METHOD AND SYSTEM FOR CHARACTERISING A BATCH OF GAIN CHIPS

Technical field of the invention

The present invention relates to a method, and an associated system, for characterizing a batch of gain chips, for example for an external-cavity laser.

Prior art

Typically, a gain chip has an anode and a cathode which can be connected to a power supply source (e.g., two poles of a voltage generator) in order to cause current to flow in the gain chip. This flow of current brings about the generation of a light beam (electromagnetic radiation), which is propagated in a guided manner within the waveguide of the gain chip. Typically, the waveguide of the gain chip extends between two end faces of the gain chip, wherein a first face has a behaviour (at least partially) reflective (e.g., it behaves like a mirror) and a second face has a semi-reflective or anti-reflective behaviour (e.g., thanks to the presence of one or more anti-reflection layers deposited on the second face and/or thanks to the non-orthogonality of the second face relative to the waveguide).

Gain chips can be used as a gain medium, for example in the production of tunable lasers and/or external-cavity lasers which are highly stable.

In the context of the application of a gain chip as a gain medium, it is important to characterize the gain chip, in order to determine the properties of the gain chip and predict its behaviour in the laser. Documents US10732105B1 and US2012268743A1 describe a respective method, and associated system, for characterizing a gain chip.

Summary of the invention

"Gain chip” means a semiconductor (gain) element with an active optical waveguide.

The terms "optical” and "light” refer to electromagnetic radiation that falls within an range extending beyond the visible optical band, and does not necessarily fall strictly within the visible optical band (i.e., approximately 400-700 nm), for example this range extending beyond the visible optical band typically includes near-infrared (for example, wavelength comprised between about 700 nm and about 2 m) and/or ultraviolet (for example, wavelength comprised between about 150 nm and about 400 nm).

"Quantum efficiency” (or more properly "external quantum efficiency”) means, for example, the number of photons that leave the gain chip for each electron-hole recombination event (further details can be found, for example, in G. P. Agrawal and N.K. Dutta, Semiconductor Lasers, e.g., Eq. (2.6.10)).

"Differential quantum efficiency” (or more properly "differential external quantum efficiency”) means, for example, the local value of the quantum efficiency, i.e., the value around a given value of supply current in a regime of linearity of the optical power-power supply current curve.

"Carrier density at transparency” means, for example, the concentration of charge carriers at which the photon emission rate is equal to the photon absorption rate (further details can be found, for example, in G. P. Agrawal and N.K. Dutta, Semiconductor Lasers).

"Modal gain coefficient” (# ₀ ) means, for example, a parameter defined by the relation g ₀ = rg _t , where r is a confinement coefficient of the optical mode considered (e.g., the fundamental transverse electric mode TE00), preferably having a value comprised between 0 and 1 (excluding the endpoints), and g _t is the gain coefficient of the material (of which the gain chip is made).

The Applicant has realized that the known methods and systems for characterizing a gain chip have several drawbacks and/or can be improved in some respects.

For example, the Applicant has observed that gain chip manufacturers supply gain chips together with the (nominal) values of several macroscopic parameters of the gain chip, such as, for example, the threshold current of the gain chip, i.e., the current value below which the gain chip (when inserted into the cavity of a laser) generates only amplified spontaneous emission, or ASE (in other words, the current value above which the gain equals the internal losses and the cavity losses, e.g., due to mirrors).

The Applicant believes that the macroscopic parameters of the gain chip may not be sufficient to provide complete indications about the behaviour of the gain chip, especially the long-term behaviour. If the device comprising the gain chip is used in applications in places that are difficult to access (e.g., at the bottom of the sea and/or inside complex operating systems) and/or require a long period of operation, the result may be a use of non-optimal chips, which, in the event of malfunctioning and/or failure, can require a complex and/or costly replacement and/or maintenance.

The Applicant has further observed that the real values of the macroscopic parameters of the gain chips in each production batch may not correspond to the nominal values declared by the manufacturer. For example, in order to reduce costs and/or production times, gain chip manufacturers carry out a sampling inspection, i.e., they conduct experimental tests on a low percentage of the gain chips produced, since such experimental tests can prove burdensome from the viewpoint of the costs and/or times of carrying them out and/or can be destructive of the gain chip tested. The Applicant has therefore addressed the problem of characterizing a batch (e.g., a production and/or sales and/or inventory batch, also a numerically large one) of gain chips in a simple and/or quick and/or reliable and/or economical way, while at the same time determining one or more gain chip parameters which are indicative of the potential real performance of the given gain chip, for example for the purpose of selecting the gain chip that is most suitable for the specific application of interest.

According to the Applicant, the aforesaid problem is solved by a method, and an associated system, for characterizing a batch of gain chips in accordance with the appended claims and/or having one or more of the following features.

According to one aspect, the invention relates to a method for characterizing a batch of gain chips, wherein each gain chip has a respective first face that is at least partially reflective and a respective second face that is at least partially transparent.

