Method and apparatus for characterizing laser amplifier chips

Abstract

A system (10, 30), comprising: a light source (16) that generates an output light signal (17) characterized by a wavelength that can be varied in response to a wavelength control signal; a mounting table (19, 39) suitable for receiving an amplifier chip (11), wherein the amplifier chip (11) has a waveguide (43) with a reflective surface (42) on a first surface of the amplifier chip and a transparent surface (12) on a second surface of the amplifier chip (11); an optical system (31) that focuses the output light signal (17) through the transparent surface (12) into the waveguide (43); and a controller (20) that causes the examination light source (16) to generate the output light signal (17) and measures a light intensity both with and without operation of the amplifier chip (11) for each of a variety of different wavelengths in order to form an amplification profile for the amplifier chip (11).

Description

background

[0001] Many measurements of interest involve measuring the response of a sample to a light beam of a known wavelength as a function of wavelength. The light source for many of these measurements is a tunable laser. Mid-infrared (MIR) spectrographic systems using quantum cascade lasers are particularly useful in identifying the chemical composition of a sample because these lasers can be tuned over a broad range of wavelengths within the spectrum of interest. For the purposes of this disclosure, MIR is defined as light between the wavelengths of 2.5 micrometers and 50 micrometers.

[0002] A tunable laser typically comprises an amplifier chip or other amplification medium that amplifies light over a range of wavelengths, and a resonator containing a wavelength filter that determines the filter's resonant wavelength. In one type of laser with an external resonator, the resonator is formed, for example, by a facet of the amplifier chip and a diffraction grating that reflects light of the desired wavelength back to the amplifier chip. In this arrangement, the wavelength of the laser light can be varied by changing the angle at which the light exiting the amplifier chip strikes the grating.

[0003] The laser tuning range is determined by details of the external resonator and the gain as a function of the wavelength of the amplifier chip. It would be advantageous to measure the gain profile as a function of wavelength for the amplifier chip without having to integrate the amplifier chip into a laser with an external resonator. US 2003 / 0161034A1 discloses a gain measurement device for an optical amplifier. Summary

[0004] The present invention comprises a system suitable for characterizing amplifier chips and a method for characterizing amplifier chips. The system comprises an inspection light source that generates an output light signal characterized by a wavelength that can be varied in response to a wavelength control signal, and a mounting stage suitable for receiving an amplifier chip characterized by a waveguide having a reflective surface on a first surface of the amplifier chip and a transparent surface on a second surface of the amplifier chip.The system also includes an optical system that focuses the output light signal through the transparent surface into the waveguide; and a controller that causes the investigation light source to generate the output light signal and to measure a light intensity both with and without operating the amplifier chip for each of a variety of different wavelengths in order to form a gain profile for the amplifier chip.

[0005] In one aspect, the investigation light source includes a tunable quantum cascade laser (QCL). In another aspect, the investigation light sources include a QCL that emits light in the MIR.

[0006] In another aspect, the optical system focuses the output light signal onto a point on the transparent surface, which is characterized by a location, with the point being at a point determined by the controller.

[0007] In another aspect, the controller causes the point to move across the transparent surface while measuring the intensity of the light leaving the amplifier chip as a function of the point's position on the transparent surface while the amplifier chip is operating.

[0008] In another aspect, the output light signal contains a collimated light beam and the optical system contains an optical element that focuses the collimated light beam onto a point on the transparent surface.

[0009] In another aspect, the optical element moves along a path that runs parallel to the transparent surface, at a distance from the transparent surface that remains constant as the optical element moves along the path.

[0010] In another aspect, the assembly table moves the amplifier chip so that the amplifier chip is parallel to the transparent surface.

[0011] In another aspect, the controller repeats the measurements for a variety of different power levels in the amplifier chip.

[0012] In another aspect, the system includes a reflection standard with a known reflectance as a function of the wavelength of the light in the output light signal, wherein the reflection standard is positioned such that the controller can cause the optical system to focus the output light signal onto the reflection standard and measure an intensity of the light reflected by the reflection standard.

