Patent Publication Number: US-11038319-B2

Title: Semiconductor laser source

Description:
The invention relates to a semiconductor laser source configured to emit at a wavelength λ Li  close to a desired wavelength λ Si . 
     Known semiconductor laser sources comprise:
         a set of waveguides optically coupled to one another and forming an optical cavity able to make an optical signal resonate at a plurality of possible resonant frequencies, the possible wavelengths λ Rj  of these possible resonant frequencies being regularly spaced apart from one another by an interval Δλ R , this set comprising:
           at least one waveguide in which a bandpass filter is produced, the transmission spectrum of this bandpass filter comprising a passband able to select, among the various possible wavelengths λ Rj , the wavelength λ Li  closest to the wavelength λ Si , and   at least one gain-generating waveguide able to generate optical gain at each wavelength λ Li  selected by the bandpass filter, this gain-generating waveguide comprising a III-V gain medium.   
               

     Such a known laser source is for example disclosed in patent application US2018261976A. 
     Prior art is also known from US20140254617A1, US20030123784A1, CN103368678B, US2016301191A1, US2012057079A1 and WO2007107187A1. 
     Patent application US20140254617A1 discloses a bandpass filter containing a Mach-Zehnder interferometer. However, the arms of the Mach-Zehnder interferometer are devoid of gain-generating sections. 
     Patent application US20030123784A1 discloses a multi-mode demultiplexer using an interferometer. 
     Patent application CN103368678B describes an optical switch using a Mach-Zehnder interferometer. This optical switch is devoid of resonant cavity. 
     It is known that the wavelength λ Li  of a semiconductor laser source varies as a function of temperature. It is desirable to limit this variation as much as possible. Many solutions have already been proposed to limit this variation. For example, in patent application US2018261976A, the bandpass filter of the laser source is made of silicon nitride in order to limit this variation as much as possible. However, the fabrication of the bandpass filter from silicon nitride complexifies the fabrication of the laser source. 
     It is also desirable to limit the bulk of such a semiconductor laser source as much as possible. 
     The invention aims to provide an architecture for the laser source that minimizes the length of the resonant cavity without however decreasing the power of the emitted optical signal. By way of secondary objective, the invention also aims to provide a laser source the variations in the emission wavelength λ Li  of which as a function of temperature are limited without it being necessary to make the bandpass filter from a material other than silicon. 
     One of the subjects thereof is therefore a semiconductor laser source according to claim  1 . 
     Embodiments of this laser source may comprise one or more of the features of the dependent claims. 
    
    
     
       The invention will be better understood on reading the following description, which is given merely by way of nonlimiting example with reference to the drawings, in which: 
         FIG. 1  is a schematic illustration, seen from above, of the architecture of a semiconductor laser source; 
         FIG. 2  is a schematic illustration in vertical cross section of the laser source of  FIG. 1 ; 
         FIG. 3  is a graph illustrating the transmission spectrum of a bandpass filter of the laser source of  FIG. 1 ; 
         FIGS. 4, 5, 8, 10, 11, 13 and 15  are schematic illustrations, seen from above, of various other possible architectures of the laser source of  FIG. 1 ; 
         FIGS. 6 and 7  are graphs illustrating transmission spectra of filters of the laser source of  FIG. 5 ; 
         FIG. 9  is a graph illustrating the transmission spectrum of filters of the laser source of  FIG. 8 ; 
         FIG. 12  is a flowchart of a method for emitting an optical signal at a wavelength λ Li  using the laser source of  FIG. 11 ; 
         FIG. 14  is a graph illustrating the transmission spectrum of filters of the laser source of  FIG. 13 . 
     
    
    
     In these figures, the same references have been used to reference elements that are the same. In the rest of this description, features and functions well known to those skilled in the art will not be described in detail. 
     SECTION I: DEFINITIONS AND NOTATIONS 
     The wavelength λ Li  is the wavelength at which the laser source emits. When the laser source is a monomode or monochromatic laser source, there is only a single wavelength λ Li . When the laser source is a multimode or polychromatic laser source, a plurality of different wavelengths λ Li  exist simultaneously. In the latter case, the index i is an identifier of the wavelength λ Li  among the various wavelengths at which the laser source emits. Typically, the wavelength λ Li  is comprised between 1250 nm and 1590 nm. 
     The wavelengths λ Rj  are the various wavelengths at which a resonant cavity is capable of resonating in the absence of bandpass filter. The bandpass filter is the filter that selects a small number of wavelengths λ Rj . These wavelengths λ Rj  are regularly spaced apart from one another by an interval Δλ R . The index j is an identifier of one particular wavelength λ Rj . 
     The interval ΔR is the smallest wavelength range containing all the possible wavelengths λ Rj . Typically, when the resonant cavity is bounded by reflectors of optical signals, the interval ΔR is equal to the reflective band of these reflectors. The interval ΔR may also be limited by the width of the amplification band or “gain band” of the optical amplifier used to amplify the optical signals that resonate in the interior of the cavity. The gain band is a frequency band that contains all the frequencies of the optical signals capable of being amplified by the optical amplifier. Optical signals the frequencies of which are outside of this gain band are not amplified by the optical amplifier. 
     The reflective band of a reflector is the −3 dB reflective band. It is a question of the wavelength range containing all the wavelengths λ Rj  able to be reflected by the reflector with a power higher than or equal to 50% of the maximum power I max  reflected by this reflector. The power I max  is equal to the power of the reflected optical signal for the wavelength λ Rj , at which this power is maximum. 
     The wavelength λ CR  is a wavelength corresponding to a maximum of the reflection spectrum of the reflector. Typically, the wavelength λ CR  is the wavelength located at the middle of the reflective band of the reflector. This wavelength λ CR  moves at the same time as the reflective band moves. 
     Here, the widths of the passbands are the widths of the −3 dB passbands. Below, the width of the passband of a filter is denoted Δλ FX , where the index FX is an identifier of the filter. 
     The wavelength that corresponds to a maximum in the transmission spectrum of a filter is denoted λ CFX  where the index FX is an identifier of the filter. Typically, the wavelength λ CFX  is the wavelength located at the middle of a passband of the filter. 
     The distance between the maximums of two successive passbands of the transmission spectrum of a bandpass filter is denoted d FSRX , where the index FX is an identifier of the filter. This distance is known as free spectral range (FSR). 
     By “athermal” filter, what is meant is the fact that the coefficient dλ CFX /dT associated with this filter is comprised between L min  and L max , where:
         λ CFX  is the wavelength that corresponds to a maximum in the transmission spectrum of the filter,   the coefficient dλ CFX /dT is the variation in the wavelength λ CFX  as a function of temperature,   L min  is equal to −50 pm/° C. and, preferably, equal to −15 pm/° C. or −7 pm/° C., and   L max  is equal to +50 pm/° C. and, preferably, equal to +15 pm/° C. or +7 pm/° C.       

