Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
  • H01S5/065—Mode locking; Mode suppression; Mode selection ; Self pulsating
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
  • H01S5/14—External cavity lasers
  • H01S5/141—External cavity lasers using a wavelength selective device, e.g. a grating or etalon
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/005—Optical components external to the laser cavity, specially adapted therefor, e.g. for homogenisation or merging of the beams or for manipulating laser pulses, e.g. pulse shaping
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
  • H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
  • H01S3/1062—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using a controlled passive interferometer, e.g. a Fabry-Perot etalon
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
  • H01S5/062—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes
  • H01S5/06233—Controlling other output parameters than intensity or frequency
  • H01S5/06246—Controlling other output parameters than intensity or frequency controlling the phase
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
  • H01S5/062—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes
  • H01S5/0625—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes in multi-section lasers
  • H01S5/06255—Controlling the frequency of the radiation
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
  • H01S5/065—Mode locking; Mode suppression; Mode selection ; Self pulsating
  • H01S5/0651—Mode control
  • H01S5/0653—Mode suppression, e.g. specific multimode
  • H01S5/0654—Single longitudinal mode emission
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
  • H01S5/065—Mode locking; Mode suppression; Mode selection ; Self pulsating
  • H01S5/0656—Seeding, i.e. an additional light input is provided for controlling the laser modes, for example by back-reflecting light from an external optical component
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
  • H01S5/14—External cavity lasers
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
  • H01S5/14—External cavity lasers
  • H01S5/141—External cavity lasers using a wavelength selective device, e.g. a grating or etalon
  • H01S5/142—External cavity lasers using a wavelength selective device, e.g. a grating or etalon which comprises an additional resonator
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/40—Arrangement of two or more semiconductor lasers, not provided for in groups H01S5/02 - H01S5/30
  • H01S5/4006—Injection locking
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S2301/00—Functional characteristics
  • H01S2301/16—Semiconductor lasers with special structural design to influence the modes, e.g. specific multimode
  • H01S2301/163—Single longitudinal mode
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/40—Arrangement of two or more semiconductor lasers, not provided for in groups H01S5/02 - H01S5/30
  • H01S5/4025—Array arrangements, e.g. constituted by discrete laser diodes or laser bar
  • H01S5/4031—Edge-emitting structures
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/40—Arrangement of two or more semiconductor lasers, not provided for in groups H01S5/02 - H01S5/30
  • H01S5/4025—Array arrangements, e.g. constituted by discrete laser diodes or laser bar
  • H01S5/4031—Edge-emitting structures
  • H01S5/4068—Edge-emitting structures with lateral coupling by axially offset or by merging waveguides, e.g. Y-couplers

Definitions

• the present invention relates to an optical source configured to obtain wavelength control in a single or plurality of optical emitters.
• DWDM dense wavelength division multiplexing
• Short-haul optical interconnect networks can also use DWDM techniques to increase the data capacity in fibre networks.
• the networks require stable control of the optical source wavelength in order to keep the optical signal within the passband of one or more optical filters within the DWDM network.
• a known solution to maintain the wavelength of a DWDM optical source is to design the optical source to emit a single longitudinal mode output, such as a distributed feedback laser (DFB) laser, and then control its wavelength by temperature or electrical current injection in the device.
• DFB distributed feedback laser
• Wavelength selectivity improvements in Fabry-Perot semiconductor lasers for wavelength- division multiplexed passive optical networks have been demonstrated using external sources to seed either amplified spontaneous emission ( H. D. Kim, S. G. Kang, and C. H. Lee, "A low-cost WDM source with an ASE injected Fabry-Perot semiconductor laser," IEEE Photonics Technol. Lett.
• an optical source comprising: a laser comprising: an optical gain section; and, an optical phase control section in optical communication with the gain section, and configured to be able to change the longitudinal mode frequency of the laser; and, an optical filter external to and in optical communication with the laser and configured to: receive light output from the laser; filter the said received light; wherein the optical source is configured to input the filtered light back into the laser.
• a laser comprising: an optical gain section; and, an optical phase control section in optical communication with the gain section, and configured to be able to change the longitudinal mode frequency of the laser
• an optical filter external to and in optical communication with the laser and configured to: receive light output from the laser; filter the said received light; wherein the optical source is configured to input the filtered light back into the laser.
• the laser may comprises a laser cavity section disposed between a first optical reflector and a second optical reflector, the laser cavity section comprising the optical gain section and optical phase control section.
• the optical source may be configured such that: at least one of the first or second optical reflectors is a partial optical reflector; and, the optical filter is configured to receive laser light from one of the said at least one partial optical reflectors; and, the optical source is configured to input filtered light into one of the said at least one partial optical reflectors.
• the optical phase control section may be controllable independently from the gain section.
• the optical phase control section or gain sections may be electronically controllable.
