IES991022A2 - Novel Embodiment of a Multisection Semiconductor laser for use as a wavelength tunable transmitter with channel location and locking. - Google Patents

Abstract

A novel hybrid, silicon waveguide device with associated method to calibrate, identify wavelength and lock a multi-section semiconductor laser for use as a wavelength tuneable transmitter. The invention exploits to full advantage an embedded Bragg grating filter whose optical throughput or transmission varies in a known way with wavelength so that the throughput 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 photo-detectors which is in turn a measure of the wavelength as modified by the filter in a known fashion. A Fabry Perot etalon slotted into the waveguide permits the laser to be locked the ITU plan for WDM optical networks. 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 sourced and a wavelength meter or analyser, while avoiding the associated time-consuming delays. A microprocessor or ASIC may be used to replace the DAQ.

Description

A novel embodiment a multi-section semiconductor laser for use as a wavelength tunable transmitter with channel location and locking. 4 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 1,000,000 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.

Optical Fibre Output Alternative PD3 site V T-Laser WG BS WG-BG BS WG FP BS V Above: PD1 PD2 PD3 Below: Silicon substrate Figure 1 OPEN TO PUBLIC INSPECTION UNDER SECTION 28 AND RULE 23 JNL No. _OF ^οψ-pca.

IE 9 9 1 0 2 2 The invention consists of a hybrid, planar array of optoelectronic devices sited upon or embedded in a silicon substrate as illustrated in figure 1. This is a hybrid of silicon with AlGaAs or InGaAsP chips. There may also be a glass or quartz Fabry Perot etalon.

A multi-section, tunable, semiconductor laser, such as the known SG-DBR or GCSR device, occupies a slot next to the output optical fibre. This fibre is positioned in a Vgroove on the substrate for ease of micro-positioning. The light from the other facet of the SG-DBR laser (or beam split light in the case of a GCSR) travels in a rib (or embedded) waveguide in the silicon. A portion of the light (10 % can be sufficient) is deflected upwards by a beam splitter, BSI, to a photodiode, PD1, where it is measured.

The beam splitter may be a simple silicon facet created in the guide by a groove. The remaining light travels along the waveguide where an embedded Bragg grating, BG, diffracts it according to the known characteristics of such an optical element. This grating is designed to have a transmission characteristic like that in figure 2. The transmitted portion rises or falls with wavelength in a close to linear fashion. This light is then split by BS2 and a similar portion (e.g. 10%) measured by PD2. The ratio of the PD1 and PD2 electrical signals is the P(%), or power percentage, shown in figure 2. The remainder of the laser beam continues along the silicon guide and reaches a Fabry Perot etalon, FP, designed to transmit only wavelengths which represent the channels of a wavelength division multiplexed, WDM, optical fibre network. This is the frequency plan for WDM communications agreed by the International Telecommunications Union, ITU. We refer to these wavelengths by λ ιτυ· The light transmitted by the FP is measured by PD3 after deflection out of the guide by mirror M. The latter may be a facet like the beam-splitters but with aluminium coating to give high (e.g. 100%) reflectivity. Alternatively the PD3 may follow a fibre pigtail sited in a V-groove like that at the far end of the silicon device.

PD3 measures light power transmitted by the FP etalon and is at a maximum when the laser is tuned to an ITU channel.

The invention exploits to full advantage a Bragg grating 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 identify wavelength in the described manner; the P(%) value is unique to each wavelength.

Figure 2 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-detectors which in tum measure 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 other 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. In this embodiment the Bragg grating is omitted and replaced with a simple waveguide.The light in the waveguide then passes through this fibre filter before reaching PD 2.

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.

The laser characterisation set-up is as follows. The three or four currents to the laser sections are automatically stepped in increments as required and the wavelength is calculated at each setting as described using the BG characteristic. A single data acquisition unit, DAQ, with D/A outputs to the laser current driver circuits and A/D input channel(s) from the photo-detector(s) can perform all of these functions under PC or microprocessor control When the currents are set correctly the laser transmits at an ITU channel and this is indicated by PD3 whose signal will be at a maximum since it measures throughput for the FP etalon. The control currents needed for each chosen wavelength channel are thereby established for use in a look-up table, LUT, to tune the laser.

An alternative embodiment for the etalon is a vacant slot in the waveguide of precise dimensions and filled with a gas such as nitrogen. This may be achieved under nitrogen at the hybrid silicon device packaging stage. Ε 9 9 1 Q 2 Figure 3 In the above process the wavelength is calculated from the two photodetector currents,PD2 being proportional to or representational of wavelength as transmitted by the BG filter, and the other, PD1, being a reference measure of optical power input to the filter. The ratio P(%) of these two allows wavelength to be calculated from the design of the BG spectral transmission. In the case where this is linear: 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 grating characteristic.

Figure 4 The calculations performed automatically may be summarised as follows and with reference to figure 4: For the Bragg grating 100x(PD2 / PD1) = P(%) which locates λ. This is channel identification or ID. For the Fabry Perot etalon At maximum signal from PD3 the drive currents lock to a λ rru· This is channel LOCK.

The full sequence of steps is as folows: (1) . Initial calibration (CAL) produces look up table (LUT). Uses LUT algorithm described above. (2) . Thereafter SET λ / LOCK / VERIFY ID cycle operates continuously and automatically at each selected channel. (3) . Occasional minor recalibration, Re-Cal, of the laser in-situ using ITU FP locker etalon to seek adjusted LUT value(s). A simple current tweaking algorithm will permit this by seeking maximum PD3 signaL SET1 / LOCK / VERIFY ID / Re-Cal / LOCK (4) . Full Re-Cal for aging. Return to 1.

Claims (1)

1. Independent daim. A novel hybrid silicon waveguide device with embedded optoelectronic components and an associated 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 embedded Bragg grating 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 identify the laser wavelength for the selected set of input drive currents without recourse to an optical wavelength meter. The laser is locked to a required wavelength channel by a Fabry Perot etalon which is sited in a slot on the silicon waveguide. Dependent claims. The embedded Bragg diffraction grating is designed to have suitable transmission spectrum. For example transmission T(%) varying 20% to 100% in the range 1520 nm to 1560 nm in a fashion close to linear is ideal· An external filter may also be deployed. This 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 optic Bragg grating filter also suffices as does a suitably selected glass filter or diffraction grating sited in a slot in the silicon waveguide. The slotted Fabry Perot etalon may be embodied as a vacant slot filled with inert gas such as Nitrogen.. 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.