     For a given filter, the coefficient dλ CFX /dT may be determined experimentally or by numerical simulation. For example, the variation in the wavelength λ CFX  as a function of temperature is measured or simulated in a temperature range extending from 20° C. to 100° C. Typically, the wavelength λ CFX  varies approximately linearly as a function of temperature in this temperature range. It is therefore possible to approximate the relationship that relates the wavelength λ CFX  to the temperature T in this range using the following linear relationship: λ CFX =aT+b, where the coefficients a and b are constants. The values of the coefficients a and b are established by determining the straight line that minimizes the deviations, for example in the least-squares sense, between this straight line and each measured or simulated point. Each measured or simulated point has as abscissa a temperature and as ordinate the wavelength λ CFX  measured or obtained by simulation for this temperature. The value of the coefficient dλ CFX /dT is then set equal to the value of the coefficient a. 
     The thermo-optical coefficient of a material is the coefficient dn r /dT, where:
         n r  is the refractive index of this material at the wavelength λ Si , and   dn r /dT is the variation in the refractive index n r  as a function of temperature in a temperature range extending from 20° C. to 100° C.
 
Similarly to what was indicated for the coefficient dλ CFX /dT, the coefficient dn r /dT is approximated by a constant in the temperature range extending from 20° C. to 100° C.
       

     SECTION II: EXAMPLES OF EMBODIMENTS 
       FIG. 1  schematically shows the general architecture of a semiconductor laser source  10  that emits at at least one wavelength λ Li  that is as close as possible to a desired wavelength λ Si . The wavelength λ Si  is constant and either set once and for all during the design of the laser source or adjustable by a user. In any case, once set, the wavelength λ Si  remains constant provided that the settings of the laser source are not modified. 
     Below, only the particularities of the laser source  10  are described in detail. For general information on the production and operation of a semiconductor laser source using waveguides made of silicon and of a III-V gain medium, the reader may consult the following article: B. Ben Bakir et al., “Hybrid Si/III-V lasers with adiabatic coupling”, 2011. 
     The laser source  10  comprises a back reflector  12  and a front reflector  14  that define the ends of a Fabry-Pérot resonant cavity. In the interior of this cavity, an optical signal may resonate, in the absence of bandpass filter, at a plurality of possible wavelengths λ Rj . The wavelengths λ Rj  are regularly spaced apart from one another by an interval Δλ R . In this embodiment, the interval ΔR that contains all the wavelengths λ Rj  is equal to the reflective band of the reflectors  12  and  14 . 
     For example, the reflector  12  has a reflectance strictly higher than that of the reflector  14 . The reflectance is equal to the ratio of the power of the optical signal reflected by the reflector to the power of the optical signal incident on this reflector. Typically, the reflectance of the reflector  12  is higher than or equal to 90% or 95% for the wavelength λ Li . The reflectance of the reflector  14  is generally comprised between 30% and 80% and is typically equal to 50%. 
     The reflectors  12  and  14  are wideband reflectors. In this embodiment, this means that the width of the reflective band of the reflectors  12  and  14  is larger than a preset lower limit. This lower limit is here:
         equal to Δλ f +DT.(dλ CR /dT) if the bandpass filter is arranged to select solely a single wavelength λ Li , and   equal to Δλ f +max {DT.(dλ CR /dT); N.d SFRf +DT.(dλ Cf /dT)} if the bandpass filter is arranged to select N wavelengths λ Li ,
 
where:
   Δλ 1  is the width of the passband of the bandpass filter of the laser source,   DT is the width of a preset operating temperature range of the laser source,   dλ CR /dT is the variation in the central wavelength λ CR  of the reflectors as a function of temperature expressed in nm/° C.,   dλ Cf /dT is the variation in the central wavelength λ Cf  of the bandpass filter as a function of temperature expressed in nm/° C.,   N is an integer number higher than or equal to two,   d SFRf  is the interval between two successive passbands of the bandpass filter expressed in nanometres,   the symbol “.” designates in this description the operation of scalar multiplication, and   max{ . . . } is the function that returns the highest of the elements that are located between the curly brackets and that are separated from each other by a semi-colon. Various embodiments of the bandpass filter of the laser source are described below.       

     The operating temperature range of a laser source is often chosen higher than 10° C. or 30° C. Here the operating temperature range is chosen as being comprised between +20° C. and +100° C. The width DT is therefore here equal to 80° C. Below, the lowest and highest temperatures of the operating temperature range are denoted T min  and T max , respectively. In this embodiment, the reflectors  12  and  14  are produced in a waveguide the core of which is made of silicon. Thus, the variation dλ CR /dT is here equal to 0.07 nm/° C. Typically, the width of the reflective band is larger than 6 nm or 15 nm or 30 nm. 
     The reflectors  12  and  14  are also designed so that, at the temperature T min , the wavelength λ Li  is closer to the upper limit λ Rmax  of the reflective band of the reflectors  12  and  14  than to its lower limit λ Rmin . For example, at the temperature T min , the wavelength λ Li  is comprised between 0.9 λ Rmax  and λ Rmax . This constraint may be relaxed if the width of the reflective band is very much larger than its lower limit such as defined above. 
     Here, the reflectors  12  and  14  are for example Bragg gratings. 
     Between the reflectors  12  and  14 , the laser source includes in succession the following photonic components in order from the reflector  12  to the reflector  14 :
         an optical waveguide  15  made of silicon in which the reflector  12  is produced,   a bandpass filter  22  able to select the wavelength λ Li  of operation of the laser source  10  from the various wavelengths λ Rj  that are able to be supported in the interior of the Fabry-Pérot cavity,   a coupler  26  that optically connects the waveguide  15  to an entrance of a waveguide  28  made of III-V gain medium,   a semiconductor optical amplifier  30  (SOA) produced in the waveguide  28  and able to generate and to amplify the optical signal resonating in the interior of the Fabry-Pérot cavity at each of the wavelengths λ Rj ,   a coupler  32  that optically connects an exit of the waveguide  28  to a waveguide  25 , and   the optical waveguide  25  made of silicon in which the reflector  14  is produced.       