Preferably, said method comprises carrying out the following operations on each gain chip of said batch: a) positioning the gain chip with said respective second face facing towards a reflective diffraction grating; b1) orienting said reflective diffraction grating in a first angular position; b2) supplying said gain chip with a plurality of first values of supply current so that said gain chip emits (from said second face) a respective first light beam for each first value of supply current; b3) making impinge each first light beam onto said reflective diffraction grating in the first angular position so as to generate a respective first zero-order diffracted beam and a respective first first-order diffracted beam, said respective first first-order diffracted beam entering (e.g., a waveguide of) said gain chip through said second face; b4) measuring a respective first value of optical power of each first zero-order diffracted beam; b5) associating each first value of optical power with the respective first value of supply current; c) as a function of a plurality of pairs of first values of supply current and of optical power and as a function of a first predetermined mathematical correlation, calculating a respective differential quantum efficiency; d1) orienting said reflective diffraction grating in a second angular position; d2) supplying said gain chip with a plurality of second values of supply current so that said gain chip emits (from said second face) a respective second light beam for each second value of supply current; d3) making impinge each second light beam on said reflective diffraction grating in the second angular position so as to generate a respective second zero-order diffracted beam and a respective second first-order diffracted beam, said respective second first-order diffracted beam not entering said gain chip (e.g., it does not enter the waveguide of the gain chip); d4) measuring a respective second value of optical power of each second zero-order diffracted beam; d5) associating each second value of optical power with the respective second value of supply current; e) as a function of a plurality of pairs of second values of supply current and of optical power and as a function of a second predetermined mathematical correlation, calculating a respective modal gain coefficient and a respective carrier density at transparency; n) verifying that said respective differential quantum efficiency, said respective modal gain coefficient and said respective carrier density at transparency satisfy a respective predetermined selection criterion.

According to one aspect, the invention relates to a system for characterizing a batch of gain chips, wherein each gain chip has a respective first face that is at least partially reflective and a respective second face that is at least partially transparent.

Preferably, said characterization system comprises:

- a test bench comprising:

I) a positioning device for a gain chip; ii) a supply source structured to supply a supply current to said gain chip positioned in the positioning device so that said gain chip emits (from said second face) a light beam; ill) a reflective diffraction grating such that (when in the first or second angular position) said light beam incident onto said reflective diffraction grating generates a respective first zero-order diffracted beam and a respective first first-order diffracted beam, iv) a first actuator structured to orient said reflective diffraction grating alternatively between a first angular position in which said first-order diffracted beam enters said gain chip through said second face, and a second angular position in which said first-order diffracted beam does not enter said gain chip (e.g., does not enter the waveguide of the gain chip); iv) an optical power measuring device structured to measure an optical power of said first zero-order diffracted beam; v) a command-and-control unit operatively connected to said supply source, said first actuator and said measuring device,

- a processing unit connected to said command-and-control unit. Preferably, said command-and-control unit is programmed and configured to carry out the following operations, with said gain chip positioned in the positioning device with said second face facing towards said reflective diffraction grating: f1) commanding said first actuator to orient said reflective diffraction grating in said first angular position; f2) commanding said supply source to supply said gain chip with a plurality of first values of said supply current so that said gain chip emits a respective first light beam for each first value of supply current, said respective first light beam generating a respective first zero-order diffracted beam and a respective first first-order diffracted beam; f3) receiving from said optical power measuring device a respective first value of optical power of each first zero-order diffracted beam (measured by said measuring device); f4) associating each first value of optical power with the respective first value of supply current; g1) commanding said first actuator to orient said reflective diffraction grating in said second angular position; g2) commanding said supply source to supply said gain chip with a plurality of second values of supply current so that said gain chip emits a respective second light beam for each second value of supply current, said respective second light beam generating a respective second zero-order diffracted beam and a respective second first-order diffracted beam; g3) receiving from said measuring device a respective second value of optical power of each second zero-order diffracted beam (measured by said measuring device); g4) associating each second value of optical power with the respective second value of supply current.

Preferably, said processing unit is programmed and configured to carry out the following operations:

11) receiving from said command-and-control unit a first plurality of pairs of first values of supply current and of optical power;

12) as a function of said first plurality and as a function of a first predetermined mathematical correlation, calculating a respective differential quantum efficiency; ml) receiving from said command-and-control unit a second plurality of pairs of second values of supply current and of optical power; m2) as a function of said second plurality and as a function of a second predetermined mathematical correlation, calculating a respective modal gain coefficient and a respective carrier density at transparency; n) verifying that said respective differential quantum efficiency, said respective modal gain coefficient and said respective carrier density at transparency satisfy a predetermined selection criterion.

The Applicant, without wishing to be limited to any theory, believes that the differential quantum efficiency, the modal gain coefficient and the carrier density at transparency are (microscopic) parameters of the gain chip which provide reliable and/or significant and/or complete indications about the (micro)structural properties of the gain chip, and that they thus allow for reliably predicting the behaviour of the gain chip, in particular the long-term behaviour. In this manner, it is possible, for example, to accurately select the most suitable, or optimal, gain chip for a given application. The Applicant also believes that the method of the present invention is highly reliable and/or simple and/or shows a high consistency of results, since the calculation of the differential quantum efficiency, of the modal gain coefficient and of the carrier density at transparency is made as a function of predetermined mathematical correlations (derived from suitable physical-mathematical models) and as a function of pairs of values of supply current and of optical power which are measurable in an accurate and/or simple and/or rapid manner and/or by exploiting easily available equipment.

The Applicant observes that, with the reflective diffraction grating in the first angular position, the first-order diffracted beam returns towards the gain chip and re-enters the waveguide of the gain chip. Subsequently, the first-order diffracted beam is (at least in part) reflected by the first face of the gain chip and is once again amplified in the waveguide. The Applicant has observed that, in this manner, it is possible to measure the optical power of the first zero-order diffracted beam under a lasing condition. When the reflective diffraction grating is oriented in the second angular position, the first-order diffracted beam does not re-enter the waveguide of the gain chip (in other words, it is lost). In this manner, it is possible to measure the optical power of the first zero-order diffracted beam under a condition of amplified spontaneous emission (ASE). The Applicant has realized that it is advantageous to characterize the gain chip under the two aforesaid different operating conditions (lasing and ASE) and that these different operating conditions are advantageously obtainable in a simple manner using a single test bench. This therefore enables a simplification and/or reduction in the characterization costs and/or times and at the same time a complete and/or comprehensive characterization of the gain chip. In particular, the Applicant has realized that the reflective diffraction grating orientable between two different angular positions allows the current operating condition of the gain chip to be changed in a simple and rapid manner.

