KR20070028463A - Thermally controlled external cavity tuneable laser - Google Patents

Abstract

The present invention relates to an external-cavity tuneable laser including a gain medium (2) and a tuneable mirror (8), wherein at least the tuneable mirror is in thermal contact, with a thermally conductive platform (10). The tuneable mirror lays substantially horizontally on the thermally conductive platform significantly thereby improving the thermal contact of the tuneable mirror with the platform. The laser beam from the gain medium is directed onto the tuneable mirror, which is mounted substantially horizontally with respect to the thermally conductive platform, by means of a deflector (6) that deflects the beam or a large part of it towards one of the principal surfaces of the tuneable mirror. The resulting laser cavity is therefore a folded cavity. The thermally conductive platform is preferably thermally coupled to a TEC (11) that provides thermal control for the platform. In a preferred embodiment, the deflector is a beam splitter that deflects part of the incoming light and transmits the remaining part. A beam splitter as deflector a more compact laser assembly can be envisaged. According to a preferred embodiment of the invention, the portion of light transmitted through the beam splitter forms the output laser beam. In other words, the external-cavity laser outputs a laser beam on the side of the wavelength selective elements (the tuneable mirror and, if any, the channel grid), i.e., on the side of the front facet of the laser diode. With this laser design, a collimating lens to collimate the output laser beam is not necessary. ® KIPO & WIPO 2007

Description

Thermally controlled external cavity tuneable laser

The use of lasers as tunable light sources can greatly enhance the reconstruction of wavelength division multiplexing (WDM) systems or newly developed high density wavelength division multiplexing (DWDM) systems. For example, other channels can be assigned to a node by simply tuning the wavelength. Tunable lasers may also be used to form wavelength routing, ie, virtual private networks based on photonic networks.

Other approaches can also be used to form a tuned laser such as a distributed Bragg Reflector Laser, a VCSEL laser with a mobile top mirror, or an external cavity diode laser. Externally tunable lasers offer several advantages such as high output power, wide tuning range, good side mode suppression and narrow line width. Various laser tuning devices have been developed to provide external cavity wavelength selection, such as mechanically adjustable or electrically activated intracavity selector elements.

U. S. Patent No. 6,526, 071 describes an externally tunable laser that can be used in telecommunications applications to generate a center wavelength for any channel on the International Telecommunications Union (ITU) grid. The disclosed tuned laser includes a gain medium, a grating generator and a channel selector, both of which are located in the optical path of the beam. The grid generator selects the cavity's periodic longitudinal mode at an interval corresponding to the channel interval and rejects the neighboring modes. The channel selector selects a channel in the wavelength grating and rejects other channels.

In order to accommodate increasing optical communication traffic, DWDM systems with channel spacings of 50 GHz and eventually 25 GHz are under development. DWDM systems with channel spacing of 50 GHz typically require a frequency accuracy of ± 2.5 GHz, while systems with 25 GHz typically require an accuracy of ± 1.25 GHz. As DWDM is used, narrower channel spacing, accuracy, and control have become more important in the positioning of tunable elements coupled with the transmitter laser over the entire tuning and operating temperature range. Non-optimal positioning of the tuned device results in loss of transmitter space and reduced output power.

Spatial misalignment of the optical components of the laser device is caused by temperature fluctuations due to expansion and contraction associated with various optical components, which degrades wavelength stability and generally degrades laser performance. Laser response generally needs to be stabilized over a relatively wide temperature range, ranging from -10 to 70 ° C. To ensure thermal stability, many communication laser devices are mounted on a common platform that exhibits high thermal conductivity and is thermally controlled by one or more thermoelectric coolers (TECs). Temperature control makes it possible to maintain the thermal alignment of the optical components.

U.S. Patent No. At 6,724,797, an external cavity laser apparatus is disclosed in which selective thermal control is applied to an optical component having a high susceptibility to thermal misalignment. Temperature sensitive optical components, gain medium and light output assembly, are mounted on a thermally conductive substrate. The TEC is coupled to the substrate to allow the gain medium and output assembly to be thermally controlled separately from the end mirrors and other optical components of the cavity laser. External components thermally insulated from the thermally conductive substrate may include a channel selector and a tunable assembly.

The Applicant noted that the thermal stability of the channel selector and / or the ITU grating element can be very important, especially when large accuracy is required in the wavelength specification of the laser output.

Laser devices that utilize active thermal control of external cavity lasers to minimize cavity losses and provide wavelength stability are disclosed in US Patent Application No. 2003/0231666. The disclosed device has a gain medium thermally coupled to a thermally conductive platform. The platform is coupled to a TEC that controls the temperature of the platform through thermal conduction.

Intel C-Band Tunable Laser is optional, as described in the Intel® C-band Tunable Laser, Performance and Design White Paper, released May 2003 at www.intel.com/design/network/papers/TunableLaser.pdf Etalon-based thermally actuated tuned lasers are used to achieve single-mode operation at possible wavelengths. The wavelength filter has two silicon etalon filters with slightly different periods to enable the "Vernier effect". The tuning filter is thermally operated and any wavelength in the C band is undertaken with the small temperature control of the individual etalons. "Vernier" tuning of the output wavelength of an external cavity laser to a selected wavelength on a wavelength grating is described in US patent application no. 2002/0126345.

Applicants have observed that, particularly for wavelength switches occurring between non-adjacent channels, a relatively long tuning time may be required to ensure that thermal tuning is within the selected wavelength. Relatively long sync times are not always compatible with the more stringent specifications of laser transmitters for WDM or DWDM applications, which can require tens of milliseconds of sync time.

Wavelength selection and tuning of the laser can be performed using an active tunable mirror. Electro-optical controlled devices that use liquid crystals and can be used as active tuned mirrors are described in US Pat. 6,215,928. The lasing wavelength of the laser is determined by the active tunable mirror to be the resonant wavelength of the mirror. The resonant wavelength can be shifted by varying the voltage or current applied to the electro-optically controlled device.