[0013] In another aspect, the controller is configured to measure the intensity of the light reflected from the transparent surface at points that do not include the waveguide.

[0014] In another aspect, the amplifier chip contains a polarized medium characterized by a chip plane of polarization, and the output light signal is polarized with a polarization plane that has a component aligned with the chip plane of polarization.

[0015] In another aspect, the investigation light source includes a polarization arrangement that adjusts the polarization plane of the output light signal in response to signals from the controller.

[0016] In another aspect, the table contains an actuator that rotates the amplifier chip around an axis perpendicular to the transparent surface in response to signals from the controller.

[0017] The present invention also comprises a method for characterizing an amplifier chip with a waveguide having a reflective surface on a first surface of the amplifier chip and a transparent surface on a second surface of the amplifier chip, wherein the method comprises generating a test light signal characterized by a test wavelength; focusing the light signal into the waveguide; and measuring the intensity of the light exiting the transparent surface for a plurality of different test wavelengths, both with and without operating the amplifier chip.

[0018] In another aspect, the wavelengths used for investigation are in the MIR range.

[0019] In another aspect, the procedure involves focusing the investigation light signal at a multitude of different points on the transparent surface while the amplifier chip is operating, and measuring the intensity of the light leaving the transparent surface at each of the multitude of different points in order to determine a position for the waveguide.

[0020] In another aspect, the amplifier chip is electrically powered by an amplifier chip power supply, and the measured intensities are repeated for a variety of different power levels from the amplifier chip power supply.

[0021] In another aspect, the procedure involves measuring the intensity of the investigation light signal reflected from the transparent surface at points that do not enclose the waveguide.

[0022] In another aspect, the procedure involves measuring the intensity of the test light reflected by a reflection standard in order to determine a reflectance for the transparent surface. Brief description of the drawings Fig. Figure 1 shows an embodiment of an amplifier chip characterization system according to the present invention. Fig. Figure 2 shows another exemplary embodiment of a chip characterization system according to the present invention. Fig. Figure 3 shows an embodiment of a scanning arrangement that can measure both the absolute and the relative reflectivity of the front of the amplifier chip. Detailed description

[0023] The manner in which the present invention provides its advantages can be more easily understood with reference to a device and a method for characterizing a QCL amplifier chip. However, it will be obvious that the method and device of the present invention can be used for characterizing other amplifier chips.

[0024] In general, the amplifier chip is characterized by a waveguide containing an active layer that amplifies the light passing through the waveguide when the amplifier chip is powered. The waveguide connects two facets of the amplifier chip. Typically, the first facet is a reflective surface, and the second facet has an antireflective coating. Light coupled into the waveguide through the second facet is reflected by the first facet. Light passing through the waveguide is subject to absorption. The degree of absorption is a function of the wavelength of the light. When the amplifier chip is powered, light coupled into the waveguide is amplified by an amount that depends on the wavelength, and some of the light in the waveguide is lost through absorption.

[0025] Now Fig. Reference is made to Figure 1, which shows an embodiment of an amplifier chip characterization system according to the present invention. The system 10 injects a light signal from the test light source 16 into the waveguide of the amplifier chip 11 through the transparent surface 12 of the amplifier chip 11. The amplifier chip 11 has two opposing surfaces, which are hereinafter referred to as the reflective surface and the transparent surface. Light enters the amplifier chip 11 through the transparent surface and is reflected back to the transparent surface by the reflective surface. When the amplifier chip 11 is powered on (energized) and the light intensity is amplified, the degree of amplification depends on the wavelength of the light. For the purposes of this application, the surface through which the light enters the amplifier chip is defined as the transparent surface.The transparent area 12 typically contains an antireflection coating. Generally, the transparent area will not be perfectly transparent, and there will be other losses within the amplifier chip. The imperfect transmission can also result from absorption within the amplifier chip or partial reflection at the reflective surface.

[0026] System 10 uses detector 18 to measure the intensity of the light exiting amplifier chip 11 when amplifier chip 11 is powered on and when amplifier chip 11 is not powered on. The ratio of these two power measurements can be used to measure the net gain provided by amplifier chip 11 at the wavelength of the light supplied by the examination light source 16.