• the optical gain section may comprise a semiconductor material.
• the optical phase control section may comprise a semiconductor material.
• the semiconductor material of the optical phase section may comprise a larger band gap than the gain section semiconductor material.
• the first reflector may be a partial reflector and configured to: output a portion of the laser light to the filter; and, receive the filtered laser light from the filter.
• the optical source may further comprise a third optical reflector external to the laser and configured to: receive filtered laser light from the optical filter; and, reflect filtered laser light back towards the optical filter.
• the third optical reflector may be a partial reflector such that it is configured to transmit a portion of the light as an output of the laser source.
• the third optical reflector may reflect substantially all the filtered light back to the optical filter; and, the second optical reflector may be a partial optical reflector configured to transmit a portion of the light as an output of the laser source.
• the first reflector may be a partial reflector and is configured to output a portion of the laser light to the filter; and, the second reflector may be a partial reflector and is configured to receive a portion of the filtered laser light from the filter.
• the optical filter may comprise a passband filter response.
• the filter bandpass full width half maximum width may be less than the longitudinal mode spacing of the laser.
• the optical filter may comprise an optical thin film filter.
• the optical source may be configured to change the central wavelength of the pass-band response of the optical filter.
• the optical source may be configured to change the said central wavelength of the pass- band response by changing the angle of incidence that the output laser light subtends with the thin film filter.
• the optical source may comprise a plurality of said lasers, wherein the optical filter comprises a multi-wavelength optical filter comprising a plurality of optical pass-bands.
• the optical filter comprises an arrayed waveguide grating, (AWG).
• AWG arrayed waveguide grating
• the optical filter may be athermalised.
• the optical filter may comprise athermalising means.
• the laser may be athermalised.
• the laser may comprise athermalising means.
• the athermalising means may comprise any of mechanical movement, thermal or stress induced refractive index change in the filter.
• Any of the laser, optical filter and any third reflector may be optically connected at least partially by free space light propagation.
• the laser, optical filter and any third reflector may be optically connected at least partially by one or more optical fibres.
• the laser, optical filter and any third reflector may be optically connected at least partially by one or more integrated optical waveguides.
• the optical phase control section may be configured to receive a current or voltage and provide a change in refractive index of the phase control section in response to the received current or voltage.
• the optical source may further comprise an optical detector optically coupled to any of: a laser output; an output from the optical filter; a transmitted portion of a third optical reflector.
• the detector may be optically coupled to the filtered output of the optical filter.
• the detector may be configured to receive light from the source and generate one or more electrical signals; and, the phase control section may be configured to receive electrical signals, based at least in part upon the detector electrical signals, for changing the refractive index of the phase section.
• the optical source may further comprise electronic processing means configured to: receive said detector electrical signals; process the said detector signals and, transmit processed electrical signals to the phase control section.
• a low-cost semiconductor laser source (Fabry-Perot laser) to form a wavelength controlled, DWDM optical source, by using a longitudinal mode frequency tuning element in the laser chip and self-injection locking the laser to a local optical filter.
• the local filter can be athermalised such that the frequency of the source does not show a large variation in frequency over an extended temperature range.
• the frequency of the laser longitudinal modes is monitored and separately controlled via the phase section so that at least one longitudinal mode is injection locked over the extended temperature range.
• Figure 1 shows the laser chip 1 with phase control section 2
• Figure 2 shows the wavelength tuning of the laser modes
• Figure 3 shows a plurality of laser chips 7 with separate phase control sections 8
• Figure 4 shows how the wavelength of each laser chip source is monitored in power and frequency via one or more multi-wavelength bandpass filters
• Figure 5 shows a plurality of laser chips 7 with separate phase control sections 8 coupled optically via lens arrays 9, 10 to a multi-wavelength bandpass filter 11
• Figure 6 shows the laser chip 16 with two reflecting mirrors 17 and two electrical contacts: one for the phase section 18 and one for the gain section 19.
• Optical sources are presented configured to output laser light.
• the optical sources preferably output light in the wavelength range between 1530-1565nm (otherwise known as the C- band), but may in principle output laser light in any one or more of the following wavelength ranges: 1260 to 1360 nm (otherwise known as the O band), 1360 to 1460 nm (otherwise known as the E band), 1460 to 1530 nm (otherwise known as the S band), 1565 to 1625 nm (otherwise known as the L band), 1625 to 1675 nm (otherwise known as the U band).
• Such wavelength ranges are preferably used for telecommunication applications.
• the optical source can be configured in any optical wavelength band.
• the optical sources are to be used for short haul optical networks, such as those with optical fibre spans not exceeding 20km, however in principle the optical sources may be used for any optical data or communications or any other application requiring a thermally stable laser source.
• the output from the optical sources is typically continuous wave (CW) light, although in principle the light output could be pulsed, for example by directly modulating the source injection current, or using any other suitable modulating means.
