GB2242327A - Optical pulse source - Google Patents

Abstract

An optical pulse source for the generation of short (typically a few tens of picoseconds) optical pulses of narrow linewidth and good contrast ratio comprises a gain-switched DFB semiconductor laser whose output is passed via a narrow linewidth filter (typically having a passband 5 Angstroms or less in width) to a laser amplifier. The narrow-band filter preferably comprises a DFB semiconductor laser biased below threshold. Both the DFB devices are preferably mutiple- contact devices so that differential bias can be applied to tune the devices. The laser amplifier is preferably a fibre device. <IMAGE>

Description

OPTICAL PULSE SOURCE This invention relates to the generation of short optical pulses.

Short optical pulses, that is pulses having a duration of hundreds of picoseconds or less, are of interest in optical communications and optical signal processing. In these and other applications it is generally desirable to have large amplitude pulses with a high contrast ratio.

Short optical pulses can be generated by mode-locked or gain-switched lasers. In general, mode-locking produces the shortest pulses, typically < lops, with sub-picosecond durations possible, but in practice, with semiconductor lasers, gain-switching is easier to implement ald is more convenient than mode-locking: see the paper by Onodera et al in Appl. Phys, Lett., Vol. 45, No. 8, 1984, pp843-845.

Unfortunately, in the areas such as optical communications and optical signal processing where short optical pulses are required, it is generally necessary for the optical pulses to be of narrow linewidth. This is unfortunate because even if one uses a narrow linewidth laser source, such as a single longitudinal mode distributed feedback (DFB) or distributed Bragg reflector (DBR) laser, gain switching results in 'chirp' and hence increased linewidth pulses. For example, in the letter by Lin and Koch, Electronics Letters, 1985, Vol. 21, No. 21, pp958-960, it was reported that even in DFB lasers which exhibit unusually low transient frequency chirp (so-called vapour-phase-transport DFB lasers) significant pulse distortion would result from chirp at bit rates of 8 to 10 Gbit/s and fibre distances of 60km or more.In fact the reported results show unacceptable chirp even at bit rates as low as 4 Gbit/s. It is also clear that one has a choice between low chirp and high pulse contrast ratio: if one wants tolerably low chirp, one has to accept low contrast ratio pulses, typically as low as 3 to 1; conversely if one wants a high contrast ratio one has to accept massive chirp and hence suffer significant dispersion and lower bit rates.

In the above-referenced paper by Onodera et al, RF modulation of a l.3um DFB laser was used to generate optical pulses having a FWHM duration of 34ps with chirp minimised to the transform limit, but this was only achieved with a prohibitively small contrast ratio of 3.

According to a first aspect the present invention provides an optical pulse source comprising a semiconductor laser source, gain switching means for producing a gain switched output from said laser source, ç distributed feedback laser structure, and laser amplifier means, wherein the optical pulse source is arranged in operation such that a gain switched output of said laser source is fed through said distributed feedback laser structure while said structure is biased below threshold, the resultant output from the distributed feedback laser structure being fed through said laser amplifier means to produce an optical output pulse.

According to a second aspect the present invention provides a method of generating optical pulses, the method comprising the steps of: gain-switching a semiconductor laser source; feeding an output of said source through a distributed feedback laser structure biased below threshold; and feeding the resultant output from the distributed feedback laser structure through a laser amplifier means to produce an optical output pulse.

Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an optical pulse source according to the invention; Figures 2(a) to 2(d) show the Optical spectra at various points in the pulse source; and Figures 3(a) to 3(c) show the pulse waveforms at points in the pulse source corresponding to the optical spectra in Figures 2(a), 2(c) and 2(d).

In Figure 1 an optical pulse source according to the invention is shown schematically in its most basic form, such as might be constructed for laboratory use for example. The three main optical components are I a laser source, in this case a multicontact I.5m ridge waveguide DFB laser; 2 a DFB laser structure with low reflectivity facets and wavelength matched to laser I; and 3 a flat-response 1.5cm amplifier, in this case a 1.5um erbium doped fibre amplifier.

The laser 1 and the laser structure 2 are, in this example, practically identical except for the reflectivities of their facets. The output facet of laser source 1 was anti-reflection coated to give 3 to 5 per cent reflectivity, the other facet not being coated and hence retaining its as-cleaved reflectivity. The DFB laser structure 2 is treated as a laser amplifier and given low reflectivity coatings on both facets to give a reflectivity per facet of 0.1 per cent or less.