[0027] The examination light source 16 preferably generates a monochromatic collimated light beam 17, characterized by a wavelength controlled by the controller 20. The collimated light beam 17 passes through the beam splitter 14 and is focused onto the transparent surface of the amplifier chip 11 at a point that ensures the light is injected into the waveguide of the amplifier chip 11. The portion of the collimated light beam 17 that does not pass through the beam splitter 14 forms a beam 21, which, in the present embodiment, is directed to a beam sink. In other embodiments, it may be advantageous to provide a detector that measures the beam intensity in beam 21 to monitor the power of the examination light source 16. The light exiting the amplifier chip 11 is collimated by the lens 13, and a portion of this light is directed through the beam splitter 14 to a detector 18.

[0028] The measurements described above assume that the amplifier chip 11 is positioned relative to the lens 13 such that the collimated light beam 17 is injected into the waveguide of the amplifier chip 11. In system 10, this is achieved by mounting the amplifier chip 11 on an XY stage 19, which moves the amplifier chip 11 relative to the lens 13. The stage is operated such that the beam is scanned across the transparent surface 12 while the examination light source 16 is switched on and the amplifier chip 11 is operating. The output of the detector 18 as a function of X and Y is an image of the transparent surface 12 in which the position of the waveguide corresponds to a bright area in the image. Controller 20 then positions the amplifier chip 11 so that the waveguide is centered on the incident light.

[0029] Once the controller 20 has correctly positioned the amplifier chip 11 relative to the lens 13, the controller 20 calculates the ratio between the output of the detector 18 when the amplifier chip 11 is off and the output of the detector 18 when the amplifier chip 11 is on. This ratio is recorded for a variety of wavelengths of the investigation light source 16 to create a gain profile as a function of wavelength for the amplifier chip 11. This profile can be used to simulate the amplifier chip 11 when it is part of a laser with an external resonator, and thus to predict the laser's behavior as a function of wavelength.

[0030] In general, the gain profile obtained by the method described above depends on the magnitude of the bias current supplied to the amplifier chip by the chip power supply 15, which is controlled by controller 20. Accordingly, controller 20 replicates the gain profile in one aspect for a number of different chip power levels.

[0031] The range of wavelengths over which the gain profile of the amplifier chip 11 can be measured depends on the range of wavelengths over which the test light source 16 can be tuned. Therefore, it is advantageous to use a test light source that can be tuned over a wider wavelength range than that required for characterizing the amplifier chip 11. In one embodiment, the test light source 16 is constructed from a QCL with an external resonator. In another exemplary embodiment, the test light source 16 is constructed from a QCL in which the gain section contains multiple amplifier chips and / or a single amplifier chip with multiple active layers.In another embodiment, the examination light source 16 is composed of several lasers with different tuning ranges, which are selected such that the combined tuning range is sufficient to provide the bandwidth required for creating the gain profile for the amplifier chip 11.

[0032] In the embodiments described above, the amplifier chip is positioned relative to the lens 13 by means of the transparent surface 12 using the xy-table 19 in order to move the amplifier chip 11 in two dimensions and generate an image of the transparent surface 12. The time required to generate such an image can be significant; therefore, the speed at which the xy-table 19 can be moved is limited by the mass of the table, which can be quite substantial. In applications where this scanning time is limited, a scanning arrangement that allows faster movement along an axis is preferred.

[0033] Now Fig. Reference is made to Figure 2, which shows a further exemplary embodiment of a chip characterization system according to the present invention. The system 30 uses a scanning arrangement 31 comprising a mirror 34 and a lens 33 to focus the collimated light beam 17 onto the transparent surface 12 at a location determined by the position of the scanning arrangement 31 along the carrier 32 on which the scanning arrangement 31 moves under the control of the controller 20. Since the mass of the scanning arrangement 31 is significantly less than that of an XY stage, such as the XY stage 19, the position of the focal point on the transparent surface 12 can be moved at a much higher speed than in the system described in Figure 2. Fig. In the embodiment shown in Figure 1, the scanning arrangement 31 moves in a direction parallel to the transparent surface 12 as it moves along the support 32. The table 39 also moves such that the focal point remains in the plane of the transparent surface 12, but in a direction orthogonal to X. Therefore, the orientation of the optical axis 35 of the light cone focused onto the transparent surface 12 remains constant as the focal point scans across the transparent surface 12. Consequently, the efficiency with which light is coupled in and out of the waveguide of the amplifier chip 11 does not change as the focal point moves across the transparent surface 12 within the waveguide.

