IE20080222A1

IE20080222A1 - Laser devices

Info

Publication number: IE20080222A1
Application number: IE20080222A
Authority: IE
Inventors: David Goulding; Stephen P Hegarty; Andrew Ellis; Guillaume Huyet
Original assignee: Univ College Cork Nat Univ Ie
Priority date: 2007-03-30
Filing date: 2008-03-27
Publication date: 2008-11-26
Also published as: WO2008120179A1

Abstract

The light from two Quantum Dot (QD) lasers (QDL1 and QDL2) are collimated and reflected from an arrangement of mirrors and a blazed diffraction grating. The grating ensures single longitudinal mode operation. The output from each of the lasers is independently coupled into optical fibres and then reformed into nearly collimated free-space beams. The two beams are combined at a beam splitter and directed onto a video camera, the camera output having interference fringes demonstrating that the two sources have locked their phases. One or other of the QD lasers may be coupled to a high-bandwidth photodiode and the intensity dynamics is analysed with an oscilloscope and a microwave electronic spectrum analyser. The oscilloscope indicates there are no significant fluctuations in the laser intensity. To provide higher sensitivity analysis, an electronic spectrum analyser generates a Fourier transform of the laser intensity, the output of which lacks any signal above the instrument noise floor, demonstrating the high stability of the locked output.

Description

The invention relates to semiconductor laser devices, and particularly to generation of useful optical signals with stably phase locked light.

At present, generation of such light output is unsatisfactory because semiconductor lasers from quantum-well materials are highly unstable and incoherent when mutually coupled. This forces the use of expensive and inefficient alternative schemes.

The invention is therefore directed towards achieving phase locked emission in a more simple and/or scaleable manner.

SUMMARY OF THE INVENTION According to the invention there is provided a light transmitter comprising as least two mutually coupled quantum dot semiconductor laser devices.

In one embodiment, the laser devices have saturated ground states.

In another embodiment, the devices are coupled by optics including at least one mirror.

In a further embodiment, the optics include a grating.

In one embodiment, the grating is a blazed grating.

In another embodiment, the grating resolution is sufficient to induce single mode emission from the laser devices will side-mode suppression.

In a further embodiment, the side mode suppression is approximately 35 to 40 dB. -2IEO 8 0 2 22 In one embodiment, the grating has in the range of 580 to 620 lines per cm.

In another embodiment, the laser devices have different emission frequencies.

In a further embodiment, the transmitter includes a frequency shifting element to transform the output of a device to be close to the natural frequencies of another device.

In one embodiment, the transmitter comprises a power splitter for coupling light from a laser device to a plurality of the other devices.

In another embodiment, the quantum-dot laser devices run in a single lateral mode.

In a further embodiment, the devices are coupled by constraining the lateral width to 3 pm or less with the use of a narrow waveguiding structures.

In one embodiment, the maximum achievable modal round-trip gain multiplied by the effective facet reflectivities lies between 1 and 1.25.

In another embodiment, at least one of the laser facets has a reflectivity between 0.34 and 0.05.

In a further embodiment, the laser device active region comprises InAs/Ino lsGao ^As quantum dot layers separated by GaAs electronic barriers.

In one embodiment, the electronic barriers are embedded between GaAs spacers.

In another embodiment, the active region is embedded in Alo.7Gao.3As cladding layers.

In a further embodiment, the device is capped by a p+GaAs layer.

DETAILED DESCRIPTION OF THE INVENTION IE 0 8 ο 2 2 2 -3Brief Description of the Drawings The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which:Fig. 1 is a diagram illustrating an optical transmitter assembly of the invention; Fig. 2 is an image of fringes obtained via interference between two quantum dot (“QD”) laser devices of the assembly of Fig. 1; Fig. 3 is a time trace of the output from a QD laser when operated in a locked regime; Fig. 4 is an ESA trace of the output from a QD laser when operated in a locked regime; Fig. 5 is a diagram of an alternative transmitter assembly incorporating a frequency shifting element; Fig. 6 is a diagram of an alternative assembly, incorporating a power splitting element; and Fig. 7 is a diagram of a further assembly, incorporating a frequency dependent multiplexer.