     Here, by waveguide made of silicon, what is meant is a waveguide the core of which is made of silicon. For example, the cladding of the waveguides made of silicon is made from another material such as, typically, silicon oxide. 
     The couplers  26  and  32  are for example adiabatic couplers. For a detailed description of an adiabatic coupler, the reader is referred to the following article: Amnon Yariv et al., “Supermode Si/III-V hybrid Lasers, optical amplifiers and modulators: proposal and analysis” Optics Express 9147, vol. 14, No. 15, 23 Jul. 2007. 
     Such an adiabatic coupler is, for example, obtained by modifying the width of the waveguide made of silicon with respect to the width of the waveguide made of III-V gain medium. Typically, for an adiabatic coupling of a waveguide made of silicon to a waveguide made of III-V material, the width of the waveguide made of silicon is gradually decreased as the waveguide made of III-V material is approached. In the opposite direction, to transfer by adiabatic coupling an optical signal from the waveguide made of III-V material to a waveguide made of silicon, the width of the waveguide made of silicon is for example gradually increased. In addition, typically toward the middle of the adiabatic coupler, the dimensions of the cross sections of the waveguides made of silicon and made of III-V material are generally such that their respective effective indices are equal. This is also true for an adiabatic coupling between hybrid Si/III-V material waveguides. 
     Preferably, the amplifier  30  is a wideband amplifier, i.e. one capable of generating and amplifying a large range of wavelengths. This range comprises the wavelength λ Si . Typically, it is centred on this wavelength λ Si  at the temperature (T max +T min )/2. The width of this wavelength range at −3 dB is for example at least 10 nm or 25 nm or 35 nm and remains wide with the increase in temperature. For example, the III-V materials from which the amplifier  30  is made are those described in the following article: Dimitris Fitsios et al. “High-gain 1.3 μm GaInNAs semiconductor optical amplifier with enhanced temperature stability for all-optical processing at 10 Gb/s”, Applied Optics, vol. 54, n° 1, 1 Jan. 2015. The fact of producing the amplifier  30  as described in this article in addition allows a wideband amplifier that is stable in temperature to be obtained. This improves the operation of the laser source and notably this allows the power emitted by the laser source to be kept almost constant in all the operating temperature range [T min ; T max ]. In this case, the waveguide  28  and the amplifier  30  take the form of a stack  34  ( FIG. 2 ) of alternating sublayers made of GaInNAs and of GaNAs interposed between a lower sublayer  35  ( FIG. 2 ) and an upper sublayer  36  ( FIG. 2 ) made of p-doped GaAs. The sublayer  35  is a sublayer made of III-V material of opposite dopant type to the upper layer  36 . Here, it is a question of a sublayer made of n-doped GaAs. 
     The amplifier  30  comprises, in addition to the waveguide  28 , a connection  37  ( FIG. 2 ) making direct mechanical and electrical contact with the segment of the sublayer  35 . The sublayer  36  makes mechanical and electrical contact with a connection  38  ( FIG. 2 ) in order to electrically connect the lower portion of the amplifier  30  to a potential. When a DC current, called the “injection current”, higher than the threshold current of the laser is applied between the connections  37  and  38 , the amplifier  30  generates and amplifies the optical signal that resonates in the interior of the Fabry-Pérot cavity. 
     In this embodiment, the filter  22  comprises solely an athermal primary filter  40 . The filter  40  is here composed of a Mach-Zehnder interferometer  42 . 
     The interferometer  42  comprises:
         two arms  44  and  46  that are each able to guide the resonant optical signal, and   two forks  48  and  50 .       

     In the direction of propagation F ( FIG. 1 ) of the optical signal travelling from the reflector  12  to the reflector  14 , the fork  48  distributes equally the optical signal reflected by the reflector  12  to the arms  44  and  46 . 
     In  FIG. 1  and the following figures, the vertical direction has been represented by a direction Z of an orthogonal coordinate system XYZ, where X and Y are horizontal directions. Here, the direction X is essentially parallel to the direction F of propagation of the optical signal. 
     In the direction F, the fork  50  combines the optical signals output from the arms  44  and  46  together to obtain an optical signal that propagates in the waveguide  25  to the reflector  14 . In the direction of propagation opposite to the direction F, the roles of the forks  48  and  50  are inverted. For example, the forks  48  and  50  are multi-mode interferometers (MMIs). 
     The arm  44  is divided into three consecutive sections S 1,1 , S 1,2  and S 1,3  in order in the direction F. Below, section n of arm m is designated S m,n , where:
         the index m is an identifier of the arm of the interferometer, and   the index n is an identifier of the section of this arm.       

     In this text, when the index m is equal to 1, this designates the lower arm  44 . When the index m is equal to 2, this designates the upper arm  46 . The index n is the numerical order of the section, starting at the fork  48  and travelling in the direction F. Thus, an index n equal to 1 corresponds to the first section encountered after the fork  48 , the index n equal to 2 corresponds to the second section encountered starting from the fork  48  and so on. 
     Each section S m,n  is a waveguide. Each of these sections therefore has an effective index Neff m,n  and a nonzero length L m,n . Typically, each section differs from the section that precedes it and from the section that follows it in:
         the dimensions of its cross section,   the one or more materials used to produce its core, and/or   the one or more materials used to produce its cladding.       

     The effective index of a waveguide depends on the materials used to form the core and the cladding of this waveguide and also on the dimensions of the core and notably on the width and thickness of the core. The effective indices of two successive sections in the direction F are therefore different. Here, the effective index Neff m,n  of each section S m,n  is considered to be constant over all its length L m,n . 
     The effective index of a waveguide may be determined, for a given temperature and for a given wavelength, by numerical simulation or experimentally. Here, when the effective indices of various sections are compared, it is a question of the effective indices determined at the same temperature and at the same wavelength. Typically, this wavelength is equal to λ Si . 
     The effective index of a waveguide varies as a function of the temperature of this waveguide. Below, the coefficient that expresses the degree of variation in the index neff, of the section S m,n  as a function of temperature is denoted dneff m,n /dT. To a first approximation, in the temperature range DT, this coefficient dneff m,n /dT may be considered to be constant. 
     Here, the section S 1,1  extends from the fork  48  to the coupler  26 . The section S 1,1  is a waveguide made of silicon. 
     The section S 1,2  extends from the coupler  26  to the coupler  32 . The section S 1,2  therefore corresponds to the waveguide  28  made of III-V gain media, in which waveguide the amplifier  30  is produced. The section S 1,2  is here called the “gain-generating section” because it has the capacity to amplify the optical signal. 
     The section S 1,3  extends from the coupler  32  to the fork  50 . The section S 1,3  is a waveguide made of silicon that is identical to the waveguide made of silicon of section S 1,1  except that the length L 1,3  made it be different from the length L 1,1 . 
     The arm  46  is divided into four consecutive sections S 2,1 , S 2,2 , S 2,3  and S 2,4  in order in the direction F. The sections S 2,1  and S 2,2  are identical to the sections S 1,1  and S 1,2 , respectively. The arm  46  therefore comprises, just like the arm  44 :
         an optical coupler  52  that optically connects the waveguide  15  to an entrance of the waveguide  54  made of III-V gain media,   an optical amplifier  56  produced in the waveguide  54 , and   an optical coupler  58  that optically connects an exit of the waveguide  54  to the waveguide  25 .       