A.S.P. Chang et al., Technical Digest, CTuM 34, of Conference on Lasers and Electro-Optics (CLEO), June 2003, "A Novel, Low-cost Tunable Laser Using a Tunable Liquid-Crystal Subwavelength." Resonant Grating Filter " discloses a tuned laser composed of a tuned liquid crystal subwavelength resonant grating as a gain medium and a wavelength selective mirror.

Applicants have observed that temperature stabilization of a tuning mirror is particularly desirable in the case of a tuning mirror comprising a liquid crystal because the properties of the liquid crystal can vary due to thermal variations.

When an external cavity laser device is assembled, the optical components in the laser cavity need to be carefully aligned with respect to the laser beam to create a controlled external cavity for the lasing and to reduce cavity loss. In tunable lasers for WDM and DWDM applications, optical components typically have to be positioned with micron alignment tolerances or even submicron tolerances. For precision optical element alignment, passive or active optical alignment can be used. Optical component attachment may be performed during or after alignment by laser welding or using soldering or bonding.

The optical parts can be fixed in an optically aligned position in front of the laser by holding and aligning the members. The retaining and aligning members are usually fixed to the platform by laser welding. U.S. Patent No. 6,690,708 describe a semiconductor laser module in which a laser chip is fixed to a substrate and a first lens and an optical isolator are disposed between the laser chip and an optical fiber for receiving light emitted from the laser chip. A lens holder fixed with the first lens and a housing fixed with the light shield are laser-welded and fixed to the holding portions of the lens holder retainer and the light shield retainer, respectively.

In order to prevent displacement shifts, the retaining and aligning members are typically made of a material having a low coefficient of thermal expansion, such as Kovar®, Invar® 36 or Alloy 42.

Japanese Patent Application No. 2000-012955 discloses a self injection synchronous laser having a phase conjugated mirror for reflecting a portion of the output light with a beam splitter. It is not necessary to adjust the angle and distance of the mirror because the phase conjugate mirror returns light which does not change when the light distance changes.

The present invention relates to an external cavity tuned laser comprising a gain medium and a tuned mirror, wherein said at least one tuned mirror is in thermal contact with a thermally conductive platform.

Preferably, the gain medium is a semiconductor gain chip. The laser diode is preferably located on the thermally conductive base to dissipate heat generated in the laser diode during operation. More preferably, both the gain medium and the tuned mirror are located on the same thermally conductive platform to further improve the accuracy of the light alignment.

Preferably, the tuned laser comprises a channel assignment grating element. The channel assignment grating element is preferably a Fabry Parrot (FP) etalon constructed and arranged to define a plurality of equally spaced transmission peaks. In applications for WDM or DWDM communication systems, the transmission peak spacing, i.e., the free spectral range of the grating element, corresponds to the ITU channel grid, e.g. 200, 100, 50 or 25 GHz.

When appearing in a laser cavity with a channel assignment grating element, the tuned mirror serves as a tuner that identifies the peaks of the channel assignment grating element. For single mode laser emission, the longitudinal cavity mode should be located above the maximum of one of the grating transmission peaks (one selected by the tunable mirror). In this way only certain frequencies pass through the grid and other frequencies competing with neighboring common modes are suppressed. The wavelength selectivity of the tuned mirror is preferably achieved by an electrical signal. The tuned mirror of the present invention preferably comprises an electro-optic tuned material, more preferably a liquid crystal (LC) material.

The tuned mirror preferably has two major surfaces that are substantially parallel to each other. One of the two major surfaces receives the incident beam. In a preferred embodiment, the tuned mirror comprises a substrate, a planar waveguide formed on the substrate and a diffraction grating optically interacting with the waveguide. The diffraction grating and the planar waveguide form a resonance structure. A cladding layer of electro-optical tunable material is placed on the planar waveguide.

Applicants have noted that microrad alignment accuracy is often needed when mounting a tuned mirror in front of a gain medium. In order to align and maintain the tunable mirror with respect to the laser beam, it may be highly desirable to use a holding member for receiving the tunable mirror to hold the tunable mirror at a position substantially perpendicular to the laser beam. Fixing by active light alignment and laser welding allows to achieve a desired alignment tolerance and long term mechanical stability. Attachment of the holding member by an adhesive such as epoxy or solder is undesirable because the adhesive or solder cannot guarantee reproducibility and long-term mechanical stability.

Materials with high thermal conductivity are generally not suitable for laser welding because the heat is dissipated away from the heat source (ie, the welding area), resulting in a relatively large molten pool. Ideally, metals with low thermal conductivity along with low coefficient of thermal expansion are suitable for laser welding. Many metal alloys are in this category, such as platinum alloys, Kovar®, Invar®, Alloy 42, and beryllium copper.

The low coefficient of thermal expansion for materials forming support structures for photons aligned to the laser beam minimizes misalignment due to thermal variations that can also occur to a small extent in the laser package when TEC is used to stabilize the temperature. desirable.

Applicants have observed that thermal contact between the tunable mirror and the thermally conductive platform can be significantly suppressed when a support member with a low thermal conductivity (eg, made of a material suitable for laser welding) is disposed between the mirror and the platform. Applicant noted that the temperature stabilization of the tuning mirror is important because the properties of the liquid crystal can vary due to thermal fluctuations, especially in the case of the tuning mirror including the liquid crystal. Preferably, the thermal variation in the tuned mirror should not exceed 0.5 ° C. in the temperature range of −10 to 70 ° C. to minimize wavelength and power drift during laser operation. In a preferred embodiment, the thermal variation within 0.2 ° C. corresponds to a frequency shift no greater than about ± 4 GHz and can accept channel spacings of 50 GHz.

The Applicant has found that the thermal contact of the tunable mirror with the platform is greatly improved by placing the tunable mirror substantially horizontally on the thermally conductive platform. The substantially horizontal position of the tunable mirror on the platform means that the mirror lies on one of the major surfaces along the same plane of the platform major surface or within degrees relative to the plane of the platform major surface. It is understood that the tunable mirror is in direct contact with the thermally conductive platform or that a substrate (or holder) of thermally conductive material can be disposed between the tunable mirror and the platform.

