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
  • H01S5/10—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
  • 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
  • H01S3/105—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the mutual position or the reflecting properties of the reflectors of the cavity, e.g. by controlling the cavity length
• • 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
  • H01S5/02—Structural details or components not essential to laser action
  • H01S5/0201—Separation of the wafer into individual elements, e.g. by dicing, cleaving, etching or directly during growth
  • H01S5/0203—Etching
• • 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
  • H01S5/02—Structural details or components not essential to laser action
  • H01S5/022—Mountings; Housings
  • H01S5/023—Mount members, e.g. sub-mount members
  • H01S5/02325—Mechanically integrated components on mount members or optical micro-benches
• • 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
  • H01S5/1039—Details on the cavity length
• • 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
  • H01S5/12—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 the resonator having a periodic structure, e.g. in distributed feedback [DFB] lasers
  • H01S5/125—Distributed Bragg reflector [DBR] lasers

Definitions

• the present invention relates to a tunable semiconductor laser with wafer emission, " comprising a laser cavity of reduced length so as to avoid mode jumps.
• Laser diodes are commonly used as tunable sources for applications in optical data transmission. In such a context, it is important to achieve great tunability, that is to say a wide range of wavelength of emission of the laser, without fear of the mode jumps which harm the quality of the optical transmission.
• the graph in Figure 1 illustrates the 'tunability' of a 'standard' DBR (Distributed Bragg Reflector) laser for this purpose.
• This graph gives the laser emission wavelength in nm as a function of the control current in mA applied to the electrode of the laser tuning section.
• the control current varies the number of carriers in the active layer of the tuning section and influences the emission wavelength of the laser.
• Such leaps in mode are also inevitable when using a “standard” tunable laser associated with a tunability technology known as MEM for “Micro-Electro-Mechanics”.
• MEM Micro-Electro-Mechanics
• One such technique is to vary the length of the resonant cavity of the laser. Indeed, it has been established that by varying the length L of the laser cavity, the emission wavelength ⁇ of the laser is varied. The variation of the length L is of the order of the variation of the wavelength ⁇ .
• Such a laser is illustrated, for example, in the publication "Tunable laser diode using a nickel micromachined external mirror" by Y.Uenishi, K.Honma and S.Nagaoka published in Electronics letters of June 20, 1996, Vol 32, No 13.
• FIGs 2a and 2b show schematically such a standard tunable laser MEM.
• the laser diode 10 comprises a cavity delimited by two reflectors, a fixed and a mobile.
• the laser diode 10 has a length of around 300 ⁇ m and has a fixed mirror, for example cleaved, at the front. It is the variation of the length of the cavity which allows the laser to be tuned.
• a mirror 20, Nickel is placed one behind the diode laser 10 mounted on a base 50 in order to vary the length of the resonant cavity by a micromechanical electrical control 25.
• tunability 20 nm could be obtained with an accuracy of 0.01 nm.
• the graph in Figure 2b clearly shows that this tunability is not continuous but presents numerous mode jumps.
• mode jumps must be avoided in applications to dense optical communication systems.
• These systems generally include wavelength multiplexers known by the English term "avelength Division Multiplexing" for WDM.
• Tunable lasers are often associated with these WDM systems, the density of which is constantly increasing.
• Tunable lasers known as, Distributed Feedback Lasers (DFB).
• DFB Distributed Feedback Lasers
• the tunability of these lasers is controlled by temperature. Their output power is large, around 30 to 40 mW, but their tunability is limited to only 2 nm, which is not enough for an application to WDM systems.
• the so-called “external cavity” lasers include a tunability mechanism consisting of an external optical mass element such as a filter or a network for example. These lasers are however too expensive and massive to be implemented in WDM telecommunication networks. In addition,> their reliability has not been demonstrated.
• Lasers multi-sections p'roposent achieve tunability by means of electrical commands applied to different sections, which may consist of DFB, DBR, SG-DBR (Sample Grating DBR for DBR sampled networks) or temperature sensitive lasers. These lasers make it possible to obtain a good tunability, of approximately 15 to 50 nm, with a correct output power, of approximately 5 to 20 mW, although these values depend essentially on the types of lasers used for each section. Multi-section lasers are however complex to implement and configure, and little is known about their resistance to aging.
• the fourth family of tunable lasers relates to lasers with vertical emission by the surface known under the term of VCSEL for "Vertical Cavity Surface Emitting Laser ”, in association with the MEM technique.