The split contact devices used were similar to those described in the following papers: Kikuchi and Tomofuji, Electronics Letters, 1989, Vol. 25, No. 2, pp162-163; Nakano et al, Electronics Letters, 1987, Vol. 23, No. 16, pp827-828; Yoshikuni et al, Journal of Lightwave Technology, Vol. LT-5, No. 4, 987, pp516-522; and Leclerc et al, Electronics Letters, 1989, Vol. 25, No. 1, pp45-47, which are all incorporated herein by this reference.

'Snake-contact' lasers as described in our co-pending British patent application 89 17564.0, which is incorporated herein by this reference, can be used in place of either or both of the above-described conventional split contact devices. Again, minimisation of chirp will in general require adjustment of the DC bias levels to the different contact segments, and possibly adjustment of the phase delay between the RF power split to the contact segments in device 1.

Because the pulse source represented in Figure 1 comprises discrete components, between the laser source 1 and the DFB structure 2 an isolator 4 and a polarisation controller 5 are inserted. The isolator 4 is a fibre isolator to minimise feedback into the laser source 1.

The polarisation controller 5 serves to maintain the orientation of the TE polarisation state from the laser source 1 to the DFB structure 2.

In a pulse source in which the laser source 1 and the DFB structure 2 are formed as an integrated unit, connected by a waveguide for example, the isolator and polarisation controller would be dispensed with.

In the embodiment represented in Figure 1 an optical fibre link is used between the laser source 1 and the DFB laser structure 2, lensed ends may be used to facilitate optimum coupling. Similarly, a lens ended fibre may be aligned to receive the output from the DFB laser structure 2, the lens ended fibre being spliced to the fibre amplifier.

In the example shown in Figure 1, both laser source 1 and DFB laser structure 2 are multiple contact (NC) devices. An advantage of using XC devices is that by varying the currents or the ratios of the currents applied to the different contacts it is possible to tune the device's output. In the present invention this flexibility is used to minimise chirp.

In one embodiment the devices 1 and 2 were each about 300;m long and were provided with 3 contacts each approximately 100;m long. For each device the outer two contacts were electrically connected together and a DC bias I1 applied thereto, with a separate DC bias I2 applied to the middle contact.

In the case of device 2, the ratio of current I1 to current I2 was approximately 2 to 1, with the device being biased below its lasing threshold. The bias currents to the device were adjusted such that the peak wavelength of the filter spectrum, shown in Figure 2b, coincided with that of the peak from the laser source 1.

(Figure 2b shows the optical output spectrum of device 2 with no optical input The FWHX of this spectrum is about 2A.) In the case of device 1, the ratio of the current I to current 12 was also approximately 2 to 1 - in one range of experiments the currents were 38.lmA and 22.6mA, but at a level at which the device was biased at threshold. In addition to the DC bias, provision is made for the supply of RF power to the electrodes for RF modulation of the source 1. A power splitter 6 enables the amount of RF power fed to the coupled electrodes 11, 11' and to the middle electrode 12 to be varied.In the course of many experiments we discovered that minimum chirp was obtained from source 1 if the ratio between the amount of RF power fed to the coupled electrodes 11, 11' and the middle electrode 12 was closely similar to the ratio of the powers fed as DC bias. In the majority of our experiments with these devices this meant that power ratios of 2.2dB or 2dB were used. Typically the total RF input to device 1 was 200mW per pulse. The RF pulses were generated by amplifying the output from a 2GHz Anritsu pattern generator.

Neans were also provided whereby the relative phase of the RF signals fed to the electrodes could be varied. It was observed that the peak wavelength of the optical output of source 1 could be tuned over a continuous span of 5A simply by varying the phase delay between the RF power split to the two sections. This tuneability is useful in that it provides another means, in addition to any tuneability of the DFB laser structure 2, whereby the peak wavelengths of the source 1 and structure 2 (which is effectively operating as a filter) can be brought into coincidence.

More information on RF modulation of semiconductor lasers can be found in the letter by Zhang, Ito and Inaba, in Electronics Letters, Vol. 24, No. 7, pp369-370, which is incorporated herein by this reference.

The output amplifier 3 is a 1.5cm doped fibre amplifier which, in this example, is pumped using a 1480nm laser. The present invention utilises the high gain and flat wavelength response of the erbium fibre amplifier but of course other laser amplifiers, while not preferred, could be used instead in so far as their gain/wavelength profiles are satisfactory. Of course fibre amplifiers typically offer much higher saturated output powers than are achievable with semiconductor laser amplifiers.

Erbium fibre amplifiers are now well known, see for example the letter by Laming et al, in Electronics Letters, 1989, Vol. 25, No. 1, ppl3-14, and the references thereto, which are herein incorporated by this reference, and hence no further description of this component will be given.