The laser beam from the gain medium is directed to a tuned mirror mounted substantially horizontally to the thermally conductive platform by a deflector that deflects most of the beam towards either the beam or the major surface of the tuned mirror. Thus, the resulting laser cavity is a clamshell cavity.

The tuned mirror can be mounted directly on the thermally conductive platform without the need to align to the laser beam and in particular without the need to use a support structure suitable for alignment. For example, a tunable mirror is fixed on the platform by adhering the back (main) surface to the platform or to a mirror holder / substrate fixed to the platform. The tuned mirror can in turn be glued to the holder / substrate. Alternatively, at least a portion of the back side of the tunable mirror or mirror holder / substrate may be metallized, such as gold plated and then soldered to the surface of the platform.

Within the laser design according to the invention, it is a deflector which is required to be aligned with the laser beam, preferably by using active light alignment. In general, fixing and aligning a structure with low thermal conductivity that was necessary to include or support a tunable mirror during alignment is unnecessary because no alignment to the laser beam is now required. Thus, good thermal contact between the tuned mirror and the thermally conductive platform can be achieved.

The thermally conductive platform is preferably thermally coupled to the TEC which provides thermal control for the platform.

In a preferred embodiment, the deflector is a beam splitter which deflects part of the incident light and transmits the remainder. Applicants have found that more compact laser assemblies can be devised using beam splitters as deflectors. According to a preferred embodiment of the present invention, a portion of the light transmitted through the beam splitter forms an output laser beam. In other words, the external cavity laser outputs a laser beam on one side of the wavelength selection element (tuned mirror and, if present, a channel grid), that is, on the front side of the laser diode. This laser design eliminates the need for a collimating lens aiming at the output laser beam, which means that both the internal cavity collimating lens is emitted from the gain medium and the beam transmitted through the beam splitter coupled to the output optics, such as an optical fiber. This is because it functions as an aiming lens. Preferably, the beam splitter has a transmittance in the range of 70-90% to ensure a relatively large laser output power.

When the output light is emitted from the reflective backside of the semiconductor gain chip (gain medium) forming one of the end mirrors of the laser cavity, the power control of the laser output is generally in the form of a laser deflector in front of a partial deflector, eg a laser diode. This can be done by placing the beam splitter outside. The deflector redirects the output light to the monitor photodetector just prior to fiber coupling. Applicant noted the following. The back side of the laser diode generally exhibits a transmittance of about 80-90% to effectively function as an end mirror for external cavities. If the beam splitter is arranged to deflect light emitted from the backside for power monitoring, it should be as small as possible so that some of the beam intensity from the light output does not disadvantage the output power. For example, the beam splitter in front of the laser diode may have a separation ratio of 98% / 2%. That is, 2% of the beam intensities make up the test beam directed to the photodetector. Thus, the optical power incident on the photodetector is relatively low, which can lead to a loss of accuracy in monitoring power. The relatively low optical power of the test beam is especially required when the wavelength adjustment of the laser beam is to be carried out using a light control system which analyzes the light filtered through a filter having wavelength dependent transmission or gain attenuation by at least one photodetector. Is particularly disadvantageous.

Applicants have found that the light transmitted by the beam separator can be monitored by one or more photodetectors using the beam separator as a deflector in the laser cavity. This is particularly advantageous in the case of a laser assembly design in which the laser output light is emitted from the light transmitted through the beam splitter. In this case, the photodetector (s) receive a light beam having an intensity almost equal to that of the output beam. In particular, when the transmittance of the beam splitter is selected in the range of 70-90%, monitoring can be performed by analyzing a beam having a relatively large optical power. Moreover, monitoring can be performed without the additional action of further deflecting the optical element to reduce the test beam. The photodetector (s) may be positioned to receive light substantially perpendicular to the output beam. A photodetector, such as a photodiode, may for example be located on one of the surfaces of the beamsplitter which does not face the tuning mirror and which does not receive the laser beam from the gain medium.

Preferably, all the elements of the laser assembly can be mounted on a common thermally conductive platform. In a preferred embodiment, an output optic comprising a focusing lens and an optical fiber is integrated into the laser assembly. That is, the output optics are mounted on the platform on which the laser assembly is mounted.

Preferably, the tuned laser has a relatively short cavity length, ie at most 12 mm.

The tuning time of the thermally controlled laser system according to the present invention may be less than 0.20 ms.

1 is a schematic diagram of a tuned laser according to a first embodiment of the present invention.

Fig. 2 is a schematic diagram of the outer longitudinal cavity mode A, the transmission mode B of the channel assignment lattice, the channel selector, i.e. the bandwidth C of the tuned mirror.

3 is a block diagram of a laser assembly according to a second embodiment of the present invention.

4 is a schematic diagram of a tuned laser according to a third embodiment of the present invention.

5A shows a support structure for a deflector in a laser system according to an embodiment of the invention.

Figure 5b shows a support structure for the deflector in the laser system according to another embodiment of the present invention.

6 is a block diagram of a portion of a laser assembly according to a fourth embodiment of the present invention.

7 is a schematic diagram of a tuned laser according to a fifth embodiment of the present invention.

8 is a schematic diagram of a tuned laser according to a sixth embodiment of the present invention.

A tunable laser system according to a preferred embodiment of the present invention is schematically illustrated in FIG. The laser system 1 is a laser assembly that fits into a package 7, such as a 14-pin butterfly package, with a boot 16 for insertion of an optical fiber, such as an optical pigtail 15. It is provided. The laser assembly comprises a gain medium 2, a collimating lens 3, a channel assignment grating element 4, a deflector 6 and a tunable mirror 8. The laser system has a thermally conductive platform 10. The gain medium 2 is disposed on a semiconductor diode, such as an InGaAs / InP multiquantum well Fabry Parrot (FP) gain chip, especially designed for external cavity applications. The diode has a back facet 22 and a front facet 23. The front face 23 of the diode is the cavity inside and has an antireflective coating. Preferably, the gain chip waveguide is curved to have an oblique incidence to the front side to further reduce back reflection. The rear face 22 is partially reflected and serves as one of the outer cavity end mirrors. The reflectivity of the backside can, for example, be between 10% and 30% to allow for relatively high laser output power.