     The couplers  52 ,  58 , the waveguide  54  and the amplifier  56  are identical to the couplers  26 ,  32 , to the waveguide  28  and to the amplifier  30 , respectively. In addition, the injection current that flows through the amplifier  56  here systematically has the same magnitude as the injection current that flows through the amplifier  30  at the same time. For example, to this end, the lower sublayer  35  and upper sublayer  36  of the amplifier  30  and of the amplifier  56  are common to these two amplifiers or are directly electrically connected to each other. Under these conditions, the indices neff 1,2  and neff 2,2  are identical and the coefficients dneff 1,2 /dT and dneff 2,2 /dT are also identical. 
     The section S 2,4  is identical to the section S 1,3  except that its length L 2,4  is larger than the length L 1,3 . In  FIG. 1 , the fact that the length L 2,4  is larger than the length L 1,3  is schematically represented by a zigzag. 
     The section S 2,3  has a coefficient dneff 2,3 /dT lower than the coefficients dneff 2,1 /dT, dneff 2,2 /dT and dneff 2,4 /dT. To this end, according to a first solution, section S 2,3  is a waveguide made of silicon the width of the core of which is smaller than the width of the core of sections S 2,1  and S 2,4 . According to a second solution, section S 2,3  is a waveguide the core of which is made from a material different from silicon such as, for example silicon nitride. In this case, the dimensions of the core of section S 2,3  may be identical to the dimensions of the cores made of silicon of sections S 2,1  and S 2,4 . A third solution consists in using an another material for the cladding of section S 2,3 , this material being different from that used for the cladding of sections S 2,1  and S 2,4 . In addition, it is possible to combine these various solutions to obtain the desired coefficient dneff 2,3 /dT. 
     In order for the interferometer  42  to have a passband that allows the wavelength λ Li  to be selected, the sections S m,n  are configured to meet the following condition (1): 
                   ∑     n   =   1       N   2       ⁢       L     2   ,   n       ⁢     neff     2   ,   n           -       ∑     n   =   1       N   1       ⁢       L     1   ,   n       ⁢     neff     1   ,   n             =       k   f     ⁢     λ   Si             
where:
         k f  is a preset integer number higher than or equal to 1,   N 1  and N 2  are the numbers of sections in the arms  44  and  46 , respectively,   L 1,n  and L 2,n  are the lengths of the nth sections of the arms  44  and  46 , respectively,   neff 1,n  and neff 2,n  are the effective indices of the nth sections of the arms  44  and  46 , respectively.       

     A high number of lengths L m,n  and of indices Neff m,n  allow condition (1) to be satisfied. 
     Here, the filter  40  is also to designed to be athermal. Thus its coefficient dλ CF1 /dT is close to zero, i.e. comprised between L min  and L max . Below, a wavelength corresponding to a maximum of the transmission spectrum of the primary filter  40  is denoted λ CF1 . To this end, the sections S m,n  are configured to also meet the following condition (2): 
               L   min     ≤         ∑     n   =   1       N   2       ⁢       L     2   ,   n       ⁢       dneff     2   ,   n       dT         -       ∑     n   =   1       N   1       ⁢       L     1   ,   n       ⁢       dneff     1   ,   n       dT           ≤     L   max           
where dneff 1,n /dT and dneff 2,n /dT are the variations in the effective indices of the nth sections of the arms  44  and  46 , respectively.
 