Description of the Embodiments The following describes various laser transmitter assemblies, all generating phase locked light arising from coupling of the outputs from at least two quantum dot (“QD”) lasers. The QD lasers are highly damped and provide a stable phase-locked output with narrow linewidth. The applications include communications, beam combining, and beam steering. Κΰ so 222 -4Referring to Fig. 1, two QD lasers QDL1 and QDL2 are mutually coupled with a wavelength-dependent filter placed between them. The QD lasers have a saturated ground state - giving high relaxation oscillation damping.

QD lasers generate of light at optical frequencies ranging from the mid-IR to the ultraviolet. A feature of many of these QD materials is that they have a well-defined number of available energy states and a finite number of QDs in a given layer. As in all semiconductor lasers, there exists a resonance between the carrier and the photon populations, termed the relaxation oscillation. When the injection of current causes the majority of the states to be filled, the damping of the relaxation oscillation is greatly enhanced and the dynamics of the laser are altered. In particular the laser operates very stably under conditions where non-QD lasers are highly unstable, for example under external optical feedback or with the external injection of an optical field. Another highly unstable arrangement for non-QD semiconductor lasers is that of ‘mutual coupling’ where two lasers are symmetrically exposed to the other’s output. Indeed this method is prescribed as a means of reliably generating chaotic dynamics in non-QD lasers.

In QD lasers however, the mutual coupling scheme can be exploited to generate phase-locked emission from two or more QD lasers. In Fig. 1 such a scheme is schematically shown. The light from two QD lasers, QDL1 and QDL2, are collimated and reflected from an arrangement of mirrors and a blazed diffraction grating. The grating is included to ensure single longitudinal mode operation. The output from each of the lasers is independently coupled into optical fibres and then reformed into nearly collimated free-space beams. The two beams are combined at a beam splitter and directed onto a video camera. The camera output is shown in Fig. 2. The generation of interference fringes proves the two sources have locked their phases.

In order to demonstrate that the emission is stable, one or other of the QD lasers is coupled to a high-bandwidth photodiode and the intensity dynamics is analysed with an oscilloscope and a microwave electronic spectrum analyser. The oscilloscope trace is shown in Fig. 3, and the steady trace indicates there are no significant fluctuations -5ISu 80 222 in the laser intensity, in contrast with the non-QD laser situation. To provide higher sensitivity analysis, the electronic spectrum analyser generates the Fourier transform of the laser intensity. The output is shown in Fig. 4 and the lack of any signal above the instrument noise floor demonstrates the high stability of the locked output.

One possible embodiment of the scheme is where the devices were fabricated from a QD wafer whose active region was composed of 10 layers, each layer consisting of 33 nm of GaAs, 0.8 nm of InAs with a 5 nm InGaAs capping layer. The waveguide thickness is 421nm in total and optical confinement is provided by 1500 nm AlGaAs layers on either side. The device substrate is gallium arsenide. The facets are uncoated and the 4 micron wide ridge could be 0.6 mm for one laser and 1.0 mm for the second laser. The laser temperature and current is adjusted so that their free-running frequencies are no more than 1 GHz different, and the loss between the two lasers is adjusted so that it is less than 6 dB.

Further embodiments are shown in Figs. 5 to 7. In Fig. 5, both lasers have a different emission frequency. A frequency shifting element PM is placed between them so that when the light from QDL1 passes through PM, it is partially transformed to be close to the natural frequency of QDL2. Similarly, when the light from QDL2 passes through PM, it is partially transformed to be close to the natural frequency of QDL1. In this fashion, the two emissions will become phase-locked though at different frequencies.

In the embodiment of Fig. 6, a power splitting element PS couples a portion of the light from QD laser QDL0 to each of a number of other QD lasers, here QDL1-3. Similarly, a portion of the output of each of QDL1-3 is coupled to QDL0. The currents and temperatures of QDL1-3 are tuned to be close to the emission frequency of QDL0. In this way a multiplicity of lasers can be phase-locked to one another.