The Applicant observes that the characterization method and system according to the present invention are the result of long, extensive analyses and tests, which have led to an accurate and intentional selection, among the countless possible choices, of the instrumental components, test method and detected microscopic parameters that are most suitable for achieving the objective of great structural and/or operational simplicity, rapidity of execution and high repeatability of the results. In this manner it is possible (e.g., for the manufacturer and/or a user of the gain chip) to test up to 100% of a batch of gain chips with industrially acceptable costs and times.

In one or more of the aforesaid aspects, the present invention can have one or more of the following preferred features. Typically, said reflective diffraction grating is of the "blazed grating” type (also known as "echelette grating”), i.e., a diffraction grating comprising a plurality of teeth on a first face of the grating, wherein said plurality of teeth has a constant pitch and, in a cross section (taken along a direction perpendicular to the first face), a step-like pattern with a triangular sawtooth shape.

Typically, said first face of each gain chip is (substantially) entirely reflective and said second face of each gain chip is (substantially) entirely transparent.

In one embodiment, said verifying comprises ordering said gain chips of said batch according to a progressive (e.g., decreasing) order of one or more of said respective differential quantum efficiency, said respective modal gain coefficient and said respective carrier density at transparency. In this manner it is advantageously possible to make a comparison among the gain chips of the batch, by identifying one or more gain chips having given performance features.

Preferably, said respective predetermined selection criterion comprises a respective predetermined threshold value. In this manner it is possible to verify, in a simple manner, which chips have one or more microscopic parameters above a desired value.

Preferably, said verifying comprises:

- as a function of said respective differential quantum efficiency, said respective modal gain coefficient and said respective carrier density at transparency of each gain chip, determining a respective statistical set for each of said differential quantum efficiency, said modal gain coefficient and said carrier density at transparency;

- identifying one (or more) gain chips whose respective differential quantum efficiency, whose respective modal gain coefficient and/or whose respective carrier density at transparency represent an outlier in the respective statistical set (e.g., based on a comparison with the interquartile range).

In this manner the Applicant has realized that it is advantageously possible to identify in a simple and/or rapid manner any gain chips not conforming to given applications because of the expected performance features (e.g., a gain chip, possibly similar to the other gain chips of the batch according to the macroscopic parameters, but having a highly different behaviour from the other gain chips of the batch).

For one or more gain chips of the batch, said operations from b1) to b5) can be carried out before said operations from d1) to d5) (said command-and-control unit being programmed and configured to carry out the operations from f1) to f4) before the operations from g1) to g4)) and, for one or more gain chips of the batch, said operations from d1) to d5) can be carried out before said operations from b1) to b5) (said command-and-control unit being programmed and configured to carry out the operations from g1) to g4) before the operations from f1) to f4)).

For one or more gain chips of the batch, said operation c) or said operation e) can be carried out respectively before said operations from d1) to d5) or said operations from b1) to b5) (said processing unit being programmed and configured to carry out the operations 11) and I2) before the operations from g1 ) to g4) or the operations ml) and I2) before the operations from f1) to f4)). In other words, the calculation of the microscopic parameters of the gain chips is carried out at the end of the respective acquisition of the measured data, and before starting the subsequent acquisition of the data under another operating condition of the chip.

For one or more gain chips of the batch, said operation c) and said operation e) can both be carried out at the end of all said operations from b1) to b5) and said operations from d1) to d5) (said processing unit being programmed and configured to carry out the operations 11), I2), and ml), m2) at the end of all the operations from f1) to f4) and from g1) to g4)). In other words, the calculation of the microscopic parameters of the gain chip is carried out at the end of both acquisitions under the two operating conditions of the chip.

Preferably, said test bench comprises a collimating lens interposed between said positioning device and said reflective diffraction grating and structured to collimate said first and said second light beam.

Preferably, prior to said operations b3) and d3), it is envisaged to collimate said first and said second light beam.

Preferably, said test bench comprises a light beam profiler, more preferably aligned with said positioning device, structured to measure a profile of a light beam (e.g., a cross-section shape of said light beam).

Preferably, said command-and-control unit is operatively connected to said light beam profiler.

Preferably, said test bench comprises a second actuator, preferably multi-axis, e.g., with five axes (e.g., three translational axes and two rotational axes), structured to move said collimating lens in space (along said axes).

Preferably, said command-and-control unit is operatively connected to said second actuator. Preferably, said test bench comprises a mechanical arm (motorized, e.g., rotating about a rotation axis) structured to move said reflective diffraction grating between an operative position (in which it intercepts said light beams), and a non-operative position, in which said reflective diffraction grating does not intercept said light beams. It is observed that the first and second angular positions are achieved with the reflective diffraction grating in the operative position (in other words, orienting said reflective diffraction grating in said first or second angular position comprises positioning said grating in said operative position).

Preferably, said command-and-control unit is operatively connected to said mechanical arm.

Preferably, preliminarily (e.g., before said operations b1) and d1 )), it is envisaged to position said reflective diffraction grating in said non-operative position. Preferably, said command-and-control unit is programmed and configured to, preliminarily (e.g., before said operations f1) and g1 )), command said mechanical arm to position said grating in said non-operative position.

