The invention relates to a semiconductor laser source and to a method for emitting an optical signal at a wavelength λLi close to a desired wavelength λSi.
Known semiconductor laser sources comprise:
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 λLi 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 and to simplify the production thereof.
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 λLi 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.
Another subject of the invention is a method for emitting an optical signal at a wavelength λLi close to a desired wavelength λSi.
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:
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.
The wavelength λLi 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 λLi. When the laser source is a multimode or polychromatic laser source, a plurality of different wavelengths λLi exist simultaneously. In the latter case, the index i is an identifier of the wavelength λLi among the various wavelengths at which the laser source emits. Typically, the wavelength λLi is comprised between 1250 nm and 1590 nm.
The wavelengths λRj 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 λRj. These wavelengths λRj are regularly spaced apart from one another by an interval ΔλR. The index j is an identifier of one particular wavelength λRj.
The interval ΔR is the smallest wavelength range containing all the possible wavelengths λRj. 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 λRj able to be reflected by the reflector with a power higher than or equal to 50% of the maximum power Imax reflected by this reflector. The power Imax is equal to the power of the reflected optical signal for the wavelength λRj, at which this power is maximum.
The wavelength λCR is a wavelength corresponding to a maximum of the reflection spectrum of the reflector. Typically, the wavelength λCR is the wavelength located at the middle of the reflective band of the reflector. This wavelength λCR 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 ΔλFX, 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 λCFX, where the index FX is an identifier of the filter. Typically, the wavelength λCFX 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 dFSRX, 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 dλCFX/dT associated with this filter is comprised between Lmin and Lmax, where:
For a given filter, the coefficient dλCFX/dT may be determined experimentally or by numerical simulation. For example, the variation in the wavelength λCFX as a function of temperature is measured or simulated in a temperature range extending from 20° C. to 100° C. Typically, the wavelength λCFX varies approximately linearly as a function of temperature in this temperature range. It is therefore possible to approximate the relationship that relates the wavelength λCFX to the temperature T in this range using the following linear relationship: λCFX=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 λCFX measured or obtained by simulation for this temperature. The value of the coefficient dλCFX/dT is then set equal to the value of the coefficient a.
The thermo-optical coefficient of a material is the coefficient dnr/dT, where:
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 λRj. The wavelengths λRj are regularly spaced apart from one another by an interval ΔλR. In this embodiment, the interval ΔR that contains all the wavelengths λRj 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 λLi. 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:
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 Tmin and Tmax, 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 dλCR/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 Tmin, the wavelength λLi is closer to the upper limit λRmax of the reflective band of the reflectors 12 and 14 than to its lower limit λRmin. For example, at the temperature Tmin, the wavelength λLi is comprised between 0.9 λRmax and λRmax. 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:
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, Jul. 23, 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 λSi. Typically, it is centred on this wavelength λSi at the temperature (Tmax+Tmin)/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 [Tmin; Tmax]. In this case, the waveguide 28 and the amplifier 30 take the form of a stack 34 (
The amplifier 30 comprises, in addition to the waveguide 28, a connection 37 (
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:
In the direction of propagation F (
In
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 S1,1, S1,2 and S1,3 in order in the direction F. Below, section n of arm m is designated Sm,n, where:
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 Sm,n is a waveguide. Each of these sections therefore has an effective index Neffm,n and a nonzero length Lm,n. Typically, each section differs from the section that precedes it and from the section that follows it in:
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 Neffm,n of each section Sm,n is considered to be constant over all its length Lm,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 λSi.
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 neffm,n of the section Sm,n as a function of temperature is denoted dneffm,n/dT. To a first approximation, in the temperature range DT, this coefficient dneffm,n/dT may be considered to be constant.
Here, the section S1,1 extends from the fork 48 to the coupler 26. The section S1,1 is a waveguide made of silicon.
The section S1,2 extends from the coupler 26 to the coupler 32. The section S1,2 therefore corresponds to the waveguide 28 made of III-V gain media, in which waveguide the amplifier 30 is produced. The section S1,2 is here called the “gain-generating section” because it has the capacity to amplify the optical signal.
The section S1,3 extends from the coupler 32 to the fork 50. The section S1,3 is a waveguide made of silicon that is identical to the waveguide made of silicon of section S1,1 except that the length L1,3 made it be different from the length L1,1.