 In the laser cavity, the beam from the diode front face 23 is aimed by the aiming lens 3 which aims the beam to form the optical path 25. The aimed beam is impinged on the channel assignment grating element 4.

The channel assignment grating element 4 is preferably an FP etalon, for example a solid or air layer etalon. The laser can be designed in such a way that the operating wavelengths are aligned with the ITU channel grating. In this case, the laser wavelength is centered on the ITU grating via FP etalons 4 that are constructed and arranged to form a plurality of equally spaced transmission peaks. In an application of a WDM or DWDM communication system, the transmission peak spacing, i.e., the FSR of the grating element, corresponds to an ITU channel grid, for example 200, 100, 50 or 25 GHz.

Preferably, aiming lens 3 is located in a cavity substantially perpendicular to light path 25. Preferably, the FP etalon 4 is disposed in the cavity slightly, for example, at an angle of inclination of 0.5 ° to the perpendicular of the optical path 25 so that the reflected light of the FP etalon does not return back to the laser diode.

Next to the FP etalon 4, the laser beam hits the deflector 6 which deflects the beam 25 on the tunable mirror 8 along the optical path 26. The tunable mirror 8 reflects the optical signal back to the deflector 6 and then deflects the optical signal back to the gain medium 2. The deflector 6 is in this embodiment a planar mirror, for example a gold coated silicon slab.

Although not shown in FIG. 1, the deflection device 6 can be housed in a holder which can be fixed to the thermally conductive platform 10 by known techniques. Preferably, the deflector is aligned with the laser beam by active light alignment technology.

The tunable mirror 8 is an electro-optical element in which tunability is achieved by using a material having a voltage dependent refractive index, preferably a liquid crystal (LC) material. The tuned mirror serves as a delicate tuner that distinguishes the peaks of the FP etalon. The tuned FWHM bandwidth is not less than the lattice etalon's FWHM bandwidth. For longitudinal single mode operation, the transmission peak of the FP etalon corresponding to a particular channel frequency must select, ie transmit, a single common mode. Thus, FP etalons should have a finesse defined as the division of FSR by FWHM that suppresses the common neighboring mode between each channel. For single mode laser emission, the longitudinal cavity mode should be positioned beyond the maximum of one of the etalon transmission peaks (one selected by the tuning element). In this way, only certain frequencies pass through the etalons and other competing neighboring common modes are suppressed. The electrical signal provided for the function of the tuned mirror comprising the LC material is an alternating voltage to prevent deterioration of the LC due to dc stress.

FIG. 2 shows various modes in a laser cavity with FP etalons having a plurality of passbands arranged in an ITU channel grating as spatially selective loss devices. (A) shows the cavity mode induced by the resonant outer cavity, (B) shows the mode of the FP etalon with the position of the peak fixed to the standard ITU lattice, and (C) the tuning element That is, the pass band of the tuning mirror. In the example shown, the FSR of the grating FP etalon corresponds to the spacing between the grating lines of the 100 GHz ITU grating.

Referring to FIG. 1, the laser cavity has an optical path length that sums the optical path 25 between the deflector 6 and the rear face 22 of the gain medium and the optical path 26 between the deflector and the tunable mirror 8. It is a folded resonant cavity.

The laser beam is coupled to the external cavity by the partially reflective back surface 22 of the laser diode 2. Preferably, the aiming lens 20 can be positioned along the optical path of the laser output beam. In this embodiment, a beam splitter 18 located behind the lens 20, for example, 98% / 2% tap, intercepts a portion of the output light as a test beam directed at the power control photodetector 19. The light focusing lens 14 directs the light passing through the light shield 17 to the optical fiber pigtail 15. The light shield 17 is used to prevent the back reflection light from passing back to the external laser cavity and is generally a selection element.

In a preferred embodiment, the laser assembly is designed to produce substantially single longitudinal and preferably single transverse mode radiation. Longitudinal mode means simultaneous lasing at multiple individual frequencies in the laser cavity. The transverse mode corresponds to the spatial change in the cross section of the beam intensity in the transverse direction of the laser radiation. In general, a suitable choice of gain medium, such as a commercially available semiconductor laser diode comprising a waveguide, ensures single space mode operation or single lateral mode operation.

The laser is configured to emit output radiation at a selected one of a plurality of equally spaced output frequencies that match equally spaced channel frequencies in a WDM or DWDM system. The laser is operated to emit a single longitudinal mode output depending on the spectral response of the optical elements in the cavity and the phase of the cavity.

The laser assembly is disposed on a common thermally conductive platform 10 that functions as a mechanical reference base for the optical device. The use of a common optical bench is desirable because it minimizes design complexity and simplifies alignment between the optical components of a tuned laser. Platform 10 may be made of any thermally conductive material, such as aluminum nitride (AlN), silicon carbide (SiC), and copper-tungsten (CuW).

Although not shown in FIG. 1, lenses 3 and 20 are mounted to the platform by respective mounts.

The tuned mirror 8 lies substantially horizontal with respect to the major surface of the thermally conductive platform 10. In a preferred embodiment, the tunable mirror is located on a thermally conductive substrate or holder that can receive the tunable mirror (indicated by 9 in FIG. 1). If the platform 10 is made of a metallic material, the substrate or holder 9 must be made of an electrically insulating material to prevent electrical contact between the synchronized mirror and the platform since the synchronous mirror is usually biased during laser operation. . In a preferred embodiment, the holder 9 is made of AlN or SiC.

By placing the tunable mirror horizontally on the platform, thermal contact with the platform is maximized, while there is no need to actively align the mirror with respect to the laser beam during the laser assembly. Preferably, during the laser assembly, the tuned mirror is bonded onto a thermally conductive platform with a thermally conductive epoxy such as silver (Ag) filled epoxy, or a silicone resin. Alternatively, the tunable mirror is disposed on a substrate that is received in a holder or bonded to a thermally conductive platform. The mirror may be attached to the substrate or the holder.

