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EP-4616496-B1 - METHOD AND SYSTEM FOR CHARACTERISING A BATCH OF GAIN CHIPS

EP4616496B1EP 4616496 B1EP4616496 B1EP 4616496B1EP-4616496-B1

Inventors

  • UBALDI, MARIA CHIARA
  • VALLONE, Marco Ernesto

Dates

Publication Date
20260506
Application Date
20231108

Claims (10)

  1. Method for characterizing a batch of gain chips, wherein each gain chip has a respective first face at least partially reflective and a respective second face at least partially transparent, wherein said method comprises carrying out the following operations on each gain chip of said batch: a) positioning said gain chip (99) with said respective second face facing towards a reflective diffraction grating (6); b1) orienting said reflective diffraction grating (6) in a first angular position; b2) supplying said gain chip (99) with a plurality of first values of supply current so that said gain chip (99) emits a respective first light beam (60) for each first value of supply current; b3) making impinge each first light beam (60) onto said reflective diffraction grating (6) in the first angular position so as to generate a respective first zero-order diffracted beam (61) and a respective first first-order diffracted beam (62), said respective first first-order diffracted beam (62) entering said gain chip (99) through said second face; b4) measuring a respective first value of optical power of each first zero-order diffracted beam (61); b5) associating each first value of optical power to 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 (6) in a second angular position; d2) supplying said gain chip (99) with a plurality of second values of supply current so that said gain chip (99) emits a respective second light beam (60') for each second value of supply current; d3) making impinge each second light beam (60') onto said reflective diffraction grating (6) in the second angular position so as to generate a respective second zero-order diffracted beam (63) and a respective second first-order diffracted beam (64), said respective second first-order diffracted beam (64) not entering said gain chip (99); d4) measuring a respective second value of optical power of each second zero-order diffracted beam (63); d5) associating each second value of optical power to 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.
  2. Method according to claim 1, wherein said operation c) comprises determining a first linear function that interpolates said plurality of pairs of first values of supply current and of optical power, and wherein said first predetermined mathematical correlation is: η d = q hν m wherein η d is said differential quantum efficiency, h is the Planck constant, v is a photon frequency, q is the electronic charge, and m is a slope of said first linear function.
  3. Method according to any one of the preceding claims, wherein said second predetermined mathematical correlation is: P opt = n sp hνv g L g 0 ln ηI 1 3 qVC 1 3 N o g 0 ln ηI 1 3 qVC 1 3 N o − α i e L g 0 ln ηI 1 3 qVC 1 3 N o − α i − 1 wherein 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, h is the Planck constant, v is a photon frequency, v g is a group velocity, L is a length of said gain chip (99), α i represents internal losses of said gain chip (99), η is an injection efficiency, C is the Auger recombination coefficient, V is a volume of said gain chip (99), q is the electronic charge
  4. Method according to any one of the preceding claims, comprising: - before said operations b3) and d3), collimating said first (60) and said second light beam (60'); - during said operations from b2) to b4) and said operations from d2) to d4), maintaining said gain chip (99) at a first temperature; - repeating said operations from b2) to b5), and from d2) to d5), while maintaining said gain chip at a second temperature, and repeating said operations c), e) and n), wherein a difference in absolute value between said first and second temperature is at least equal to 4°C.
  5. Method according to any one of the preceding claims, comprising before said operations b1) and d1): - positioning said reflective diffraction grating (6) in a non-operative position in which said reflective diffraction grating (6) does not intercept said light beams (60, 60'); - supplying said gain chip (99) with a test value of supply current so that said gain chip (99) emits a test light beam (70); - collimating said test light beam (70) for obtaining a collimated test light beam (71); - measuring a profile of said collimated test light beam (71); - verifying that said profile satisfies a predetermined profiling criterion.
  6. Method according to any one of the preceding claims, wherein said verifying comprises ordering said gain chips of said batch according to a progressive order of one or more of said respective differential quantum efficiency, said respective modal gain coefficient and said respective carrier density at transparency, wherein said respective predetermined selection criterion comprises a respective predetermined threshold value, and/or wherein 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 a gain chip 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.
  7. System (100) for characterizing a batch of gain chips, wherein each gain chip has a respective first face at least partially reflective and a respective second face at least partially transparent, wherein said system (100) for characterizing comprises: - a test bench (1) comprising: i) a positioning device (2) for a gain chip (99); ii) a supply source (3) structured to supply a supply current to said gain chip (99) positioned in the positioning device (2) so that said gain chip (99) emits a light beam (60, 60'); iii) a reflective diffraction grating (6) structured such that said light beam (60, 60') incident onto said reflective diffraction grating (6) generates a respective zero-order diffracted beam (61, 63) and a respective first-order diffracted beam (62, 64); iv) a first actuator (7) structured to orient said reflective diffraction grating (6) alternatively between a first angular position in which said first-order diffracted beam (62) enters said gain chip (99) through said second face, and a second angular position in which said first-order diffracted beam (64) does not enter said gain chip; v) an optical power measuring device (12) structured to measure an optical power of said zero-order diffracted beam (61, 63); vi) a command-and-control unit (80) operatively connected to said supply source (3), said first actuator (7) and said measuring device (12), - a processing unit (90) connected to said command-and-control unit (80), wherein said command-and-control unit (80) is programmed and configured to carry out the following operations, with said gain chip (99) positioned in the positioning device (2) with said second face facing towards said reflective diffraction grating (6): f1) commanding said first actuator (7) for orienting said reflective diffraction grating (6) in said first angular position; f2) commanding said supply source (3) to supply said gain chip (99) with a plurality of first values of said supply current so that said gain chip (99) emits a respective first light beam (60) for each first value of supply current, said respective first light beam (60) generating a respective first zero-order diffracted beam (61) and a respective first first-order diffracted beam (62); f3) receiving from said optical power measuring device (12) a respective first value of optical power of each first zero-order diffracted beam (61); f4) associating each first value of optical power to the respective first value of supply current; g1) commanding said first actuator (7) to orient said reflective diffraction grating (6) in said second angular position; g2) commanding said supply source (3) to supply said gain chip (99) with a plurality of second values of supply current so that said gain chip (99) emits a respective second light beam (60') for each second value of supply current, said respective second light beam (60') generating a respective second zero-order diffracted beam (63) and a respective second first-order diffracted beam (64); g3) receiving from said optical power measuring device (12) a respective second value of optical power of each second zero-order diffracted beam (63); g4) associating each second value of optical power to the respective second value of supply current, and wherein said processing unit (90) is programmed and configured to carry out the following operations: i1) receiving from said command-and-control unit (80) a first plurality of pairs of first values of supply current and of optical power; i2) as a function of said first plurality and as a function of a first predetermined mathematical correlation, calculating a respective differential quantum efficiency; m1) receiving from said command-and-control unit (80) 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 respective predetermined selection criterion.
  8. System (100) according to claim 7, wherein said test bench (1) comprises a thermostat (20) structured to maintain said gain chip (99), positioned in said positioning device (2), stable in temperature, wherein said command-and-control unit (80) is operatively connected to said thermostat (20) and is programmed and configured to: - during said operations f2), f3) and g2), g3), command said thermostat (20) to maintain said gain chip (99) at a first temperature; - repeat at least said operations from f2) to f4) and said operations from g2) to g4) while maintaining said gain chip (99) at a second temperature, wherein a difference in absolute value between said first and second temperature is at least equal to 4°C, and wherein said processing unit (90) is programmed and configured to repeat said operations i1), i2), m1), m2), and n).
  9. System (100) according to claim 7 or 8, wherein said test bench (1) comprises: - a collimating lens (4) interposed between said positioning device (2) and said reflective diffraction grating (6) and structured to collimate said first (60) and said second light beam (60'); - a second multi-axis actuator (5) structured to move said collimating lens (4) in space; - a light beam profiler (9) aligned with said positioning device (2) and structured to measure a profile of a light beam; - a mechanical arm (8) structured to move said reflective diffraction grating (6) between an operative position, and a non-operative position in which said reflective diffraction grating (6) does not intercept said light beams (60, 60').
  10. System (100) according to claim 9, wherein said command-and-control unit (80) is operatively connected to said light beam profiler (9), to said second actuator (5), and to said mechanical arm (8), wherein said command-and-control unit (80) is programmed and configured to, preliminarily: - command said mechanical arm (8) to position said reflective diffraction grating (6) in said non-operative position; - command said second actuator (5) to position said collimating lens (4) in space; - command said supply source (3) to supply said gain chip (99) with a test value of supply current so that said gain chip (99) emits a test light beam (70) which is collimated through said collimating lens (4) to obtain a collimated test light beam (71); - receive from said light beam profiler (9) a measure of a profile of said collimated test light beam (71), and wherein said processing unit (90) is programmed and configured to receive from said command-and-control unit (80) said profile of said collimated test light beam (71) and verifying that said profile satisfies a predetermined profiling criterion.

Description

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. Document VIZBARAS AUGUSTINAS ET AL: "GaSb Swept-Wavelength Lasers for Biomedical Sensing Applications", IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, IEEE, USA, vol. 25, no. 6, November 2019 (2019-11), pages 1-12, discloses a method for characterizing a gain chip by providing indications of selected microstructural properties of the 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 µm) 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" (g0) means, for example, a parameter defined by the relation g0 = Γgt, where Γ 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 gt 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