Preferably, preliminarily (and after positioning said reflective diffraction grating in said non-operative position), said command-and-control unit is programmed and configured to command said second actuator to position said collimating lens in space.

Preferably, preliminarily (and after said positioning of said collimating lens in space), it is envisaged (said command- and-control unit is programmed and configured to command said supply source) to supply said gain chip with a test value of supply current so that said gain chip emits a test light beam, it is envisaged (e.g. by means of said collimating lens) to collimate said test light beam (to obtain a collimated test light beam), and it is envisaged to measure (said command-and-control unit is programmed and configured to receive from said light beam profiler a measurement of) a profile of said collimated test light beam.

Preferably, it is envisaged (said processing unit is programmed and configured to receive from said command-and- control unit said profile of said collimated test light beam) to verify that said profile satisfies a predetermined profiling criterion.

In this manner it is advantageously possible to transform the light beam emitted by the gain chip, which typically has high divergence, into a light beam with low divergence.

Preferably, said test bench comprises a mirror structured to reflect said respective first (and possibly said second) first- order diffracted beam towards said reflective diffraction grating. Preferably, (at least) during said operations from b1) to b5) and said operations from d1) to d5) (or said operations from f1) to f4) and said operations from g1) to g4)) it is envisaged to maintain said mirror fixed in space.

Preferably, said test bench comprises a third actuator structured to modify a spatial position of said mirror.

Preferably, said command-and-control unit is operatively connected to said third actuator.

In this manner, for every gain chip under examination, it is possible to create an external laser cavity by acting on the actuators of the grating and the mirror.

Preferably, said first predetermined mathematical correlation is based on the following first formula: where P _opt is said optical power, I is said supply current, l _th is a threshold current of said gain chip (typically predetermined), q _d is said differential quantum efficiency, h is the Planck constant, v is a photon frequency (typically predetermined, e.g. corresponding to a wavelength at which a gain peak of said gain chip occurs, this wavelength being, for example, a central wavelength in a tuning range of the gain chip), and q is the electron charge.

The Applicant, without wishing to be limited to any theory, has observed that the use of the aforesaid formula, which correlates the optical power and the supply current with the reflective diffraction grating in the first angular position (i.e. , with the gain chip that creates a mirror of the external laser cavity), provides reliable and repeatable results on the detected values of differential quantum efficiency, in a simple and rapid manner. Therefore, the Applicant believes to have made a selection, among the various available physical-mathematical models, of a model that is particularly advantageous for the pre-established purpose.

Preferably, said operation c) (or said operation 12)) comprises determining a first function, preferably linear, that interpolates said (first) plurality of pairs of first values of supply current and of optical power.

Preferably, said first linear function is determined by linear regression of said plurality of pairs of first values of supply current and of optical power. In this manner the line that interpolates the pairs of values of supply current and of optical power is obtained in a way that is computationally simple and/or highly accurate.

Preferably, said first linear function is determined at said first values of supply current which are greater than said threshold current of said gain chip. In this manner the linear regression is conducted in the regime of lasing currents (for values of supply current below the threshold current, the optical power is the ASE power, which is substantially negligible). Preferably, said first linear function is determined at said first values of supply current below an upper reference value (e.g., above which the linear correlation between the optical power and the supply current might no longer be true).

Preferably, said first predetermined mathematical correlation is obtained by deriving said first formula as a function of said supply current.

Preferably, said first predetermined mathematical correlation is: where m is an angular coefficient of said first linear function.

Preferably, said second predetermined mathematical correlation is: where P _opt is said optical power, I is said supply current, g ₀ is said modal gain coefficient, N _o is said carrier density at transparency, n _sp is a population inversion factor (typically predetermined), h is the Planck constant, v is the photon frequency, v _g is a group velocity (in the medium), L is a length of said gain chip (taken along a prevalent direction of extension of the gain chip which goes from said first to said second face), represents internal losses of said gain chip, rj is an injection efficiency (preferably measurable by means of appropriate experimental tests or, alternatively, predeterminable), C is the Auger recombination coefficient (expressed in cm ⁶ s~ ¹ , and preferably measurable by means of appropriate experimental tests or, alternatively, predeterminable), V is a volume of said gain chip, and q is the electron charge.

The Applicant, without wishing to be limited to any theory, has made the following observations. First, the Applicant believes that when the gain chip is supplied with a supply current, the electrons and the holes recombine radiatively. The optical power generated becomes a guided mode and the photon density distribution (<J>) along the direction z

(i.e. , the direction of the length L of the gain chip) can be expressed by the following formula (4): where r is the confinement coefficient of the optical mode (e.g., measurable by means of appropriate experimental tests or, alternatively, predeterminable), and g ) is a gain of the material (of which the gain chip is made). The Applicant also believes that any retro-reflection effects at the first and/or second face of the gain chip may be disregarded. By integrating the formula (4) and assuming the boundary condition < >(/, 0) = 0, it is possible to obtain the photon density < >(/).

The optical power can be expressed by means of the relation P _opt where S is a surface area of the first and/or second face of the gain chip.