     For example, here the lengths L m,n  and the coefficients dneff m,n /dT meet condition (2) with L min =−7 pm/° C. and L max =+7 pm/° C. and, preferably, with L min =−2 pm/° C. and L max =+2 pm/° C. 
     There are a high number of sets of values for the lengths L m,n  and the indices neff m,n  that meet both of the above conditions (1) and (2). Here, among all of these sets, it is the set of values or one of the sets of values that in addition allows a width Δλ 40  to be obtained for the passband of the primary filter  40  larger than Δλ R  that is chosen. 
     Under these conditions, because the interferometer  42  meets condition (2), the passband of the filter  22  does not move or practically does not move as a function of temperature. In contrast, in this embodiment, the wavelengths λ Rj  move as a function of temperature. In particular, the variation in temperature may be such that a wavelength λ Rj  precedingly located in the interior of the passband of the filter  22  moves and exits from this passband. In this case, since the width Δλ 40  is larger than or equal to the interval Δλ R , when a wavelength λ Rj  exits from the passband of the filter  22 , another wavelength λ Rj−1  or λ Rj+1  enters into the interior of this passband. Thus, in this embodiment, it is not necessary to employ a tuning device that moves the wavelengths λ Rj  so that there permanently exists a wavelength λ Rj  located at the centre of the passband of the filter  22 . 
     To configure the sections S m,n  so that they meet the various above conditions, one solution consists, in a first step, in choosing the transverse dimensions of each section and the materials used for the core and the cladding of each section. Thus, the effective indices neff m,n  and the coefficients dneff m,n /dT of each section are defined. Next, in a second step, the lengths L m,n  allowing the various preceding conditions to be met are sought. If it proves to be impossible to find satisfactory lengths, then the method returns to the first step in order to modify the effective indices of one or more sections. Such sections are for example dimensioned in accordance with the teaching given, in a different context, in the following article: Biswajeet Guha et al: “Minimizing temperature sensitivity of silicon Mach-Zehnder interferometers”, Optics Express, 15/01/2010, pages 1879-1887. 
       FIG. 2  shows the laser source  10  in a vertical cross section passing through the arm  44 . In this figure, the path followed by the optical signal between the reflectors  12  and  14  is represented by a double-headed arrow. 
     The laser source  10  comprises a substrate  70  that mainly lies in a horizontal plane called the “plane of the substrate”. The thickness of the substrate  70  is for example larger than 500 μm. 
     A layer  72  of single-crystal silicon encapsulated in silicon oxide is stacked on the upper face of the substrate  70 . The reflectors  12  and  14 , the waveguides  15  and  25  and the sections S 1,1 , S 1,3 , S 2,1 , S 2,3  and S 2,4  and the lower portions of the couplers  26 ,  32 ,  52  and  58  are produced in the encapsulated silicon of this layer  72 . 
     A thin layer  74 , for example made of silicon oxide, is deposited on the layer  72 . Lastly, the laser source  10  comprises a layer  76  made of gain medium encapsulated in, for example, silicon oxide. The waveguides  28  and  54 , the amplifiers  30  and  56  and the upper portions of the couplers  26 ,  32 ,  52  and  58  are produced in this layer  76 . 
       FIG. 3  shows the transmission spectrum  80  of the primary filter  40 . The ordinate axis represents the transmittance of the filter  40  and the abscissa axis the wavelength associated with this transmittance. In this figure and the other figures showing a transmission spectrum, the transmittance of the filter is plotted on a logarithmic scale. Thus, a maximum of the transmission spectrum corresponds to the value 0 or to a value close to 0 in these graphs. In this example, the width Δλ 40  of a passband of the filter  40  is for example 5 nm. Since this width Δλ 40  is larger than the interval Δλ R , the laser source  10  is multimode. 
       FIG. 4  shows a laser source  100  that is identical to the laser source  10 , except that the filter  22  has been replaced by a filter  102 . The filter  102  is identical to the filter  22 , except that the primary filter  40  has been replaced by a primary filter  104  that comprises an interferometer  106  instead of the interferometer  42 . The interferometer  106  is identical to the interferometer  42  except that the arm  46  has been replaced by an arm  108 . 
     The arm  108  is identical to the arm  46  except that it comprises only three sections S 2,1 , S 2,2  and S 2,3 . Section S 2,1  of the arm  108  is identical to section S 1,1  of the arm  44 . Section S 2,2  is identical to section S 2,2  of the interferometer  42  except that the amplifier  56  has been replaced by an amplifier  110  shorter than the amplifier  30 . Thus, the length L 2,2  of section S 2,2  of the arm  108  is shorter than the length L 1,2 . Section S 2,3  is identical to section S 1,3  except that the length L 2,3  is larger than or equal to the length L 1,3 . 
     As in the preceding embodiment, the various sections S m,n  are arranged so that the lengths L m,n  and the indices neff m,n  meet both of conditions (1) and (2) described above. This laser source  100  operates just like the laser source  10  but without it being necessary to implement an additional section in one of the arms the effective index of which is lower than the indices neff 2,1  and neff 2,2 . 
       FIG. 5  shows a laser source  130  that is identical to the laser source  10 , except that the filter  22  has been replaced by a filter  132  that comprises a primary filter  134  instead of the primary filter  40 . The filter  134  comprises a Mach-Zehnder interferometer  136  and a secondary filter  138 . 
     The filter  138  is a bandpass filter typically comprising a plurality of passbands. The width Δλ 138  of each of these passbands is smaller than or equal to the interval Δλ R . Here, the width Δλ 138  is equal to Δλ R . This filter  138  has a coefficient dneff 138 /dT higher than zero and generally higher than 15 pm/° C. or 50 pm/° C., where neff 138  is the effective index of the waveguide in which the filter  138  is produced. Here, the filter  138  is formed in a waveguide made of silicon. Thus, its coefficient dneff 138 /dT is higher than 50 pm/° C. 
     In this embodiment, the filter  138  comprises a ring resonator  140 . The ring  140  is optically coupled to section S 2,3  of the arm  46  by way of an optical coupling  142 . Thus, a portion of the optical signal that passes through the arm  46  also passes through the filter  138 . The coupling  142  is for example an evanescent coupling. 
       FIG. 6  shows the transmission spectrum  150  of the filter  138 . This spectrum  150  comprises a plurality of passbands that are regularly spaced apart from one another by the distance d FSR138 .  FIG. 6  also shows the transmission spectrum  152  of the interferometer  136 . The spectrum  152  presents a plurality of transmission maximums that are regularly spaced apart from one another by the distance d FSR136 . 
     Here, the filter  138  is arranged so that the distance d FSR138  is such that at a given time, in the interior of the interval ΔR, it is possible for there to be only a single passband of the filter  138  centred on a transmission maximum of the spectrum  152 . To achieve this, either the filter  138  is configured so that the distance d FSR138  is larger than ΔR or the filter  138  is configured so that the distance d FSR138  is a non-integer multiple of the distance d FSR136 , as shown in  FIG. 6 . In  FIG. 6 , this band of the filter  138  centred on a transmission maximum of the spectrum  152  has been framed by a rectangle  154 . Under these conditions, the filter  132  selects a single wavelength λ Li  contained in the interior of the passband framed by the rectangle  154 . The laser source  130  is therefore a monomode laser source. 
     Here, the interferometer  136  is identical to the interferometer  42  except that the various sections S m,n  are arranged so as to meet condition (1) and the following condition (3): 
     
       
         
           
             
               
                 L 
                 min 
               
               - 
               
                 
                   dneff 
                   138 
                 
                 dT 
               
             
             ≤ 
             
               
                 
                   ∑ 
                   
                     n 
                     = 
                     1 
                   
                   
                     N 
                     2 
                   
                 
                 ⁢ 
                 
                   
                     L 
                     
                       2 
                       , 
                       n 
                     
                   
                   ⁢ 
                   
                     
                       dneff 
                       
                         2 
                         , 
                         n 
                       
                     
                     dT 
                   
                 
               
               - 
               
                 
                   ∑ 
                   
                     n 
                     = 
                     1 
                   
                   
                     N 
                     1 
                   
                 
                 ⁢ 
                 
                   
                     L 
                     
                       1 
                       , 
                       n 
                     
                   
                   ⁢ 
                   
                     
                       dneff 
                       
                         1 
                         , 
                         n 
                       
                     
                     dT 
                   
                 
               
             
             ≤ 
             
               
                 L 
                 max 
               
               - 
               
                 
                   dneff 
                   138 
                 
                 dT 
               
             
           
         
       