Preferably, the deflector 6 is required for aligning the laser beam by photoactive alignment technology. The deflector may be protected in the cavity, for example by a support structure secured to the platform 10 (not shown in FIG. 1). The deflector may be attached to the support structure, or may be bonded, if at least partially metallized.

If the thermally conductive platform is made of a reflective material, such as a gold (Au) plated ceramic base, an FP cavity can be formed between the tunable mirror and the platform, which causes pseudo reflection in the laser cavity. To minimize this problem, the tilting mirror is slightly tilted with respect to the major top layer of the platform such that a small angle, such as 0.5 ° or 1 °, is formed between the plane of the principal surface of the tunable mirror and the plane along the platform plane. It is preferable. Alternatively, the reflectivity of the platform face can be reduced at least at the bottom and in the peripheral region of the tunable mirror, for example by roughening the surface of the platform by laser cutting.

In order to stabilize the temperature, the FP etalon 4 is preferably housed in the thermally conductive housing 5 to facilitate thermal contact with the platform 10. The housing 5 of the etalon preferably exhibits a thermal conductivity not less than 150 W / mK and is welded directly onto the platform, for example by die attach using a silver filled epoxy resin, a low temperature weld alloy or a thermally conductive epoxy. For example, the etalon holder can be made of CuW alloy. If the FP etalon or its holder, if any, is bonded or welded to the platform, adjustment of the optical alignment of the etalons to the laser beam can be performed by actively aligning the collimating lens 3.

The aiming lens 3 can be mounted to the platform 10 by a support structure (not shown), which can also be used for alignment of the lens to the laser beam.

The gain chip 2 is preferably positioned by, for example, bonding to the thermally conductive submount 21 to position the emitted beam at a convenient height relative to other optical elements and to further improve heat dissipation. The thermally conductive submount 21 is located on the thermally conductive platform 10.

The thermally conductive platform 10 is preferably thermally coupled to the TEC 11 which provides thermal control to the platform. For example, platform 10 may be bonded or welded to TEC 11. Thermal conductivity by means of a thermal sensor device 12, such as a thermistor or thermocouple, located on the platform and operatively coupled to the TEC to provide a control signal for cooling or heating the platform 10. Temperature monitoring of the platform is provided. The additional thermal sensor device may optionally be located on one or more optical elements of the laser assembly, for example a tunable mirror or FP etalon, if specific thermal control of the optical element is required.

Numerical simulations show that even when the dissipated power of the tuned mirror is as high as 50mW due to the applied voltage, the maximum temperature change of the laser assembly remains below 0.1 ° C over the temperature operating range of -10 to 70 ° C.

3 is a schematic diagram of a laser assembly according to a second preferred embodiment of the invention. The laser assembly 30 includes a gain medium 31, a collimating lens 32, an FP etalon 33, a deflector 34, and a tuning mirror 35. The gain medium 31 is disposed on a semiconductor diode having a rear face 41 and a front face 42. The front face 42 of the diode is antireflective coated while the rear face 41 is partially reflected. Preferably, the gain medium is a semiconductor gain chip having a gain chip waveguide bent to have an oblique incidence on the front surface to further reduce back reflection. The deflector of the clamshell laser cavity according to the second embodiment is a beam separator (BS) 34. Light emitted from the gain medium 31 is aimed by the aiming lens 32 and passes through the FP etalon 33. After passing through the FP etalon, light traveling along the optical path 37 hits the BS 34, where the light is partially redirected along the path 38 to the tunable mirror 35. Depending on the reflectivity of the tuning mirror, most of the light hitting the tuning mirror is reflected back to the BS, which in turn sends some back to the laser diode. The laser cavity is a clamshell with an optical path that combines the optical paths 37 and 38. The rear surface of the laser diode 41 and the rear surface of the tunable mirror 40 form an end mirror of the laser external cavity.

Thus, the BS reflects a portion of the incident light beam 37 which is deflected into the tunable mirror and is then sent back to the BS which returns the light to the laser diode. The remaining (preferably significant) portion of the light is transmitted by the BS in the direction of the optical path 39 and constitutes a laser output beam. Preferably, the transmission axis of the BS 39 is substantially parallel to the optical axis of the laser beam, ie the optical path 37. For example, the BS is a cube beam splitter that reflects an incident beam at 90 degrees. Preferably, the BS is sensitive to polarization of incident light.

Within this laser design, BS 34 preferably has a relatively large transmittance, preferably in the range of 70% to 90%, to ensure a relatively high output laser power. The optimal value of the transmittance of the beam splitter can be selected as a trade-off between cavity loss and minimization of gain ripple. The reflectivity of the back surface 41 of the laser diode 31 is preferably about 100%. Although the ratio of light reflected from the BS appears to be small in a preferred embodiment, good laser amplification in the laser cavity may be achieved by a semiconductor gain chip having a differential gain by a high output gain medium, for example at least 200 cm ⁻¹ .

According to the embodiment of Figure 3, the second transmission axis of the BS is substantially perpendicular to the incident beam. The light beam transmitted through the BS along the light path 43 may be used as a test beam to monitor the output power by simply placing a photodetector 36, such as a photodiode along the light path 43. For example, the semiconductor photodiode can be attached directly to the BS optical component by adhesive or soldering.

The photodiode 36 receives a light beam having the same intensity as the laser output beam along the path 39. Power monitoring in the laser design of FIG. 3 has the advantage that a relatively large proportion of the laser beam is detected by the photodetector, thereby increasing the accuracy of power variations or fluctuations during laser operation.

4 shows a laser system according to a third embodiment of the present invention. The same reference numerals are given to the elements of the tunable laser corresponding to the elements shown in FIG. 1 and the detailed description is omitted.

After passing through the FP etalon 4, the light impinges on the beam separator (BS) 45, where the light is partially redirected to the tuned mirror 8. Photodetector 46 receives a portion of the laser light, which is returned to the BS by a tunable mirror and then transmitted through the BS as a test beam for power monitoring.