• Such a laser is described in the publication "2mW CW single mode tunable operation 1550 nm vertical cavity emitting laser with 50 nm tuning. Range" by D. Vakhshoori, P.Tayebati, Chih-Cheng Lu, M.Azimi, P.Wang, Jiang-Huai Zhou and E. Canoglu published in Electronics Letters of May 27, 1999, Vol.35, No 11.
• An illustration is given in figure 3.
• Such a laser 10 comprises a substrate 8 (InP for example) and a region active 11 crossing the cavity framed by two reflectors, a fixed 28 and a mobile 25.
• the mobile mirror is electrically controlled by a suspended membrane 26 making it possible to vary the length of the cavity so as to tune the emission wavelength of the laser.
• the objective of the present invention is to solve the drawbacks of the prior art.
• the invention provides a tunable laser having a tunability of approximately 30 nm continuously which is easy to produce, to implement and to control.
• the laser according to the invention with emission by the wafer, has optical losses at the acceptable threshold for a significant tunability without mode jumps.
• the length Li of the active section is between 5 and 12 ⁇ m.
• the length L 2 of the tunable section depends on the tunability of the laser according to the following relationship:
• ⁇ + 1 ⁇ 2/2 (n ⁇ L ⁇ + n 2 L 2 )
• ⁇ the laser tunability ⁇ the laser emission wavelength, ni, n 2 the respective refractive indices of the first and second section of the laser cavity.
• the laser has a continuous tunability ⁇ greater than or equal to 30nm.
• the two reflectors fixed and mobile, each have a reflectivity greater than or equal to 90%.
• the fixed reflector is an engraved mirror which is located on the front face of the active section.
• the etched mirror of the fixed reflector is an alternation of semiconductor and air or an alternation of polymer and air or an alternation of semiconductor and polymer.
• the rear face of the active section includes an anti-reflective treatment.
• the movable reflector is a mirror external to the laser cavity.
• the movable reflector is made of etched silicon or nickel or dielectric deposited on silicon. According to one feature, the movable reflector is controlled by a micro-electro-mechanical control (MEM)
• MEM micro-electro-mechanical control
• the tunable section is an air zone.
• the tunable section is a gas zone.
• the invention also relates to a method for manufacturing a tunable semiconductor laser with wafer emission, characterized in that it comprises the following steps: production of a laser chip comprising at least one substrate and one layer active consisting of a gain medium, the length Li of the gain medium being between 5 and 12 ⁇ m, production of a fixed etched mirror on the front face of the laser chip, transfer of the laser chip to a base, realization of a movable reflector on the base at the back of the laser chip.
• the production of the etched mirror comprises the following steps: etching of the active layer of the laser chip, deposition of a polymer in the etched area, - etching of the polymer to form a mirror.
• the production of the etched mirror comprises the following stages: etching of the active layer of the laser chip, resumption of epitaxy in the area etched by a non-doped semiconductor transparent to the length of emission wave, etching of the transparent non-doped semiconductor to form a mirror.
• the production of the etched mirror further comprises a step of depositing a polymer in the etchings of the transparent non-doped semiconductor.
• the invention cleverly makes it possible to produce a laser with emission by the wafer, the cavity of which is sufficiently short to avoid mode jumps on a high tunability while limiting optical losses at the threshold.
• Wafer emission lasers are also easier to produce and implement than surface emitting lasers.
• tunability using MEM technology is simple to control and stable over time and at outside temperature conditions. '
• FIG. 1 is a graph of the emission wavelength of a standard DBR laser as a function of the control current.
• FIG. 2a is a diagram of a standard laser associated with the known MEM technology according to the prior art.
• FIG. 2b is a graph of the emission wavelength of the laser of FIG. 2a as a function of the control voltage.
• - Figure 3 is a schematic sectional view of a MEM-VCSEL laser of the prior art.
• Figure 4 is a schematic sectional view of the tunable laser according to the invention.
• FIG. 5 is a graph of the gain of the material of the active section as a function of the emission wavelength of the laser according to the invention.
• FIG. 6 is a graph of the emission wavelength of the laser according to the invention as a function of the control voltage.
• the following description relates to a single-mode tunable laser allowing tunability of its emission wavelength of approximately 30 n continuously.
• the laser according to the invention illustrated in FIG. 4, has a cavity 5 of length L delimited by two reflectors 15 and 20.
• the cavity 5 is divided into two sections, a first active gain section 1 comprising an active layer 11 of a length Li and a second section 2 of length L 2 tunable.
