed to separate two wavelength bands at 1480 nanometers and above was found to be very suitable in that it has an almost linear transmission power characteristic, decreasing with increasing wavelength. This can be calibrated against a wave-meter and then used repeatedly without further access to the optical instrument.

Another filter is a fibre optic with embedded diffraction Bragg grating. Another is a coloured glass filter, for example a type used with NdYAG lasers.

INT CL -^//o/S 3/co - . Gtoi ΣΓ 9/00 OPEN TO PUBLIC INSPECTION UNDER SECTION 28 AND RULE 23 JNL No. .flM-.....OF SNSDOCID: IE990865 The laser characterisation set-up is illustrated in figure 1. It comprises a suitable optical filter (transmission linear or otherwise with wavelength), with an optical splitter to divide the beam. One portion passes through the filter and the photo-detector current is a measure of transmitted power. The other portion is directly measured by the same or a second photo-detector and acts as a reference. The three or four currents to the laser sections are automatically stepped in increments as required and the wavelength is calculated at each setting. A single data acquisition unit, DAQ, with D/A outputs to the laser current driver circuits and A/D input channels) from the photo-detector(s) can perform all of these functions under PC or microprocessor control. The control currents needed for each chosen wavelength channel are thereby established for use in a look-up table to tune the laser.

Figure 1.

The wavelength is calculated from the two photodetector currents, one being proportional to or representational of wavelength as transmitted by the filter, and the other being a reference measure of optical power input to the filter.

WAVELENGTH = (Power through filter / power into filter) x (Cl) + C2.

The correction factors Cl and C2 are derived from the initial filter throughput spectrum. If the filter is not linear then this calculation is modified accordingly to match the calibrated function.

Claims

1. A novel characterization method to find the set of control currents required to tune a multisection semiconductor laser with respect to wavelength. Changes in output wavelength are converted to optical power information by a suitable optical filter in a known way. The filter is characterised once with a wavemeter for transmission spectrum which may but need not be linear. The relative power is then sufficient to calculate the laser wavelength for the selected set of input drive currents without recourse to an optical wavelength meter. Dependent claims. The filter can be of many embodiments. A fibre optic coupler for seperating wavelength bands near the tuning range of interest has been found to work very well. A fibre Bragg grating filter also suffices as does a suitably selected glass filter or diffraction grating.. For the controller the electrical input drive currents and the photodetector output current can be embodied in a data acquisition card with multiple input/output channels. A microprocessor with memory also suffices. An application specific circuit (ASIC) with embedded or connected memory is another embodiment.