The laser beam, ie, the laser light transmitted through the BS 45 along the optical axis direction of the laser output beam, is directed through the light shield 17 and then focused by the lens 14 onto the optical fiber 5. The design of the laser cavity according to the embodiment of FIG. 4 does not require a second collimating lens (lens 20 of FIG. 1) to aim the output beam at the light output assembly to couple to the optical fiber, thus compacting the cavity. It is important to note that there are benefits to improving Moreover, if power supervision is implemented, there is no need for another BS to output the output power.

Preferably, in the laser design of FIG. 4, the back side of the laser diode has a reflectivity greater than 90% and the front side has a 10 ^{− in} order to minimize gain ripple that can cause laser emission into an unselected channel. Has a reflectivity of less than ³ .

5A and 5B show two possible support structures for the beam separator 45 of FIG. 4. In this embodiment, the BS is a cube beam splitter. It should be appreciated that the support structure may be used to hold and secure the beam splitter in place according to another embodiment of the present invention. Similar support structures can also be used to hold and secure the planar mirror of FIG. 1 in place. In FIG. 5A, the support structure 80 consists of a U-shaped holder 81 and a U-shaped rectangular frame 82 to hold the beam separator 45, and is mounted and fixed in place on the thermally conductive platform 10. do. The beam separator 45 may be attached to the holder 81 in advance by epoxy or adhesive. The holder 81 can be fixed to the rectangular frame 82 after alignment with the laser beam. The inner surface of the rectangular frame 82 opens to leave space for the tunable mirror 8 lying horizontally on the platform 10. The rectangular frame 82 surrounds the synchronized mirror 8 while the holder 81 is located above the synchronized mirror.

5B shows another support structure for the BS 45. The support structure 85 consists of a U-shaped holder 83 for receiving the beam splitter 45 and a U-shaped rectangular frame 86 surrounding the tunable mirror 8. The holder 83 and the rectangular frame 86 are fixed to each other by the U bolt 87 and the U bolt support 88. For clarity, the U bolt 87 is also shown away from the rest of the support structure. A portion of the support structure 85 is fixed by laser welding through weld spots 84. The beam separator 45 may be attached to the holder 83 in advance with epoxy or adhesive. The support structure 85 enables alignment with five degrees of freedom (three axes and two radial directions) after securing the U-shaped rectangular frame 86 to the platform 10. Rectangular frame 86 may be attached to the platform by adhesive, soldering, or laser welding.

The alignment of the beam splitter by the support structure 80 or 85 is preferably performed by an active light alignment method known per se.

It is to be appreciated that the support structure of FIGS. 5A and 5B is given by way of example and many other geometric configurations for the support structure of the deflector are possible.

In a preferred embodiment, the tuned mirror is an electro-optical device comprising a waveguide formed on a substrate and a diffraction grating made of the same material as the waveguide, for example, formed on the waveguide. Above the diffraction grating, a cladding layer is formed which at least fills the gap of the diffraction grating. The cladding layer is made of a liquid crystal material having a wide range of refractive index that is electrically selectable. Two conductors, at least one of which is transparent, are located on opposite sides of the liquid crystal layer. A voltage source or current source is connected across two transparent conductors. An example of the structure of a tuning mirror is described in US Patent No. 6,215,928. Depending on the voltage or current applied to the voltage applied to the conductor, the tuned mirror reflects radiation only at a given wavelength (λ _TM ). Radiation at all other wavelengths passes through the tuned mirror. Thus, the tunable mirror functions both as a tunable selection element and as a cavity end mirror. The tuned mirror is driven with an alternating voltage V _TM to prevent degradation of the liquid crystal due to dc stress. The frequency of the applied voltage can range from 20 kHz to 200 kHz. The spectral response of the tuned mirror is a spectral line, e.g., with a linear form of a Lorentzian curve with a (FWHM) TM bandwidth centered at λ _TM and can range from about 50 GHz to about 250 GHz, preferably 70 to 100 GHz. Have a similar linearity. In certain embodiments, λ _TM can be tuned over a 80 nm range.

Laser having a lasing output wavelength, that is, the lasing channel is selected to correspond to the amplitude of the tunable mirror resonance wavelength λ _TM and the corresponding, in turn applied to the tunable mirror voltage V _TM of.

In other words, the selection of the emission wavelength (frequency) of the tuning laser by the tuning mirror is achieved by selecting a value corresponding to the applied voltage V _TM . The microwavelength adjustment can be derived from the analysis of the laser output power, for example by slightly varying the voltage V _TM applied to the tunable mirror and finding the maximum value in the output power. Preferably, the AC component of the optical power at the laser output and associated phase is such that the wavelength difference between the comode wavelength λ _CM and the peak wavelength of the tuned mirror λ _TM , ie the magnitude and signal for Δλ = λ _CM −λ _TM . It is measured for evaluation. In order to reduce or eliminate the wavelength difference Δλ, minimization of the AC component of the optical power is sought by changing the voltage V _TM .

For mode stability in the laser cavity, alignment of the cavity mode of λ _CM at the center of the transmission peak of the etalon in λ _FP must be achieved. Closed loop control to align the selected etalon peak and the lasing mode can be performed, for example, by adjusting the injection current I _LD of the gain medium, for example a laser diode. The change in the injection current of the laser diode induces a change in the refractive index of the gain medium and thus a change in the laser output phase. Centering of etalon peaks with cavity modes can be obtained by adjusting the I _LD and monitoring the laser output power. An algorithm that maximizes the laser output power can be executed for fine tuning of the cavity mode below the peak of the etalon mode.

Preferably, in order to align the tuned mirror to the selected cavity mode, the AC component of the output power is analyzed while maximizing the integrated output power is sought to align the cavity mode to the etalon peak. As described above, laser output power can be measured by a photodetector from a test beam separated from the output laser beam. In a preferred embodiment, the monitoring of the laser output power is performed by the photodiode of the configuration shown in FIGS. 3 and 4 for alignment of the tunable mirror to the cavity mode and the cavity mode to the etalon peak. Two control algorithms that operate continuously can be performed for this purpose.