In the embodiment of Fig. 7, a multiplicity of QD lasers of natural frequencies differing by an approximately constant amount, F, are combined in a frequency dependent coupler AWG. The combined output is transformed in frequency shifter PM and returned to the AWG from a broadband mirror M. When the frequency shifter ΙΕ υ θ ο 2 2 2 -6is set to approximately F or a multiple of F or to a sub-multiple of F, phase locking can ensue between a multiplicity of lasers with different frequencies.

Example Referring again to Fig. 1, in one example the laser structure was grown on Si-doped (100) GaAs substrate by Molecular Beam Epitaxy (MBE) technique. The active region consisted of six stacked InAs/Ino ^Gao ^As QD layers separated by five 40 nmthick GaAs electronic barriers and embedded between two 55 nm-thick GaAs spacers. The 2.8 monolayers of InAs, the Ino.isGao.82As and the first 5 nm-thick GaAs barrier were grown at low temperature and the remaining 35 nm of GaAs barrier were grown at high temperature (600 °C). From an uncapped sample, the QD density measured by atomic force microscopy (AFM) resulted to be 3χ101θ cm-2. The active region was embedded in two Al0.7Ga0.jAs cladding layers. The n-type and p-type layers were 1.5 and 1.3 pm-thick respectively. The structure was finally capped by a 300 nm p+-GaAs layer. The doping level of the p-type layers was decreased from the contact layer to the undoped active region, with a doping range varying from 3xl019 cm-3 to 4xl017 cm-3. Narrow stripe ridge waveguides were formed by optical lithography followed by selective wet chemical etching, which was stopped above the active region. Thin silicon oxide was then deposited by plasma-enhanced chemical-vapour deposition in order to open a contact window over the narrow ridges through the dry etching of the oxide. Ti/Au p-type contact was thermally deposited on the topmost layer. Then the substrate was thinned down to around 100 pm and finally n-type metal AuGe/Ni/Au was deposited on the wafer backside. Laser cavities with different lengths were cleaved and no coatings were applied on the facets. The blazed grating BG is included to ensure single mode operation. Optical isolators (ISO) prevent external reflections from perturbing the system.

Experimental Arrangement Two QD lasers, one 600 pm long and one 1 mm long, were mounted on temperature controlled stages that gave optical access to both laser facets. The wavelength of one mode of one laser was measured with an optical spectrum analyser and chosen as a reference wavelength. The temperature and injection current of the second laser were then varied so that the wavelength of one of its modes matched the reference £θ8θ222 -τwavelength. The laser outputs from the facets facing one another with approximately 30 cm separation were collimated in free-space and coupled to one another. The laser outputs from the non-facing facets were passed through optical isolators and used to analyse the laser characteristics. The mutual coupling caused the devices to now emit a common spectrum. As the devices were not the same length, only the modes common to both free-running spectra were permitted. For the 1 mm device, this meant that every fifth mode was allowed in the coupled configuration, for the 600 pm device, every third mode lased. The side-mode suppression of modes adjacent to shared modes could be used to optimise the laser detuning, when this was done the relative suppression of non-common modes was on the order of 35 dB. The total laser intensities were coupled to high-frequency photodiodes and examined on an electrical spectrum analyser (ESA) and an oscilloscope. No intensity fluctuations were observed on either instrument. The laser output was combined with a tunable laser source using a fibre coupler and the beat tone examined on the ESA. The linewidth of an individual mode was 4 MHz (significantly narrower than the 30 MHz linewidth measured when the mutual coupling was removed) without any conspicuous noise peaks or bands. These multi-longitudinal mode results are greatly at variance with typically chaotic QW performance in comparable configurations, though they are insufficient to demonstrate phase-locking.