Therefore, the optical power at the second face (i.e., output from the gain chip, i.e., with z=L) is given by the relation:

The gain of the material g ) can be expressed as a function of a carrier density (N) in the active layer of the gain chip

(i.e., the region of the gain chip comprising quantum wells), with that gain of the material which can be expressed by the relation: where g _t is the gain coefficient of the material (depending on the characteristics of the active layer of the gain chip, such as, for example, the number and thickness of the quantum wells). The Applicant has observed that for low values of supply current (e.g., less than 20 mA), the carrier density (N) is low (N<No) and the stimulated emission can be disregarded. In fact, under that condition the gain chip absorbs more photons than it emits, i.e., g(N) < 0. As the value of supply current increases, there is a progressive increase in the charge carrier density until arriving at a value of that charge carrier density (i.e., the carrier density at transparency) at which an inversion in the behaviour of the gain chip occurs, that is, the photon absorption rate equals the photon emission rate (i.e., g(N) = 0).

The Applicant has also observed that it is possible to define the modal gain, i.e., an effective gain that a determined optical mode undergoes (e.g., the fundamental electric transverse mode), as the product between the confinement coefficient (r) and the gain of the material (g(N)). The Applicant has identified that at transparency (i.e., for N=No), the modal gain is defined by the coefficient (defined as modal gain coefficient g ₀ ) expressed by the relation g ₀ = rg _t . Moreover, in order to express the carrier density (N) as a function of the supply current (I), the Applicant has observed that it is possible to use a transport model (e.g., the model known as "ABC”), which, in the high-density approximation (specific case of local density approximation, or LDA) can be substantially expressed by the formula:

Therefore, the carrier density N can be substantially expressed according to the relation:

Based on such observations, the Applicant has arrived at the formulation of the aforesaid second predetermined mathematical correlation according to the formula (3). The Applicant has verified that the use of the aforesaid formula (3) provides reliable and repeatable results on the detected values of the modal gain coefficient and carrier density at transparency, in a simple and rapid manner. Therefore, the Applicant believes to have made a selection, among the various available physical-mathematical models, of a model that is particularly advantageous for the pre-established purpose.

Preferably, said calculating of said respective modal gain coefficient and said respective carrier density at transparency comprises calculating a function for the minimization of a residual sum of squares, said minimization function being a function of said second values of optical power and of a plurality of theoretical values of optical power calculated as a function of said second predetermined mathematical correlation. The Applicant has realized that in this manner it is possible to favour a reliable calculation of the modal gain coefficient and of the carrier density at transparency. In other words, said values of modal gain and carrier density at transparency are the values that make said residual sum of squares minimum.

Preferably, said test bench comprises a thermostat structured to maintain said gain chip, positioned in said positioning device, stable in temperature.

Preferably, said command-and-control unit is operatively connected to said thermostat. Preferably, said command- and-control unit is programmed and configured to, during (at least) said operations f2), f3) and g2), g3), command said thermostat to maintain said gain chip at a first temperature.

Preferably, (at least) during said operations from b2) to b4) and said operations from d2) to d4), it is envisaged to maintain said gain chip at a first temperature.

The Applicant has realized that temperature has a large influence on the behaviour of the gain chip and is therefore able to influence (at least in part) the calculation of the microscopic parameters of the gain chip. Consequently, it can be advantageous to carry out the method of the present invention maintaining the temperature of the gain chip constant, at least during the operations of acquiring the optical power (under the two operating conditions), in order to limit the risk that the values calculated for the microscopic parameters might deviate from the real values.

Preferably, said command-and-control unit is programmed and configured to repeat at least said operations from f2) to f4) and said operations from g2) to g4) while maintaining said gain chip at a second temperature, and said processing unit is programmed and configured to repeat said operations 11), I2), ml), m2), and n) (starting from the optical power values measured with the gain chip at the second temperature).

Preferably, it is envisaged to repeat said operations from b2) to b5), and from d2) to d5) while maintaining said gain chip at (at least) a second temperature, and repeating said operations c), e) and n) (starting from the optical power values measured with the gain chip at the second temperature).

The Applicant believes that is advantageous to carry out the acquisitions of the optical power under the two operating conditions (lasing and ASE) with the gain chip at (at least) a second temperature (different from the first temperature), thereby obtaining - the supply current being equal - first and second values of optical power that are different from the ones obtained with the gain chip at the first temperature. Consequently, the value of the microscopic parameters subsequently calculated will also differ, which allows for evaluating the behaviour of the gain chip with variations in temperature. This makes it possible to simulate a condition in which an overheating and/or cooling of the environment surrounding the gain chip occurs, verifying that such temperature variations do not influence (at least not significantly) the behaviour of the gain chip.

Preferably, a difference (in absolute value) between said first and second temperatures is at least equal to 4°C, more preferably at least equal to 8°C. In this manner it is possible to evaluate the behaviour of the gain chip when there are significant temperature variations.

Brief description of the figures figure 1 schematically shows a system for characterizing a batch of gain chips according to one embodiment of the present invention; figure 2 schematically shows the characterization system in figure 1 in a preliminary step of an example of the characterization method according to the present invention; figure 3 schematically shows the characterization system in figure 1 in a first step of an example of the characterization method according to the present invention; figure 4 schematically shows the characterization system in figure 1 in a second step of an example of the characterization method according to the present invention; figure 5 shows an example of pairs of values of optical power and supply current obtained in the first step of the method, at two different temperature values of the gain chip; figure 6 shows an example of pairs of values of optical power and supply current obtained in the second step of the method, at two different temperature values of the gain chip.

Detailed description of some embodiments of the invention

The features and advantages of the present invention will be further clarified by the following detailed description of some embodiments, presented by way of example and not by way of limitation of the present invention, with reference to the appended figures.

In the figures, the reference number 100 denotes in its entirety a system for characterizing a batch of gain chips according to one embodiment of the present invention.