Initial operating points for all channels on the ITU grid are stored in a lookup table. Every channel in the lookup table is associated with the voltage V _TM applied to the tuned mirror and thus the selectable channel wavelength λ _TM . The lookup table may also store the initial operating value of the injection current I _LD associated with the channel frequency.

The loop controller can execute control algorithms for frequency and mode control. For example, the loop controller includes drive optical feedback circuitry that includes a processor having appropriate program code and lookup tablet (s).

When the laser is on or the channel is switched, the driver reads from the lookup table the current I _LD applied to the laser diode and the voltage V _TM applied to the tunable mirror. The Applicant has observed that the parameters corresponding to a given lasing channel may vary for the parameters stored in the lookup table (s). If the laser is drift for some reason, eg by aging or changes in environmental conditions, the stored values may not match the selected channel frequency. This can occur in particular when the laser system is switched on after a very long period of inactivity and in particular switched to a temperature significantly different from the temperature at which it was initially aimed.

According to the fourth embodiment of the present invention, an absolute reference of the frequency value of the lasing channel is obtained. 6 shows a part of a laser assembly according to a fourth embodiment of the invention. Tuned mirror 52 and BS 53 are shown as elements of the laser assembly. As described above, a portion of the laser light incident on the BS is transmitted in a direction 55 opposite to the direction of travel toward the tuned mirror 52. The first photodetector 51 receives a portion of the light deflected along the direction 55 and monitors the optical power. To monitor the wavelength of the laser light, filter 54 receives the laser light and outputs filtered light having transmission that varies with the wavelength. More specifically, filter 54 has gain attenuation, for example, as a function of wavelength where the gain increases linearly as the wavelength increases. The filtered light is received by the second photodetector 50. By monitoring the amplitude ratio of the laser light received by the second photodetector to the amplitude of the laser light received by the first photodetector, the lasing wavelength can be determined with reasonable accuracy. By subtracting the lasing frequency, the lasing channel can be derived.

Preferably, the first and second photodetectors are photodiodes. In order to improve the compactness of the laser assembly, the first photodetector 51 and filter 54 may be attached to the surface of the beam separator by an adhesive. The second photodetector 50 may be adhered on the backside of the filter 54.

Determining the lasing wavelength by the configuration shown in Fig. 6 requires that the light output of the laser light incident on the photodetector be relatively large so as to have sufficient accuracy in the wavelength evaluation over the target wavelength range, for example, the C band. Should be. This is also due to the fact that the filtered light has wavelength dependent transmission or attenuation, that is, the gain of the filter can be significantly attenuated corresponding to the predetermined wavelength. For example, for a laser output beam having a power of 10 dBm, at least about 10% of the light incident on the beam splitter must be converted as a test beam corresponding to a BS having a separation ratio of 90% / 10% or less. The use of a BS with a separation ratio of 90% / 10% or less is hardly acceptable (i.e., because the laser output power can be quite disadvantageous when the light output assembly is aligned as in the embodiment shown in FIG. 1). The test beam deviates from the laser output beam by a beam splitter located in front of the rear of the laser diode).

7 illustrates a tunable laser system according to another embodiment of the present invention. The same reference numerals are given to the elements of the tunable laser corresponding to the elements shown in FIG. 4 and the detailed description is omitted. The light control system 60 includes a first photodetector 61, a second photodetector 62 and a filter 63 (having wavelength dependent transmission). The configuration of the light control system 60 is similar to the system described in more detail with reference to FIG. 6. Light control system 60 is preferably located on the beam splitter. The laser system of FIG. 7 includes a feedback system that allows closed loop control to ensure wavelength alignment and stability of the laser output beam. Thermal control of the thermally conductive platform is achieved using the thermal sensor device 12, such as a thermosistor, to provide the temperature feedback signal 69 to the microprocessor 68. In response to a temperature change or external input detected by the thermosistor 12, a control command for cooling or heating the platform 10 is provided by the feedback signal 71 and the variable current generator 66 of the TEC 11. Given through temperature control. The injection current I _LD of the laser diode 2 is controlled by the variable current generator 67 via the feedback signal 70. Wavelength tuning is performed by varying the voltage 64 applied to the tunable mirror 8 and controlled by the microprocessor by the feedback signal 73. A piezoelectric substrate 69 is under the tuned mirror 8 and is in thermal contact with the piezoelectric substrate layer substantially horizontally on a thermally conductive platform 10, for example, the platform. The piezoelectric substrate is thermally conductive, such as piezoelectric ceramic, to ensure good thermal contact between the tunable mirror and the platform. For example, piezoelectric ceramics are single layer ceramic sheets made of lead zircon titanate (PbZrTiO ₃ ) having a thickness of several mm. The change in voltage V _PZ applied to the piezoelectric material induces mechanical deformation in the material, which is generally the size of the micron ratio with respect to tens of volts voltage. This causes a change in the optical path influence of the outer laser cavity. That is, the optical path length can change in response to an electrical input to the piezoelectric material. Thus, in this embodiment, the centering of the etalon peak with the common mode can be obtained by adjusting V _PZ and by monitoring the laser output power. An algorithm that maximizes the laser output power can be performed for tuning of the cavity mode below the peak of the etalon mode. To this end, a feedback signal 72 can operate the variable power supply 65 on the piezoelectric plate 69 under the control of the microprocessor 68. The lookup table can store the initial operating value of V _PZ , which is related to the channel frequency. In this way, it is not necessary to adjust the injection current I _LD of the laser diode for tuning the cavity mode. However, I _LD can be used to control the output power. While the adjustment of V _PZ can be used to align the cavity mode to the etalon peak, the adjustment of I _LD can be performed simultaneously to maximize the output power. Two control algorithms that are subsequently operated may be performed for this purpose. By controlling both V _PZ and I _LD, output control enables another degree of freedom for co-mode centering and optimization of output power. Thus, power uniformity across the channel can be achieved.