• the total length of the cavity L L ⁇ + L 2 is less than or equal to 20 ⁇ .
• WDM optical systems typically require continuous ⁇ tunability of at least 30nm. In other words, it is necessary that the spacing between two resonant modes is at least 30 n.
• This condition over the total length L of the cavity 5 strongly depends on the conditions of manufacture, emission and the desired tunability.
• a laser chip 10 is produced so that the length Li of the active section 1 is between 5 and 12 ⁇ m.
• the length L 2 of the tunable section 2 then depends on the desired tunability according to the following relationship:
• ⁇ ⁇ 2/2 (n ⁇ L ⁇ + n 2 L 2 )
• ⁇ 1 the laser tunability
• ⁇ 2 the laser emission wavelength
• ni, n 2 the respective refractive indices of the first and second section of the laser cavity.
• This relation expresses that the laser cavity is defined in such a way that there is only one resonant mode over the range of tunability ⁇ .
• One consequence of this relationship is the single mode operation of the laser.
• the gain of the material g of the active section 1 has been plotted as a function of the emission wavelength of the laser ⁇ .
• the material gain for these points B and C is less than -250cm "1 while it is + 6cm _1 for the main mode at point A.
• the laser will have an emission in only one mode since 'A generally accepted criterion for qualifying a single mode laser is to have a gain difference of only 5cm "1 between the main mode and the" secondary "modes.
• the problem to be solved by the invention then consists in making such a small cavity (Li between 5 and 12 ⁇ m) for a wafer emission laser. Indeed, the cavity 5 is delimited by two reflectors 15 and 20. However, on an active section 1 of length Li between 5 and 12 ⁇ m, it is not possible to produce a fixed mirror 15 by the conventional technique of cleavage which has a. precision of the order of ⁇ 5 ⁇ m.
• the known Bragg stacking technique used to produce mirrors on the surface of components such as VCSELs, is not applicable to the production of mirrors on the wafer of components such as wafer emission lasers which concern us. .
• the fixed reflector 15 must therefore be an engraved mirror. Several techniques can then be envisaged.
• a first technique consists in etching the active layer 11 of the laser diode 10 to the substrate 8, then in carrying out an epitaxy resumption, also known by the English term "butt coupling" in a non-active material. transparent to the emission wavelength of the laser in which can etch the mirror 15.
• the material used for epitaxial regrowth is preferably 1 'undoped InP (index 3.17). Etching, dry or wet according to known techniques, is then carried out in the epitaxial semiconductor to create a reflecting mirror.
• a second technique consists in etching the active layer 11 of the laser diode 10 to the substrate 8, then in depositing a polymer, for example benzocyclobutene (BCB) which is etched to obtain a reflecting mirror .
• a polymer for example benzocyclobutene (BCB) which is etched to obtain a reflecting mirror .
• the two reflectors, the fixed 15 and the mobile 20, each have a reflectivity R greater than or equal to 90% in order to compensate for the short length Li of the active section 1.
• the movable mirror 20 is associated with a MEM command, the implementation of which has been described previously.
• This mobile mirror 20 can be produced according to various known techniques, such as anisotropic etching on silicon or micro-polishing on nickel or a dielectric deposit on silicon.
• the laser chip 10 is transferred to a base 50, made of silicon for example, and the movable mirror 20 is fixed on this base 50 so as to be placed opposite the rear face of the chip 10 to complete the cavity 5.
• the rear face of the chip '10 advantageously comprises an anti-reflective coating. The tunability of the laser strongly depends on the materials used in its manufacture since they determine the refractive indices.
• the active section 1 of the cavity 5 consists of the gain medium of the active layer 11. It may be, for example, a mass material such as InGaAsP, InGaAs, InGaAlAs or a structure with quantum wells preferably forced.
• the tunable section 2 of the cavity is air, or a gas for application to gas detectors for example, the presence of a gas modifying the refractive index of the. tunable section and thus modifying the laser emission wavelength.
• the graph of the. FIG. 6 illustrates well the continuous tunability of the laser according to the invention.
• the variation in the emission wavelength of the laser is directly linked to the variation in the length L 2 of the tunable section 2 of the cavity 5 of the laser.
• This variation in length L 2 is itself linked to the square of the control voltage applied to the MEM.

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• Physics & Mathematics (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Optics & Photonics (AREA)
• Semiconductor Lasers (AREA)

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