The characterization system 100 comprises a test bench 1 and a processing unit 90.

The test bench 1 comprises a positioning device 2 (shown in a purely schematic manner) which typically comprises a positioning cavity 2' for positioning a gain chip and a plurality of stop elements (not shown) which, when in action, prevent the translation and/or the rotation (about any axis) of the gain chip positioned in the positioning cavity 2'. Although the positioning device 2 is shown in the figures as made up of two distinct bodies, this division has the sole purpose of enabling a clearer representation of the characterization system 100 in its entirety and does not reflect the real structure of the positioning device. By way of example, the positioning device 2 can further comprise, moreover, one or more actuators (not shown) structured to move the stop elements (typically in the range of several micrometres) so that it is possible to reposition the gain chip inserted in the positioning cavity 2'. In this manner, it is possible to simplify the alignment of the components of the characterization system 100.

The test bench 1 comprises a supply source 3 structured to supply a supply current into the gain chip positioned in the positioning device 2. The supply source 3 is for example a voltage generator comprising two electrodes 3' and 3” which are connected at two opposite poles (e.g., anode and cathode) of the gain chip so that the latter is supplied with current. By way of example, the test bench 1 comprises a thermostat 20 (shown only schematically) structured to maintain the gain chip, positioned in the positioning device 2, at a fixed temperature.

For example, the thermostat 20 and the supply source 3 are physically integrated in a single physical device, such as, for example, the LDC-3726 Laser Diode Controller sold by the Newport Corporation.

The test bench 1 comprises a reflective diffraction grating 6, typically of the "blazed grating” type (also known as "echelette grating”), and a first actuator 7 structured to orient (with micrometric precision) the reflective diffraction grating 6, e.g., alternatively between a first angular position and a second angular position. By way of example, the diffraction grating 6 is rotatable about an axis perpendicular to the plane of the sheet (as shown by the respective rotation arrow in figure 4).

By way of example, the test bench 1 comprises one (or more) collimating lenses 4, typically aligned with the positioning device 2 (in particular with the positioning cavity) and interposed between the positioning device 2 and the reflective diffraction grating 6.

By way of example, the test bench 1 also comprises a second actuator 5 structured to move the collimating lens 4 in space. For example, the second actuator 5 is capable of adjusting the position of the collimating lens 4 in space along the three translational axes and is capable of rotating the collimating lens 4 about two rotation axes (typically, the collimating lens 4 does not rotate about the rotation axis substantially perpendicular to the main faces of the collimating lens itself).

By way of example, the test bench 1 comprises a motorized mechanical arm 8 comprising a member 8' connected, at a first end, directly to the reflective diffraction grating 6 and, at a second end, to a respective actuator 8” which enables a rotation of the member 8' (for example about a rotation axis perpendicular to the plane of the sheet).

By way of example, the test bench 1 comprises a light beam profiler 9, for example aligned with the positioning device 2 and positioned distally from the positioning device 2 relative to the reflective diffraction grating 6. The profiler 9 is structured to measure a profile of a light beam, for example a cross-section shape of the light beam (i.e., the shape of the light beam on the plane transverse to the direction of propagation of the light beam).

By way of example, the test bench 1 comprises a mirror 10, for example a flat mirror, and, for example, a third actuator 11 structured to orient the mirror 10 in space (e.g., by rotating it about a respective rotation axis, for example perpendicular to the plane of the figures).

The test bench 1 comprises an optical power measuring device 12.

The test bench 1 comprises a command-and-control unit 80 operatively connected (for example by means of a respective wired or wireless communication line) to: - the supply source 3 (via the communication line A); - the first actuator 7 (via the communication line B); and

- the measuring device 12 (via the communication line C).

By way of example, the command-and-control unit 80 is also operatively connected (e.g., by means of a respective wired or wireless communication line) to:

- the thermostat 20 (via the communication line D);

- the second actuator 5 (via the communication line E);

- the further actuator 8” (via the communication line F);

- the light beam profiler 9 (via the communication line G); e

- the third actuator 11 (via the communication line H).

The characterization system 100 also comprises a processing unit 90 connected to the command-and-control unit 80 (via a respective wired or wireless communication line, indicated by the letter M).

The present invention envisages any arrangement and logical and/or physical distribution both of the processing unit and of the command-and-control unit. Each of such units can for example consist of a respective single physical and/or logical unit or be made up of a respective plurality of physical and/or logical units distinct from and cooperating with each other. Moreover, for the purposes of the present invention, the division between the processing unit and command-and-control unit (as shown in the figures) is purely for illustrative purposes, as such units can be integrated into a single physical and/or logical unit.

Described below is an example of the method for characterizing a batch of gain chips, implementable through the characterization system 100 described above. For example, each gain chip comprises a first entirely reflective face and a second face that is entirely transparent (e.g., thanks to the depositing of anti-reflection layers).

First, the method envisages positioning a first gain chip 99 in the positioning cavity 2' of the positioning device 2 with the second face facing towards the light beam profiler 9, and securing it in the cavity 2' by means of the stop elements. At this point, the method envisages, by way of example, preliminarily carrying out a profiling test (schematically shown in figure 2). The profiling test envisages first activating the mechanical arm 8 (via a command of the command-and- control unit 80) to move the reflective diffraction grating 6 into a non-operative position in which the grating 6 is located outside a line joining the positioning device 2 and the profiler 9. In this manner, only the collimating lens 4 is interposed between the positioning device 2 and the light beam profiler 9. Subsequently, the second actuator 5 is activated in order to position the collimating lens 4 in a first spatial position.