The arm 46 is divided into four consecutive sections S2,1, S2,2, S2,3 and S2,4 in order in the direction F. The sections S2,1 and S2,2 are identical to the sections S1,1 and S1,2, respectively. The arm 46 therefore comprises, just like the arm 44:
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 neff1,2 and neff2,2 are identical and the coefficients dneff1,2/dT and dneff2,2/dT are also identical.
The section S2,4 is identical to the section S1,3 except that its length L2,4 is larger than the length L1,3. In
The section S2,3 has a coefficient dneff2,3/dT lower than the coefficients dneff2,1/dT, dneff2,2/dT and dneff2,4/dT. To this end, according to a first solution, section S2,3 is a waveguide made of silicon the width of the core of which is smaller than the width of the core of sections S2,1 and S2,4. According to a second solution, section S2,3 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 S2,3 may be identical to the dimensions of the cores made of silicon of sections S2,1 and S2,4. A third solution consists in using an another material for the cladding of section S2,3, this material being different from that used for the cladding of sections S2,1 and S2,4. In addition, it is possible to combine these various solutions to obtain the desired coefficient dneff2,3/dT.
In order for the interferometer 42 to have a passband that allows the wavelength λLi to be selected, the sections Sm,n are configured to meet the following condition (1):
where:
A high number of lengths Lm,n and of indices Neffm,n allow condition (1) to be satisfied.
Here, the filter 40 is also to designed to be athermal. Thus its coefficient dλCF1/dT is close to zero, i.e. comprised between Lmin and Lmax. Below, a wavelength corresponding to a maximum of the transmission spectrum of the primary filter 40 is denoted λCF1. To this end, the sections Sm,n are configured to also meet the following condition (2):
where dneff1,n/dT and dneff2,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 Lm,n and the coefficients dneffm,n/dT meet condition (2) with Lmin=−7 pm/° C. and Lmax=+7 pm/° C. and, preferably, with Lmin=−2 pm/° C. and Lmax=+2 pm/° C.
There are a high number of sets of values for the lengths Lm,n and the indices neffm,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 Δλ40 to be obtained for the passband of the primary filter 40 larger than ΔλR 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 λRj move as a function of temperature. In particular, the variation in temperature may be such that a wavelength λRj precedingly located in the interior of the passband of the filter 22 moves and exits from this passband. In this case, since the width Δλ40 is larger than or equal to the interval ΔλR, when a wavelength λRj exits from the passband of the filter 22, another wavelength λRj−1 or λRRj+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 λRj so that there permanently exists a wavelength λRj located at the centre of the passband of the filter 22.
To configure the sections Sm,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 neffm,n and the coefficients dneffm,n/dT of each section are defined. Next, in a second step, the lengths Lm,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, Jan. 15, 2010, pages 1879-1887.
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 S1,1, S1,3, S2,1, S2,3 and S2,4 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.
The arm 108 is identical to the arm 46 except that it comprises only three sections S2,1, S2,2 and S2,3. Section S2,1 of the arm 108 is identical to section S1,1 of the arm 44. Section S2,2 is identical to section S2,2 of the interferometer 42 except that the amplifier 56 has been replaced by an amplifier 110 shorter than the amplifier 30. Thus, the length L2,2 of section S2,2 of the arm 108 is shorter than the length L1,2. Section S2,3 is identical to section S1,3 except that the length L2,3 is larger than or equal to the length L1,3.
As in the preceding embodiment, the various sections Sm,n are arranged so that the lengths Lm,n and the indices neffm,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 neff2,1 and neff2,2.
The filter 138 is a bandpass filter typically comprising a plurality of passbands. The width Δλ138 of each of these passbands is smaller than or equal to the interval ΔλR. Here, the width Δλ138 is equal to ΔλR. This filter 138 has a coefficient dneff138/dT higher than zero and generally higher than 15 pm/° C. or 50 pm/° C., where neff138 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 dneff138/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 S2,3 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.
Here, the filter 138 is arranged so that the distance dFSR138 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 dFSR138 is larger than ΔR or the filter 138 is configured so that the distance dFSR138 is a non-integer multiple of the distance dFSR136, as shown in
Here, the interferometer 136 is identical to the interferometer 42 except that the various sections Sm,n are arranged so as to meet condition (1) and the following condition (3):
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, Feb. 3, 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
The filter 154 is an athermal filter the transmission spectrum 156 of which (
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 (
Here, the filter 154 is an athermal Mach-Zehnder interferometer. For example, the filter 154 is identical to the filter 40 except that:
Thus, the arm 44 of the filter 154 comprises a single section S1,1 between the forks 48 and 50. This section S1,1 is a waveguide the core of which is made of silicon. This section S1,1 is identical, for example, to the concatenation of sections S1,1 and S1,3 of filter 40. The arm 46 of the filter 154 comprises three consecutive sections S2,1, S2,2 and S2,3. Sections S2,1, S2,2 and S2,3 of the filter 154 are for example identical to sections S2,1, S2,3 and S2,4 of the filter 40, respectively.