The optical signals at various points (A, B and C in Figure 1) through a pulse source according to the invention were analysed and the pulse width, amplitude and contrast ratio were determined. The sampling optical oscilloscope which was used for this analysis had a time resolution (pulse width) of lops. Wavelength chirp was measured at the same three points using an optical spectrum analyser having a resolution of IA. The relevant optical spectra for points A, B and C are shown in Figures 2(a), 2(c) and 2(d). The related pulse waveforms are shown in Figures 3(a), 3(b) and 3(c), and the results summarised in Table 1.

The output of the laser source 1, at point A, produced 59ps pulses with an amplitude of 7 arbitrary units and large -20dB bandwidth (10. it). The theoretical bandwidth of the fundamental lobe for 60ps pulses is 33GHz (2.?A), so the additional wavelength chirp is 7.4A.

The output at B from the laser structure 2, with the input from the laser source 1 corresponding to Figure 2(a), is shown in Figure 2cur, The -20dB chirp bandwidth has been reduced to 4.3A from lO.lA. However, the pulse FWHX has increased to 75ps and the amplitude of the pulses has dropped to 3 arbitrary units. This reduction in bandwidth correlates well with the increase in pulse width - the narrow passband of the DFB laser structure 2 attenuates wavelength components which fall outside the fundamental lobe of the original pulse spectrum. The fundamental lobe bandwidth for 75ps pulses is 2.it, and hence at this point there is an 'excess' chirp of 2.2A, down from 7.4A. This chirp reduction has, however, been accompanied by an unacceptable decrease in pulse amplitude.

Figure 2(d) shows the output at C from the fibre amplifier 3 with the chirp narrowed pulses in Figure 2(c) as the input. Even with the fibre amplifier not driven hard the amplitude of these pulses has increased by over lOdB as compared to the original pulses, at A, output from the laser source 1. The variations in the height of alternate pulses is probably due to reflections back into the DFB structure 1.

Additionally, and surprisingly, within experimental error neither the FWHM of the pulses nor the chirp bandwidth is affected by the increase in amplitude. Thus the pulse source according to the present invention has overcome the previously accepted compromise between chirp and pulse amplitude which has previously been accepted in gain-switched semiconductor lasers.

An optical pulse source according to the present invention finds application in, inter alia, an optical time division multiplex (OTDN) system, and such a system could have an increased path length for a given amount of pulse broadening and interpulse interference at the receiving terminal(s).

The laser source 1 and DFB laser structure (or equivalent) 2 could be integrated onto a single photonic integrated circuit to obtain chirp reduced optical pulses. The output from the integrated device could then be simply coupled into a fibre amplifier to give large amplitude, low chirp picosecond (loops or less) optical pulses. Shorter pulses can be obtained by decreasing the length of the laser source 1 to reduce the photon lifetime.

Claims (10)

1. An optical pulse source comprising a gain-switched semiconductor laser source, a narrow passband filter means and laser amplifier means, wherein an output of said laser source is fed through said narrow linewidth filter means, the resultant output from the filter means being fed into said laser amplifier means.

2. A pulse source as claimed in claim 1 wherein said filter means comprises a passive waveguide grating device.

3. A pulse source as claimed in claim 2 wherein the waveguide grating device comprises a semiconductor, and means are provided whereby the refractive index of the semiconductor adjacent the grating can be varied.

4. A pulse source as claimed in claim 3 wherein said means for varying the refractive index comprise two or more electrodes adjacent the grating.

5. A pulse source as claimed in claim 1 wherein said filter means comprises a distributed Bragg reflector laser structure.

6. A pulse source as claimed in claim 1 wherein said filter means comprises a distributed feedback laser structure.

7. A pulse source as claimed in any one of the preceding claims wherein said filter means has an effective passband of 5 Angstroms or less.

8. An optical pulse source comprising a semiconductor laser source, gain-switching means for producing a gain-switched output from said laser source, a distributed feedback laser structure, and laser amplifier means, wherein the optical pulse source is arranged in operation such that a gain-switched output of said laser source is fed through said distributed feedback laser structure while said structure is biased below threshold, the resultant output from the distributed feedback laser being fed through said laser amplifier means to produce an optical output pulse.

9. A method of generating optical pulses, the method comprising the steps of: gain-switching a semiconductor laser source; feeding an output of said source through a narrow passband filter means; and feeding the resultant output from the narrow passband filter means through a laser amplifier means to produce an optical output pulse.

10. A method of generating optical pulses, the method comprising the steps of: gain switching a semiconductor laser source; feeding an output of said source through a distributed feedback laser structure biased below threshold; and feeding the resultant output from the distributed feedback laser through a laser amplifier means to produce an optical output pulse.