Subsequently, the profiling test envisages supplying the gain chip 99 (by applying a potential difference between the two electrodes 3' and 3”, controlled by the command-and-control unit 80) with a test value of the supply current (e.g., 150 mA) so that the gain chip 99 emits a test light beam 70. This light beam is collimated by the collimating lens 4 into a collimated test light beam 71 which is captured by the profiler 9 (without impinging on the reflective diffraction grating 6, which is in the non-operative position).

The light beam profiler 9 measures the profile of the collimated test light beam 71 and sends the result of the measurement to the command-and-control unit 80, which in turn sends this result to the processing unit 90. The processing unit 90 verifies that the profile of the collimated test light beam 71 satisfies a predetermined profiling criterion, for example that the profile of the collimated test light beam 71 is the desired one (e.g., substantially circular). If this profiling criterion is not satisfied, the profiling test envisages repositioning the collimating lens 4 in space, in a second spatial position (e.g., by translating and/or rotating it) and repeating the previous operations. If, by contrast, the profiling criterion is satisfied, the profiling test is considered concluded.

After that, the characterization method envisages activating the mechanical arm 8 (via a command of the command- and-control unit 80) to move the reflective diffraction grating 6 into an operative position in which the grating 6 intersects the line joining the positioning device 2 and the profiler 9 (see figures 3 and 4).

At this point (with reference to figure 3), the characterization method envisages, by way of example, a first step in which the following operations are carried out.

The reflective diffraction grating 6 is oriented in a first angular position (by means of a command signal sent by the command-and-control unit 80 to the first actuator 7).

The gain chip 99 is supplied (e.g., through the application of an appropriate voltage at the electrodes 3' and 3”) with a plurality of first values of supply current, for example comprised between 0.05 mA and 310 mA, endpoints included, with increments of 10 mA or 20 mA.

In this manner, the gain chip 99 emits a respective first light beam 60 for each first value of supply current.

Subsequently, each first light beam 60, coming out of the second face of the gain chip 99 with high divergence, is collimated by the collimating lens 4 (positioned in the spatial position determined with the profiling test) and, subsequently, impinges on the reflective diffraction grating 6 in the first angular position (and in the operating condition). This brings about, for each first light beam 60, the generation of a first zero-order diffracted beam 61 and a first first- order diffracted beam 62.

Each first first-order diffracted beam 62 is thus directed towards the mirror 10, is reflected back by the latter towards the reflective diffraction grating 6 along the same direction (as represented by the double arrow that goes from the grating 6 to the mirror 10), and then re-enters the gain chip through the second face (as represented by the arrow inside the gain chip 99).

This operating condition, defined as the lasing condition (i.e. where the gain chip behaves like a laser diode), is possible thanks to the specific angular position of the reflective diffraction grating 6 and the spatial position of the mirror 10, which are such that the grating 6, the mirror 10, and the first reflective face of the gain chip 99 create an external laser cavity. In an example operating embodiment, the actuators 7 and/or 11 are commanded to rotate the grating 6 and/or mirror 10, while simultaneously monitoring the optical power detected by the device 12 on the first zero-order diffracted beam for a given supply current, for example 150 mA. The desired angular position of the grating 6 and of the mirror 10 is the one in which the detected power is maximum (under that condition, the maximum coupling of the first-order diffracted beam with the gain chip is achieved).

It is also possible to create the external laser cavity using alternative configurations of a known type, such as, for example, the Littrow or the Littman-Metcalf configuration.

Each first zero-order diffracted beam 61 is reflected towards the optical power measuring device 12 in order to measure a respective first value of optical power thereof. The first values of optical power are subsequently acquired by the command-and-control unit 80 (via the communication line C), which associates them with the respective first value of supply current, resulting in a first plurality of pairs of first values of supply current and of optical power. The first plurality of pairs of first values of supply current and of optical power is subsequently transmitted by the command-and-control unit 80 to the processing unit 90 (via the communication line M). The first step of the method concludes with this action.

A second step of the characterization method which, by way of example, is carried out at the end of the aforesaid first step, will now be described with reference to figure 4. Alternatively, the second step can be carried out before the first step.

By way of example, in the second step the following operations are carried out.

The reflective diffraction grating 6 is maintained in the operative position and is oriented in a second angular position (by means of a command signal sent by the command-and-control unit 80 to the first actuator 7). In the passage from the first to the second angular position, the grating 6 undergoes an anticlockwise rotation (represented by the respective arrow in figure 4), preferably of an extent greater than or equal to 1 ° and/or less than or equal to 10°, more preferably less than or equal to 5°, for example equal to about 3°.

Subsequently, the gain chip 99 is supplied (e.g., through the application of an appropriate voltage at the electrodes 3' and 3”) with a plurality of second values of supply current, for example the same values of supply current as used in the first step (I ,e. , the aforesaid first values of supply current).

In one embodiment, the second values of supply current can be different from the first values of supply current. For example, each second value of supply current can be contained within a respective range of values centred on the respective first value of supply current and having a width, for example, of 5mA.

As mentioned above, the gain chip 99 emits a respective second light beam 60' for each second value of supply current. Each second light beam 60', coming out of the second face of the gain chip 99 with high divergence, is collimated by the collimating lens 4 (maintained in the aforesaid spatial position determined with the profiling test) and, subsequently, impinges on the reflective diffraction grating 6 in the second angular position. This brings about, for each second light beam 60', the generation of a second zero-order diffracted beam 63 and a second first-order diffracted beam 64.