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.
In contrast, in this embodiment, the injection currents I1 and I2 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 S1,2 and S2,2 are structurally identical, their effective indices neff1,2 and neff2,2 are different. Specifically, since the magnitudes of the currents I1 and I2 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:
L2,2neff2,2−L1,2neff1,2=kfλSi
where the lengths L2,2 and L1,2 are equal.
Since sections S1,2 and S2,2 are structurally identical, the coefficients dneff1,2/dT and dneff2,2/dT are equal. Thus, condition (2) described above is also met provided that the lengths L2,2 and L1,2 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
The interferometer 246 is identical to the interferometer 186 except that the lengths L1,2 and L2,2 are different. As described above with reference to
L2,2neff2,2−L1,2neff1,2=kfλSi
Condition (3) for the primary filter 244 to be athermal is written in the following simplified form:
where the coefficients dneff1,2/dT and dneff2,2/dT are equal.
To find the magnitudes of the currents I1 and I2 and the lengths L1,2 and L2,2 that meet the above conditions (1) and (3), it is possible, for example, to first set the magnitudes of the currents I1 and I2 and then to seek the lengths L1,2 and L2,2 that meet the above conditions. It is also possible to proceed in the opposite way, i.e. for the lengths L1,2 and L2,2 to be set first and then the magnitudes of the currents I1 and I2 sought.
In the embodiment of
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 Sm,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 dλCF3/dT is equal or practically equal to the coefficient dλCF1/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 S2,1 or S2,4 and the ring 140, or even between section S2,2 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
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 S1,2 and S2,2 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.
The fact of injecting currents I1 and I2 of different magnitudes into the amplifiers 30 and 56 may be implemented in all the embodiments described here. In particular, currents I1 and I2 of different magnitudes may be injected through the amplifiers 30 and 56 of the embodiments of
In the laser source 180, it is not necessary for sections S1,1 and S1,3 to be identical to sections S2,1 and S2,3, respectively. For example, as a variant, section S1,1 is longer than section S2,1 and, to compensate, section S1,3 is shorter than section S2,3. In fact, it is enough for the following condition (4) to be met for the laser source 180 to operate correctly: L1,1neff1,1+L1,3neff1,3=L2,1neff2,1+L2,3neff2,3. This condition (4) may be generalized without difficulty to the case where each arm comprises more than three sections Sm,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 L1,2 and L2,2 to be equal. In this case, the laser source 108 operates correctly provided that sections S1,2 and S2,2 are configured so that the following condition is met:
L2,2neff2,2−L1,2neff1,2=kfλSi
The above condition may be met even with different lengths L1,2 and L2,2.
The core of the gain-generating sections such as sections S1,2 and S2,2 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 λLi 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:
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 λLi 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 λLi 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 λLi 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 λLi 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 λLi 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 ΔλR 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 ΔλR 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 ΔλR 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 ΔλR allows a passive thermal stabilization of the wavelength λLi to be obtained. Specifically, if following heating of the resonant cavity the wavelength λRj selected by the bandpass filter moves and exits from this passband, at the same time the preceding wavelength λRj−1 or the following wavelength λRj+1 enters into the interior of this passband. Thus, even in the absence of an active component for keeping one of the wavelengths λRj at the centre of the passband of the bandpass filter, the variations in the wavelength λLi as a function of temperature are limited.
The fact of injecting currents I1 and I2 of different magnitudes into the amplifiers 30 and 56 makes it possible to obtain sections S1,2 and S2,2 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.
Number | Date | Country | Kind |
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18 60520 | Nov 2018 | FR | national |
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Entry |
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French Preliminary Search Report dated Sep. 24, 2019 in French Application 18 60519 filed on Nov. 15, 2018 (with English Translation of Categories of Cited Documents & Written Opinion). |
French Preliminary Search Report dated Sep. 24, 2019 in French Application 18 60520 filed on Nov. 15, 2018 (with English Translation of Categories of Cited Documents & Written Opinion). |
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Number | Date | Country | |
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20200161832 A1 | May 2020 | US |