To address the issue of phase-locking, the separation between the two lasers was increased to approximately 1 m to accommodate a blazed grating with 600 lines per cm. The grating resolution was sufficient to induce single-mode emission from the two lasers with a side-mode suppression ratio of 37 dB. The linewidth was again measured using the tunable laser source and ESA, and was approximately 1 MHz, the extra narrowing a consequence of the increased photon lifetime we believe. The fibre coupled outputs of the two lasers were collimated and interfered on a video camera. Interference fringes shown in Fig. 2 are high contrast, indicating good phase locked operation. The single-mode laser output was again coupled to a high-speed photodetector, and the time-trace and intensity spectrum examined on an oscilloscope and ESA respectively. The deep intensity fluctuations of mutually coupled QW lasers are again absent and no increase in laser relative intensity noise was observed. -8IE Ο 8 Ο 2 2 2 The tolerance of the phase-locked state to detuning between the laser modes is critical to potential technological exploitation of this effect. The mutual coupling between the lasers was removed and one of the free-running laser modes was beaten with the tunable laser source. The beating frequency was recorded as a function of laser injection current, where the dominant effect will be Joule heating of the device. The coupling between the lasers was restored and the stable range of injection current recorded which corresponded to approximately 4 GHz when each laser was operated at twice threshold current. Outside this stable detuning range large amplitude instabilities could be observed though their description is outside the scope of this work.

It will be appreciated that the high stability of InAs/GaAs QD lasers when mutually coupled is a consequence of the highly saturated ground-state typical of this material system. This offers the possibility of manipulating the laser spectrum, of making coherent arrays of devices and of narrowing the laser linewidth.

The invention is not limited to the embodiments described but may be varied in construction and detail. In general terms, it is preferable that the QD lasers run in a single lateral mode, by constraining the lateral width to 3 pm or less with the use of a narrow waveguiding structures; that the maximum achievable modal round-trip gain multiplied by the effective facet reflectivities should lie between 1 and 1.25; that at least one of the laser facets should have a reflectivity between 0.34 and 0.05.

Claims

1. A light transmitter comprising as least two mutually coupled quantum dot semiconductor laser devices.

2. A light transmitter as claimed in claim 1, wherein the laser devices have saturated ground states.

3. A light transmitter as claimed in claims 1 or 2, wherein the devices are coupled 10 by optics including at least one mirror.

4. A light transmitter as claimed in claim 3, wherein the optics include a grating.

5. A light transmitter as claimed in claim 4, wherein the grating is a blazed 15 grating.

6. A light transmitter as claimed in claims 4 or 5, wherein the grating resolution is sufficient to induce single mode emission from the laser devices will sidemode suppression.

7. A light transmitter as claimed in claim 6, wherein the side mode suppression is approximately 35 to 40 dB.

8. A light transmitter as claimed in any of claims 4 to 7, wherein the grating has 25 in the range of 580 to 620 lines per cm.

9. A light transmitter as claimed in any preceding claim, wherein the laser devices have different emission frequencies. 30

10. A light transmitter as claimed in claim 9, wherein the transmitter includes a frequency shifting element to transform the output of a device to be close to the natural frequencies of another device. -10Κϋ8ΰ2 22

11. A light transmitter as claimed in any preceding claim, wherein the transmitter comprises a power splitter for coupling light from a laser device to a plurality of the other devices.

12. A light transmitter as claimed in any preceding claim, wherein the quantumdot laser devices run in a single lateral mode.

13. A light transmitter as claimed in claim 12, wherein the devices are coupled by constraining the lateral width to 3 pm or less with the use of a narrow waveguiding structures.

14. A light transmitter as claimed in any preceding claim, wherein the maximum achievable modal round-trip gain multiplied by the effective facet reflectivities lies between 1 and 1.25.

15. A light transmitter as claimed in any preceding claim, wherein at least one of the laser facets has a reflectivity between 0.34 and 0.05.

16. A light transmitter as claimed in any preceding claim, wherein the laser device active region comprises InAs/Ino.isGao ^As quantum dot layers separated by GaAs electronic barriers.

17. A light transmitter as claimed in claim 16, wherein the electronic barriers are embedded between GaAs spacers.

18. A light transmitter as claimed in claims 16 or 17, wherein the active region is embedded in AlojGaojAs cladding layers.

19. A light transmitter as claimed in any of claims 16 to 18, wherein the device is capped by a p+GaAs layer.