• CW continuous wave
• phase control means may include using a thermo optical effect, for example where a current is supplied to a heating element adjacent to the phase control medium. Additionally or alternatively carrier (current) injection into a phase control medium may cause the refractive index change.
• phase control section comprises an electrical contact configured to apply a suitable current or voltage signal to the phase control medium of the phase control section to give rise to the desired refractive index change.
• the gain section comprises a semiconductor gain medium.
• the phase control section comprises a semiconductor phase control medium.
• any of the gain or phase control sections may comprise a plurality of different gain or phase control mediums respectively.
• any one or more of the semiconductor mediums comprise InP, or InGaAsP or AllnGaAs or InGaAs or any other suitable semiconductor material including quantum dots or organic semiconductors.
• any of the sections of the laser may comprise a waveguide.
• Any such waveguide may be any of a rib waveguide, ridge waveguide, buried waveguide or any other suitable waveguide structure comprising suitable semiconductor and/or dielectric materials that provide the functionality required by each of the said sections.
• the semiconductor medium in the phase control section preferably comprises a larger band gap than the semiconductor medium in the gain section.
• the bandgap of the phase control section may be any bandgap, for example the bandgap in the phase control section may be the same as the gain section.
• the phase control section and gain section are separately electronically controlled such that the current or voltage applied to the gain control medium is not applied to the phase control medium and vice versa.
• the gain medium is preferably electronically coupled to two electrical contacts.
• the phase control medium is preferably electronically coupled to two electrical contacts. One of the contacts may be a common ground or anode shared with the gain section.
• the laser cavity section may comprise a plurality of gain sections and/or a plurality of phase control sections.
• At least one of the first and second mirrors is partially reflecting in the wavelength range of interest.
• both of the mirrors may be partially reflecting.
• a partially reflecting mirror reflects any of 80% to 99% of the light incident upon it, more preferably 85- 95%.
• the reflectors reflect substantially the same amount of light across the wavelength range of operation such that the reflectors are broadband in optical response.
• the laser cavity comprises a semiconductor chip and the reflectors are physically attached to the ends of the chip, for example by coating the ends of the chip or by any other suitable means.
• the optical source comprises a laser optically coupled to an external optical filter that feeds back filtered output laser light into the laser, thus forming an external optical cavity.
• the external optical cavity may comprise a further optical reflector that reflects filtered light back into the optical filter.
• the further reflector is at least a partial optical reflector, or can be an optical reflector that substantially reflects 100% of the light incident upon it in the wavelength range of source operation.
• the external optical cavity is optically coupled to the laser such that light exiting the laser from a partially reflecting reflector is incident upon the optical filter.
• the optical filter may be a single optical filter device or a plurality of optical filters with a combined optical filtering characteristic.
• the optical filter preferably comprises a pass band filtering characteristic as shown in figure 2.
• an optical source comprising a semiconductor laser coupled optically to a narrow bandpass optical filter and reflector, wherein the filtered optical reflection injection-locks the laser to operate at a single wavelength determined by the optical filter passband.
• the laser contains a method of longitudinal mode frequency tuning wherein the frequency of at least one of the laser modes can be tuned to the same frequency as the optical filter. This enables the optical source wavelength to be controlled in such a way that the laser source emits a single wavelength.
• the laser is a Fabry-Perot semiconductor laser comprising at least two sections.
• the said laser is optically coupled to a narrow bandpass optical filter and a partial optical reflector.
• One section of the laser is used to control the optical gain of the laser chip, for example by injecting current into the said section, and the other section is used to vary the frequency and/or gain of the laser longitudinal modes.
• the phase control section and gain section are independently controllable.
• the bandpass of the optical filter is preferably narrower in frequency than the laser longitudinal mode spacing such that the filter only passes one of the laser longitudinal modes.
• the wavelength of the optical filter can be tuned, for example, by altering the angle of the filter with respect to the incident optical beam output from the laser.
• the optical filter is preferably a thin-film coated device coated on a plane parallel substrate such that the emergent angle of the optical beam is not deviated as the optical filter is angle tuned.
• the substrate for the coated filter is preferably a thermally compensated (athermalised) material such that the centre wavelength of the filter has a small variation with temperature, for example ⁇ 3pm/ °C.
• the wavelength of the optical source output can be maintained over temperature by controlling the phase section of the laser to vary the absolute frequency of the longitudinal modes. If the temperature range extends beyond the tuning range of a particular longitudinal mode, then an adjacent longitudinal mode can be used alternatively to comprise the optical source output.
• the Fabry-Perot cavity can also be athermalised by using an additional optical polymer waveguide section so that the longitudinal variation in mode frequency is controlled to be less than the mode frequency adjustment produced by controlling the phase section.