     
     In this embodiment, condition (2) is replaced by condition (3) above. When conditions (1) and (3) are met, the interferometer  136  compensates to a large extent the movement of the transmission spectrum of the filter  138  due to the temperature variations. The filter  134 , which combines this interferometer  136  and the filter  138 , is therefore athermal. Such sections are for example dimensioned in accordance with the teaching given, in a different context, in the following article: Biswajeet Guha et Al: “CMOS-compatible athermal silicon microring resonators”, Optics Express, Mar. 2, 2010, pages 3487-3493. 
     Here, the filter  134  also comprises a device  144  for adjusting the position of the passbands of the filter  138  with respect to the transmission maximum of the spectrum  152 . For example, the device  144  is a heater able to locally heat the ring  140  without heating the arms  44  and  46  of the interferometer  136 . As illustrated in  FIG. 7 , this device  144  allows the spectrum  150  to be moved with respect to the spectrum  152  until another passband of the filter  138  is centred on a transmission maximum of the spectrum  152 . This allows, with a small movement of the spectrum  150 , the value of the desired wavelength λ Si  of the laser source  130  to be substantially changed. In the example of  FIGS. 6 and 7 , a movement of less than 4 nm of the spectrum  150  allows the value of the wavelength λ Si  to be made to vary by about 10 nm. 
       FIG. 8  shows a laser source  150  that is identical to the laser source  10 , except that the filter  22  has been replaced by a filter  152  that comprises, in addition to the primary filter  40 , a tertiary filter  154  that is optically connected in series with the filter  40 . To simplify  FIG. 8 , the details of the filter  40  have not been illustrated again. 
     The filter  154  is an athermal filter the transmission spectrum  156  of which ( FIG. 9 ) comprises a plurality of passbands that are regularly spaced apart from one another by a distance d SFR154 . Preferably, the filter  154  is arranged so that the distance d SFR154  is equal to p.d SFR40 , where p is an integer number higher than or equal to 2. 
     In addition, the filter  154  is arranged so that at least one transmission maximum of its spectrum  156  is coincident with a transmission maximum of the spectrum  80  of the filter  40 . Under these conditions, the distance between two consecutive maximums of the transmission spectrum  158  ( FIG. 9 ) of the filter  152  is equal to the distance d SFR154 . The width Δλ 152  of the passband of the filter  152  remains equal to the width Δλ 40  of a passband of the filter  40 . Thus, the use of the filter  154  allows the passbands of the filter  152  to be spaced apart without increasing the width of these passbands. This therefore allows a number of wavelengths λ Li  lower than the number of wavelengths λ Li  selected by the filter  22  of the laser source  10  to be selected. By virtue of this, the laser source  150  may be made monomode. 
     Here, the filter  154  is an athermal Mach-Zehnder interferometer. For example, the filter  154  is identical to the filter  40  except that:
         it is devoid of gain-generating section comprising a core made of gain medium,   the various sections are adjusted to meet conditions (1) and (2) and in addition to obtain a distance d SFR154  equal to p times the distance d SFR40 .       

     Thus, the arm  44  of the filter  154  comprises a single section S 1,1  between the forks  48  and  50 . This section S 1,1  is a waveguide the core of which is made of silicon. This section S 1,1  is identical, for example, to the concatenation of sections S 1,1  and S 1,3  of filter  40 . The arm  46  of the filter  154  comprises three consecutive sections S 2,1 , S 2,2  and S 2,3 . Sections S 2,1 , S 2,2  and S 2,3  of the filter  154  are for example identical to sections S 2,1 , S 2,3  and S 2,4  of the filter  40 , respectively. 
       FIG. 10  shows a laser source  170  that is identical to the laser source  10 , except that the reflectors  12  and  14  have been replaced by the reflectors  172  and  174 , respectively. The reflectors  172  and  174  are reflectors the reflective bands of which are narrow and narrower than the reflective bands of the reflectors  12  and  14 . Here, the width of the reflective bands of the reflectors  172  and  174  is comprised between Δλ R  and d SFR40 . Thus, the reflectors  172  and  174  permit only wavelengths λ Rj  contained in the interior of a single passband of the filter  22 . The reflectors  172  and  174  are, for example, Bragg gratings. The production of such narrow-reflective-band reflectors is for example described in the following article: G. H. Duan et al.: “Hybrid III-V on Silicon Lasers for Photonic Integrated Circuits on Silicon”, Journal of Selected Topics in Quantum Electronics, vol. 20, n° 4, July 2014, pages 1-13. 
     Preferably, the reflectors  172  and  174  are also insensitive to temperature variations. For example, to this end, the core of the reflectors  172  and  174  is made from a material of low thermo-optical coefficient such as silicon nitride. 
       FIG. 11  shows a laser source  180  that is identical to the laser source  10 , except that the filter  22  has been replaced by a filter  182  in which the primary filter  40  has been replaced by a primary filter  184 . The filter  184  is identical to the filter  40  except that the interferometer  42  has been replaced by an interferometer  186 . The interferometer  186  is identical to the interferometer  42  except that the arm  46  is structurally identical to the arm  44 . Thus, the arm  46  comprises only three consecutive sections S 2,1 , S 2,2  and S 2,3 . These sections S 2,1 , S 2,2  and S 2,3  of the interferometer  186  are identical to sections S 1,1 , S 1,2  and S 1,3  of the arm  44  of the same interferometer, respectively. Under these conditions, the total lengths of the arms  44  and  46  are equal. In addition, the transverse dimensions of the cores of sections S 1,2  and S 2,2  are identical. Here, by “transverse dimensions”, what is meant is the dimensions in a plane orthogonal to the direction of propagation of the optical signal. It is therefore in particular a question of the width and height of the core. 
     In contrast, in this embodiment, the injection currents I 1  and I 2  that flow through the amplifiers  30  and  56 , respectively, are not identical. To this end, here, the connections  37  of the amplifiers  30  and  56  are electrically connected to respective power sources  190  and  192 . Under these conditions, even though sections S 1,2  and S 2,2  are structurally identical, their effective indices neff 1,2  and neff 2,2  are different. Specifically, since the magnitudes of the currents I 1  and I 2  are different, the temperatures of the amplifiers  30 ,  56  are different and therefore their effective indices are different. Here, the sources  190  and  192  are adjusted so the condition (1) is met. In the particular case of the laser source  180 , condition (1) is then written in the following form:
 
 L   2,2   neff   2,2   −L   1,2   neff   1,2   =k   f λ Si  
 
where the lengths L 2,2  and L 1,2  are equal.
 