Applicants have observed that mechanical stress on a package containing a laser assembly (7 in FIG. 7) can result in a reduction in optical coupling between the optical fiber 15 and the laser output signal, thereby reducing the output optical power. In order to improve the mechanical stability of the laser system, the light output assembly can be mounted on the thermally conductive platform 10 on which the optical elements of the laser cavity are mounted, as shown in FIG. 8. The light shield 75 and the focusing lens 77 are mounted on the holding structure 76 fixed on the thermally conductive platform 10. A joint tube 78 fixed to the holding structure 76 holds the optical fiber 15. Thus, according to the present invention, the optical element of the output light is integrated with the wavelength selection element 4, 8 and the gain medium 2 of the laser cavity so that misalignment between the laser output beam and the output optics that focus the output beam to the optical fiber is minimized. Is formed. Moreover, with this design, the thermal stability of the elements of the light output assembly can be ensured. Preferably, the optical fiber 15 is soldered to the fitting tube 78 by a hermetic sealant, such as glass solder. An external boot 79 integrated with or fixed to the package 7 surrounds the soldered optical fiber for extra mechanical support and to reduce fiber strain. Preferably, the fitting tube 78 is soldered to the outer boot 79 by a sealing sealant such that the optical fiber fitting and the outer boot form a sealing optical feed through.

Included in the Detailed Description of the Invention.

Claims (32)

A gain medium (2, 31) having an external cavity for forming a plurality of cavity modes and capable of emitting a laser beam to said external cavity; Tuned mirrors 8,35 aligned to the external cavity for placement of the laser emission frequency, substantially horizontally on thermally conductive platform 10, and in thermal contact with the thermally conductive platform. 52) A reflector R, arranged in the optical path of the laser beam emitted by the gain medium in the outer cavity to deflect a portion of the laser beam towards the tunable mirror 6, 34, 45, 55 A tunable laser system configured to emit output radiation comprising. The method of claim 1, And the deflector is arranged in the outer cavity such that the light reflected by the deflector to the tunable mirror has a direction substantially perpendicular to the direction in which the laser beam emitted by the gain medium moves. The method of claim 3, wherein Further comprising support structures (80,85) for holding the deflector, the support structures being positioned on a thermally conductive platform and having a U-shaped frame (82,86) surrounding the tunable mirror Laser system. The method of claim 2 or 3, And wherein said deflector is formed with a reflective layer tilted at 45 [deg.] With respect to the direction of the laser beam traveling from the gain medium. The method of claim 1, And a channel assignment grating element (4) arranged in said outer cavity to form a plurality of pass bands substantially aligned with corresponding channels of a selected wavelength grating. The method of claim 5, And the tuned mirror selects a channel arranged in the outer cavity to variably select one of the passbands to tune the light beam. The method of claim 6, And the channel assignment grating element is arranged in the laser cavity in the optical path of the laser beam emitted by the gain medium between the gain medium and the tuned mirror. The method according to any one of claims 5 to 7, And wherein said selected wavelength grating has a channel spacing in the range of 25 to 200 GHz. The method of claim 5, The channel assignment grating element (4) is a Fabry-Perot etalon (Fabry-Perot etalon) tuned laser system. The method of claim 9, The Fabry Parrot Etalon is a tuned laser system in thermal contact with a thermally conductive platform (10). The method of claim 10, The Fabry Parrot Etalon (4) is housed in a thermally conductive housing (5) in thermal contact with a thermally conductive platform (10). The method of claim 1, The gain medium (2) is in thermal contact with the thermally conductive platform (10). The method of claim 1, And a light output assembly, wherein the light output assembly is located on a thermally conductive platform. The method of claim 1, And said gain medium (2) is a semiconductor gain chip. The method of claim 1, The thermally conductive platform (10) is a tuned laser system in thermal contact with the thermoelectric cooler (11). The method of claim 15, And a thermal device sensor (2) thermally coupled to the platform (10) to provide temperature control of the platform. The method of claim 1, The deflector is a planar mirror (6) having a reflectivity R not less than 70%. The method of claim 1, The deflector comprises beam splitters 34, 45 for separating the laser beam emitted from the gain medium 2 into a beam portion reflected toward the tunable mirrors 8, 35, 52 and a beam portion transmitted through the beam splitter. 53) phosphorous laser system. The method of claim 18, And the beamsplitter is arranged in the external cavity such that the transmitted portion of the light beam is directed along the optical path direction of the laser beam emitted by the gain medium. The method of claim 18, And wherein said transmitted portion of said light beam generates said laser output light. The method of claim 18, And a first photodetector (36, 46, 51, 61) for monitoring the portion of the beam transmitted through said beam splitter. The method of claim 21, A filter (54,63) for receiving the laser light transmitted through the beam splitter and outputting the filtered light having transmission varying according to the wavelength of the received light; And a second photodetector (50, 62) for monitoring said filtered light. The method of claim 22, And said filter has gain attenuation as a function of wavelength of said laser light. The method according to any one of claims 18 to 23, And reflectance R of the beam splitter is comprised between 10% and 30%. The method of claim 1, The tunable mirrors 8, 35, 52 are electro-optical elements comprising a diffraction grating and a planar waveguide that optically interacts with the diffraction grating, wherein the diffraction grating and the planar waveguide form a resonant grating. system. The method of claim 25, The tuned mirror further comprises a light transmissive material having a refractive index that changes in response to an electric field applied to the light transmissive material that makes the tuned mirror electrically tunable, wherein the light transmissive material is tunable to the planar waveguide. A tuned laser system for forming a cladding layer. The method of claim 26, And the light transmissive material comprises a liquid crystal material. The method of claim 1, The tunable mirror is located on a holder (9) made of a thermally conductive material, and the holder is located on a thermally conductive platform (10). The method of claim 28, The holder comprises a piezoelectric plate (69). The method of claim 29, And a variable power supply (65) electrically connected to a piezoelectric plate capable of applying a voltage to the substrate to induce mechanical deformation. The method of claim 1, The laser system is configured to emit output radiation at a laser emission frequency in a single longitudinal mode. Emitting a laser beam to an external cavity; Deflecting the emitted beam to tunable mirrors (8, 35, 52); Thermally stabilizing the tunable mirror by thermal contact with a thermally conductive platform 10; Reflecting the emitted beam using the tunable mirror in a direction substantially perpendicular to the thermally conductive platform.

Images