• a multi-wavelength optical source comprising a plurality of semiconductor lasers coupled optically to a multi-channel optical filter and common reflector, wherein the filtered optical reflection induces each laser to operate at a single and distinct wavelength determined by the optical filter frequency response.
• Each laser is configured to be able to tune the laser longitudinal mode frequency.
• the frequency of the laser modes of each laser can be tuned, independently of other lasers, to a particular central frequency of a passband of the multi-channel optical filter.
• Preferably each laser is tuned to a different passband central frequency of the filter.
• the Fabry-Perot laser cavity of any one or more of the lasers can also be athermalised by using an additional optical polymer waveguide section so that the longitudinal variation in mode frequency is controlled to be less than the mode frequency adjustment produced by controlling the phase section.
• a plurality of Fabry-Perot semiconductor lasers is provided, each comprising at least two sections.
• the said lasers are optically coupled to a multi-wavelength bandpass optical filter and a common partial reflector.
• One section of each laser is used to control the optical gain of the laser chip and the other section is used to vary the frequency (and gain) of each laser longitudinal modes.
• the bandpass of each channel of the multi-wavelength optical filter is preferably narrower in frequency than the laser longitudinal mode spacing such that the filter only passes one of the laser modes for each wavelength channel.
• the centre frequency of each channel of the multi-wavelength optical filter is determined by the design of the filter.
• the multi-wavelength optical filter is preferably thermally compensated such that the centre wavelength of each channel of the filter has a small variation with temperature.
• the wavelength of the optical source output can be maintained over temperature by controlling the phase section of the laser to vary the absolute frequency of the longitudinal modes. If the temperature range extends beyond the tuning range of a particular longitudinal mode, then an adjacent mode can be used alternatively to comprise the optical source.
• an optical source with a plurality of lasers, optically coupled to a first optical filter and optical reflector wherein a second optical filter is inserted in the optical path between the lasers and the first optical filter to allow a predefined selection of wavelengths to be reflected back into the lasers.
• Figure 1 shows the laser chip 1 with phase control section 2 coupled optically via a lens 3 to an optical bandpass filter 4 and a partial reflector 5.
• the output beam from the laser can be collimated or focussed with a lens 3 to allow retro-reflection of part of the beam from the partial reflector 5.
• the intensity of the reflected light is designed such that the reflected beam self-injection locks the laser to a wavelength determined by the optical filter 4.
• An optical detector 6 after the partial reflector 5 can be used to monitor the frequency of a longitudinal mode with respect to the optical filter frequency as part of an electronic control loop.
• the filter 4 can be tuned by position, angle, temperature or electrically to vary the wavelength of the optical source.
• the phase and frequency of the laser chip longitudinal modes are adjusted via the phase section 2 so that the frequency of one mode overlaps with the optical filter frequency.
• the optical filter 4 can be fabricated by thin- film deposition on a thermally matched substrate such that the variation in the centre frequency with temperature is below 3pm/ °C.
• Figure 2 shows the wavelength tuning of the laser modes 30 with respect to a bandpass filter at frequency ⁇ 1.
• Figure 2 shows two graphs showing laser modes 30 of a laser and the frequency dependent bandpass characteristic 32 of the optical filter having a central bandpass frequency o1.
• the left hand graph of figure 2 shows none of the initial laser modes of the laser having a frequency equal to ⁇ 1.
• the multi-wavelength bandpass filter 1 1 can be in the form of an optical waveguide device, such as an arrayed waveguide grating, or a free-space device such as an Eschelle grating.
• the multi-wavelength bandpass filter 11 has thermal compensation, such as mechanical movement or optical polymer infill, such that the centre wavelength of each filter channel has a low variation in centre frequency with temperature, for example below 3pm/ °C.
• Figure 5 shows a plurality of laser chips 7 with separate phase control sections 8 coupled optically via lens arrays 9, 10 to a multi-wavelength bandpass filter 1 1 , common partial reflector 12 and monitoring filters 13, where a separate optical filter 15 is inserted between the laser chips and the multi-wavelength bandpass filter 1 1.
• the filter 15 can be positioned between two sets of lens arrays 9, 10 such that the beam from the laser chips is collimated through the thin-film coated filter 15.
• the filter transmission response is designed to allow a defined range of wavelengths to be passed through the filter and hence select one or more wavelength ranges to be reflected back into the laser chips from the common partial reflector 12.
• the filter 15 can be used to select a single free-spectral range of the periodic array waveguide grating response.
• the filter 15 can be coated on a thermally matched substrate such that the variation in the optical filter response with temperature is small, for example below 3pm/ °C.
• An optical source comprising a semiconductor laser optically coupled to a local
• optical filter and optical reflector wherein the laser has a phase control section for tuning the laser longitudinal mode frequency with respect to the optical filter frequency.
• An optical source according to feature 4 where the wavelength of the source can be tuned using an optical tuning element.

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