     Since sections S 1,2  and S 2,2  are structurally identical, the coefficients dneff 1,2 /dT and dneff 2,2 /dT are equal. Thus, condition (2) described above is also met provided that the lengths L 2,2  and L 1,2  are equal. Therefore, the primary filter  184  is also athermal. The way in which the laser source  180  works may be deduced from the explanations given with respect to the laser source  10 . 
     More precisely, as shown in  FIG. 12 , in a step  200  of emitting an optical signal at the wavelength λ Li  close to the desired wavelength λ Si  the sources  190  and  192  inject the currents I 1  and I 2  of different magnitudes through the amplifiers  30  and  56 , respectively. 
       FIG. 13  shows a laser source  220  that is identical to the laser source  130  except that the bandpass filter  132  has been replaced by a bandpass filter  222 . The filter  222  is identical to the filter  132  except that the primary filter  134  has been replaced by a primary filter  224 . The primary filter  224  is identical to the primary filter  134  except that the secondary filter  138  has been replaced by a secondary filter  228 . The filter  228  is identical to the filter  138  except that the ring  140  has been replaced by a ring  230 . The ring  230  is configured so that:
         the width Δλ 228  of each passband of the filter  228  is equal to Δλ R ,   a maximum of the transmission spectrum  234  ( FIG. 14 ) of the filter  228  coincides with a maximum of the transmission spectrum  152  of the interferometer  136 , and   the distance d FSR228  between two consecutive maximums of the spectrum  234  ( FIG. 14 ) is equal to p.d SFR136 , where p is an integer number.       

       FIG. 14  shows the transmission spectra  152  and  234  in the particular case where p is equal to 1. As illustrated in  FIG. 14 , the filter  222  simultaneously selects a plurality of wavelengths λ Li  that are separated from one another by the distance d FSR228 . The laser source  220  is therefore multimode. 
       FIG. 15  shows a laser source  240  that is a combination of the embodiments of  FIGS. 5 and 11 . The laser source  240  is identical to the laser source  130  except that the bandpass filter  132  has been replaced by a bandpass filter  242 . The filter  242  is identical to the filter  132  except that the primary filter  134  has been replaced by a primary filter  244 . The primary filter  244  is identical to the primary filter  134  except that the interferometer  136  has been replaced by an interferometer  246 . 
     The interferometer  246  is identical to the interferometer  186  except that the lengths L 1,2  and L 2,2  are different. As described above with reference to  FIG. 11 , the magnitudes of the DC currents I 1  and I 2  are different. Thus, the effective indices neff 1,2  and neff2,2 are different. In contrast, the coefficients dneff1,2/dT and dneff2,2/dT are equal since the materials and the cross section of the cause of sections S 1,2  and S 2,2  are identical. In this particular case, condition (1) is then written in the following form:
 
 L   2,2   neff   2,2   −L   1,2   neff   1,2   =k   f λ Si  
 
     Condition (3) for the primary filter  244  to be athermal is written in the following simplified form: 
                 L   min     -       dneff   138     dT       ≤         L     2   ,   2       ⁢       dneff     2   ,   2       dT       -       L     1   ,   2       ⁢       dneff     1   ,   2       dT         ≤       L   max     -       dneff   138     dT             
where the coefficients dneff 1,2 /dT and dneff 2,2 /dT are equal.
 
     To find the magnitudes of the currents I 1  and I 2  and the lengths L 1,2  and L 2,2  that meet the above conditions (1) and (3), it is possible, for example, to first set the magnitudes of the currents I 1  and I 2  and then to seek the lengths L 1,2  and L 2,2  that meet the above conditions. It is also possible to proceed in the opposite way, i.e. for the lengths L 1,2  and L 2,2  to be set first and then the magnitudes of the currents I 1  and I 2  sought. 
     In the embodiment of  FIG. 15 , the tuning device  144  has been omitted. 
     SECTION III: VARIANTS 
     Variants of the Bandpass Filter: 
     As a variant, the laser source comprises a plurality of primary filters that are connected in series one after the other. 
     In another embodiment, the primary filter is not athermal. In this case, the various sections S m,n  have no need to be arranged to meet condition (2) or (3). If the primary filter is not athermal and this primary filter is connected in series with a tertiary filter, this tertiary filter is itself also not athermal. More precisely, in this case, the tertiary filter is arranged so that its coefficient dλ CF3 /dT is equal or practically equal to the coefficient dλ CF1 /dT of the primary filter. 
     The optical coupling  142  between the ring  140  and the arm  46  may occur elsewhere. For example, as a variant, this optical coupling occurs between section S 2,1  or S 2,4  and the ring  140 , or even between section S 2,2  and the ring  140 . 
     A tertiary filter such as the filter  154  may be implemented in the other embodiments described here and, in particular, in the embodiments of  FIGS. 1, 4, 5, 10, 11 and 13 . 
     Other embodiments of the tertiary filter are possible. For example, as a variant, the tertiary filter is identical to the primary filter  134  but the sections S 1,2  and S 2,2  comprising gain media are omitted. In another example embodiment, the tertiary filter is made athermal not using a Mach-Zehnder interferometer but using other means. For example, the core of the tertiary filter is made from a material the thermo-optical coefficient of which is low, i.e. at least two times lower than the thermo-optical coefficient of silicon. Thus, in another example, the tertiary filter is a ring resonator formed in a waveguide made of silicon nitride and coupled to the waveguide  15  or  25 . 
     Other embodiments of the secondary filter  138  are possible. More precisely, any other filter having a transmission spectrum similar to the spectrum  150  and capable of being optically coupled to one of the sections of the arm  46  may be employed. 
     The secondary filter may also be produced in waveguides the core of which is made from a material the thermo-optical coefficient of which is low. For example, as a variant, the filter  138  is produced in a waveguide the core of which is made of silicon nitride and the cladding of which is made of silicon oxide. 
     Other Variants: 
     The fact of injecting currents I 1  and I 2  of different magnitudes into the amplifiers  30  and  56  may be implemented in all the embodiments described here. In particular, currents I 1  and I 2  of different magnitudes may be injected through the amplifiers  30  and  56  of the embodiments of  FIGS. 1, 4, 5, 8, 10 and 13 . In this case, the other sections of the arms  44  and  46  are arranged so that conditions (1) and (2) or conditions (1) and (3) are met. The way in which these variants work is then the same as that of the laser source  180 . 
     In the laser source  180 , it is not necessary for sections S 1,1  and S 1,3  to be identical to sections S 2,1  and S 2,3 , respectively. For example, as a variant, section is longer than section S 2,1  and, to compensate, section S 1,3  is shorter than section S 2,3 . In fact, it is enough for the following condition (4) to be met for the laser source  180  to operate correctly: L 1,1 neff 1,1 +L 1,3 neff 1,3 =L 2,1 neff 2,1 +L 2,3 neff 2,3 . This condition (4) may be generalized without difficulty to the case where each arm comprises more than three sections S m,n . 
     In the laser source  180 , if it is not necessary for the bandpass filter to be athermal, then it is also not necessary for the lengths L 1,2  and L 2,2  to be equal. In this case, the laser source  108  operates correctly provided that sections S 1,2  and S 2,2  are configured so that the following condition is met:
 
 L   2,2   neff   2,2   −L   1,2   neff   1,2   =k   f λ Si  
 
The above condition may be met even with different lengths L 1,2  and L 2,2 .
 