Each second first-order diffracted beam 64 is reflected towards the mirror 10 (arrow pointing towards the mirror) and is then reflected back towards the reflective diffraction grating 6 (return arrow pointing towards the grating), with an angle of incidence such that, after the reflection, it does not impinge on (e.g., it gets lost in the air) the second face of the gain chip 99.

This operating condition, defined as the ASE condition, is possible to thanks to the orientation of the reflective diffraction grating 6 in the second angular position, which causes the opening of the aforesaid external laser cavity.

Each second zero-order diffracted beam 63 is reflected towards the optical power measuring device 12 in order to measure a respective second value of optical power thereof. The second values of optical power are subsequently acquired by the command-and-control unit 80 (via the communication line C), which associates them with the respective second value of supply current, resulting in a second plurality of pairs of second values of supply current and of optical power.

The second plurality of pairs of second values of supply current and of optical power is subsequently transmitted by the command-and-control unit 80 to the processing unit 90 (via the communication line M). The second step of the method concludes with this action. By way of example, the aforesaid first and second steps are carried out while maintaining (via the thermostat 20 commanded by the command-and-control unit via the communication line D) the gain chip 99 at a first temperature, for example equal to 23°C. Subsequently, the first and second steps described above are preferably repeated, maintaining the gain chip 99 at (at least) a second temperature greater than the first temperature by at least 4°C, and for example equal to 28°C.

In this manner one obtains a further first plurality of pairs of first values of supply current and of optical power and a further second plurality of pairs of second values of supply current and of optical power.

Figures 5 and 6 graphically show a purely illustrative example of pairs of values of optical power and supply current obtained respectively in the first step of the method and in the second step of the method, at two different temperature values of the gain chip (solid and dashed circles). In figure 5, for current values below the threshold current l _t h of the gain chip, equal, for example, to 50 mA, the values of optical power are negligible (non-lasing condition).

At this point, once all the aforesaid (e.g., four) pluralities of pairs of values of supply current and of optical power have been acquired, the processing unit 90 carries out the following operations.

In one embodiment, the operations described below for one of the four pluralities of pairs of values can be carried out immediately after the acquisition of the respective plurality of pairs of values, and before obtaining one of the other pluralities of pairs of values.

By way of example, the processing unit 90 determines a respective first linear function which interpolates, respectively, the first and the further first plurality of pairs of first values of supply current and of optical power (i.e., acquired under the lasing operating condition). One example of such linear functions is shown in figure 5, where the reference number 200 indicates the linear function which interpolates the first plurality of pairs, and the reference number 200' indicates the linear function which interpolates the further first plurality of pairs (in the zone of linearity of the curve). These interpolations are, by way of example, carried out for current values above the threshold current of the gain chip (indicated purely schematically by the label lth) and below an upper reference value (indicated purely schematically by the label l _sup )

At this point, the processing unit 90 calculates a respective differential quantum efficiency of the first gain chip 99 as a function of a first predetermined mathematical correlation, for example the following: where q _d is the differential quantum efficiency (dimensionless), h is the Planck constant, v is a photon frequency, for example calculated as the ratio between the value of the speed of light in a vacuum and the wavelength at which the gain peak of the gain chip occurs (this wavelength can be known for the given gain chip, e.g., provided by the gain chip manufacturer, or, alternatively, it is measurable by means of suitable experimental tests, e.g., spectroscopy), e.g., the wavelength is equal to 1064 nm, q is the electron charge (in absolute value), and m is a angular coefficient of the first (or respective first) linear function (expressed in Watts/Amperes).

For example, for the gain chip 99, q _d is equal to about 0.047 at the first temperature (23°C) and equal to about 0.042 at the second temperature (28°C).

At this point, the processing unit 90 calculates a respective modal gain coefficient and a respective carrier density at transparency of the gain chip as a function of a second predetermined mathematical correlation, which for example is: where P _opt is the optical power, I is the supply current, g ₀ is the modal gain coefficient (expressed in cm- ¹ ), N _o is the carrier density at transparency (expressed in cm ³ ), n _sp is a population inversion factor (e.g., equal to about 1), h is the Planck constant, v is the photon frequency, v _g is a group velocity in the medium (i.e., the air), L is a length of the gain chip 99 taken along a prevalent direction of extension of the gain chip 99 which goes from the first to the second face, at represents the internal losses of the gain chip (equal, for example, to 15 cm ¹ ; such internal losses are for example determined experimentally, or estimated as a function of the material of the gain chip, or known, e.g., provided by the manufacturer), q is an injection efficiency (equal, for example, to about 0.5, and for example determined experimentally, or estimated as a function of the material of the gain chip), C is the Auger recombination coefficient (equal, for example, to about 10 ^-28 cm ⁶ s ^-1 ), V is a volume of the gain chip 99, and q is the electron charge.

For example, for the first gain chip 99, g ₀ is equal to about 63 cm ¹ at the first temperature (23°C) and equal to about 68 cm ¹ at the second temperature (28°C), and N _o is equal to about 8.89*10 ¹⁷ cm ³ at both the first and second temperatures.

At this point, the method envisages repeating (carrying out all the aforesaid operations, profiling test included) the calculation of the differential quantum efficiency, of the modal gain coefficient and of the carrier density at transparency for each of the other gain chips of the batch.

At this point it is possible to verify that the differential quantum efficiency, the modal gain coefficient, and the carrier density at transparency of a given gain chip satisfy a predetermined respective selection criterion, for example they are above/below a predetermined threshold value. Alternatively, or in addition, it is also possible to make a combined evaluation of the three parameters.