     The core of the gain-generating sections such as sections S 1,2  and S 2,2  may be produced using other materials. For example, the stack  34  may also be a stack of sublayers in alternation made of InP and InGaAsP. As a variant, the gain medium is deposited directly on an extension made of silicon of the waveguide  15  or  25 . This superposition of the gain medium on the extension made of silicon then forms the core of an optical-gain-generating waveguide. In this case, the optical-gain-generating waveguide comprises III-V material but also silicon. 
     There are also other embodiments of the resonant cavity and, in particular, embodiments devoid of reflectors. For example, the cavity may take the form of a ring in which the optical signal rotates in such a way as, in each rotation, to pass through the gain medium of the interferometer. For example, this amounts to replacing the reflectors  12  and  14  with an additional waveguide that connects the left end of the resonant cavity to the right end while circumventing the gain media. In this case, the additional waveguide is, for example, produced in the layer  72 . 
     Whatever the embodiment described here, to limit the consequences of the shift in the reflective band of the reflectors as a function of temperature, these reflectors may be made from a material of low thermo-optical coefficient such as silicon nitride. 
     The reflectors are not necessarily Bragg gratings. For example, a reflector may also be produced using a Sagnac loop. 
     Lastly, the system allowing the variations in the wavelength λ Li  as a function of temperature to be limited is not necessarily a passive system as described in the preceding embodiments. Thus, as a variant, the laser source may comprise:
         a tuning device able to move the wavelengths λ Rj  as a function of an electrical control signal,   a sensor able to measure a physical quantity representative of the difference between the wavelength λ Cf  of the bandpass filter and the closest of the wavelengths λ Rj , and   an electronic circuit able to generate the electrical control signal so as to permanently keep one of the wavelengths λ Rj  at the centre of the passband of the bandpass filter.
 
The wavelength λ Cf  of the bandpass filter corresponds to a maximum of the transmission spectrum of this filter. Embodiments and implementations of these various components in a laser source are, for example, identical to those described in patent application US20180261976A.
       

     SECTION IV: ADVANTAGES OF THE EMBODIMENTS 
     The fact of producing the gain-generating waveguide in each of the arms of an interferometer allows the length of the resonant cavity to be substantially decreased with respect to the case of a resonant cavity that allows an identical optical signal of the same power to be generated, but in which the gain-generating waveguide is located outside of the bandpass filter. Specifically, here, the functions of filtering and amplifying the optical signal are interleaved and carried out by one and the same component, namely a Mach-Zehnder interferometer. Thus, at equal characteristic and, in particular, at equal power, the resonant cavity of the laser sources described here is much shorter. This therefore allows, at equal characteristic, the bulk of the laser source to be decreased. 
     The fact of using a Mach-Zehnder interferometer arranged so as to meet condition (2) or (3) allows the movement of the passband of this filter as a function of temperature to be limited. This limits the variations in the wavelength λ Li  as a function of temperature. In particular, this limitation is obtained without it being necessary to make some of this bandpass filter from materials of low thermo-optical coefficient such as silicon nitride. Production of this bandpass filter is therefore simpler. In addition, the limitation of the movement of the passband of the filter as a function of temperature that it is possible to obtain is better than the limitation that it is possible to obtain by making, at least partially, the bandpass filter from materials of low thermo-optical coefficient. 
     When the arms of the interferometer comprise no optical coupling with a secondary filter such as the filter  138  and when this interferometer is arranged to meet condition (2), its transmission spectrum varies very little as a function of temperature. Therefore, the position of the passband of its transmission spectrum is practically constant. By virtue of this, it is possible to limit the variations in the wavelength λ Li  of the laser source as a function of temperature. 
     When the arm of the interferometer comprises an optical coupling with a secondary filter such as the filter  138  and when this interferometer is arranged to meet condition (3), the obtained primary filter is also athermal. Thus, this embodiment of the bandpass filter for its part also allows the variations in the wavelength λ Li  as a function of temperature to be limited. In addition, the secondary filter allows the width of the one or more passbands of the bandpass filter to be decreased or certain passbands to be limited with respect to the case where this bandpass filter is devoid of such a secondary filter. This therefore allows a monomode laser source to be obtained 
     The fact that the secondary filter is a ring resonator allows the bulk of the laser source to be decreased. 
     The use of an athermal tertiary filter connected in series with the primary filter allows the passbands of the bandpass filter to be spaced apart while preserving small variations in the wavelength λ Li  as a function of temperature. 
     The use of an athermal Mach-Zehnder interferometer to produce the tertiary filter allows a tertiary filter the spectrum of which varies very little as a function of temperature to be obtained and therefore the variations in the wavelength λ Li  as a function of temperature to be very effectively limited. 
     The fact that the width of the passband of the bandpass filter is smaller than or equal to the interval Δλ R  allows a monomode laser source to be obtained. In contrast, the fact that the width of the passband of the bandpass filter is larger than the interval Δλ R  allows a multimode laser source to be obtained. 
     The use of reflectors the reflective band of which is narrow allows a monomode laser source to be obtained. 
     The presence in the transmission spectrum of the bandpass filter of a plurality of passbands spaced apart from one another by an interval equal to an integer multiple of the interval Δλ R  allows a multimode laser source to be obtained. 
     The fact that the passband of the bandpass filter has a width substantially identical to the interval Δλ R  allows a passive thermal stabilization of the wavelength λ Li  to be obtained. Specifically, if following heating of the resonant cavity the wavelength λ Rj  selected by the bandpass filter moves and exits from this passband, at the same time the preceding wavelength λ Rj−1  or the following wavelength λ Rj+1  enters into the interior of this passband. Thus, even in the absence of an active component for keeping one of the wavelengths λ Rj  at the centre of the passband of the bandpass filter, the variations in the wavelength λ Li  as a function of temperature are limited. 
     Advantages of the Currents I 1  and I 2  of Different Magnitudes: 
     The fact of injecting currents I 1  and I 2  of different magnitudes into the amplifiers  30  and  56  makes it possible to obtain sections S 1,2  and S 2,2  the effective indices of which are different without having to modify the transverse dimensions of these sections or the materials from which they are made. This therefore simplifies the production of the bandpass filter. 
     If, in addition, the arms  44  and  46  are structurally identical and these arms are not coupled to a secondary filter, then the simple fact of meeting condition (1) necessarily leads to condition (2) being met. This therefore facilitates the production of a primary filter that is in addition athermal.