[0034] If the amplifier chip is to be operated in a pulsed mode, it is advantageous to provide a pulsed examination light source and to operate the amplifier chip in a pulsed mode.

[0035] The device described above can also be used to measure the reflectivity of the transparent surface of the amplifier chip. While ideally the transparent surface 12 of the amplifier chip 11 is coated with an antireflective layer, the surface may still exhibit limited reflectivity. In one aspect of the device and method of the present disclosure, the device is used to measure this reflectivity. Now, it is referred to Fig. Reference is made to Figure 3, which shows an embodiment of a scanning arrangement capable of measuring both the absolute and relative reflectivity of the front face of the amplifier chip. To simplify the following discussion, those elements of the scanning arrangement 40 that perform functions analogous to those in Figure 3 were identified as follows: Fig. The elements shown in section 1 are provided with the same numerical designations and are not further explained here unless the context requires otherwise.

[0036] To measure the reflectance of the transparent surface 12, the amplifier chip 11 is positioned such that the light from the test light source 16 is focused onto a point 42 on or near the surface of the transparent surface 12 at a location away from the waveguide 43. The scanning procedure described above can be used to map the transparent surface 12 as a function of X and Y. For the reflectance measurements, a point is chosen that is away from the bright area corresponding to the waveguide 43. The light returning from point 42 to the detector 18 when the test light is focused onto point 42 depends on the reflectance of the surface 42. The absolute value of the reflectance of the transparent surface 12 at point 42 can be determined by measuring the light reflected by a calibration standard 44 with a known reflectance.

[0037] In many amplifier chips, the light they produce is polarized. For characterizing such chips, the test light must have a significant complement polarized in the same direction. QCLs that generate a polarized output beam can be used as the test light source. Since the polarization direction of the output beam is known, the amplifier chip is preferably mounted on the stage such that its polarizations are correctly aligned with the test light beam. Alternatively, embodiments can be used in which the test light source incorporates a polarization rotator. In such embodiments, the rotation angle of the test light source is varied to maximize the light output of the amplifier chip when the chip is positioned so that the test light beam enters the waveguide.In other embodiments, the table on which the amplifier chip is mounted includes an actuator for rotating the amplifier chip around the position shown in the diagram. Fig. Figure 3 shows the z-axis, in addition to the movement of the chip in the X and Y directions. The rotation angle is varied until the amplifier chip's performance is maximized when the amplifier chip is positioned so that the examination light beam enters the waveguide. In this position, the z-axis is perpendicular to the transparent surface of the amplifier chip.

[0038] The embodiments of the present invention described above have been provided to illustrate various aspects of the invention. However, it should be understood that different aspects of the present invention, shown in various specific embodiments, can be combined to provide other embodiments of the present invention. Furthermore, various modifications of the present invention will become apparent from the preceding description and the accompanying drawings. Accordingly, the present invention is limited only by the scope of the following claims.

Claims

[1] A system (10, 30) comprising: an examination light source (16) producing an output light signal (17) characterized by a wavelength that can be varied in response to a wavelength control signal; a mounting table (19, 39) suitable for receiving an amplifier chip (11), the amplifier chip (11) having a waveguide (43) with a reflective surface (42) on a first surface of the amplifier chip and a transparent surface (12) on a second surface of the amplifier chip (11); an optical system (31) that focuses the output light signal (17) through the transparent surface (12) into the waveguide (43); and a controller (20) that causes the examination light source (16) to generate the output light signal (17) and measures a light intensity both with and without driving the amplifier chip (11) for each of a plurality of different wavelengths to form a gain profile for the amplifier chip (11). [2] The system of claim 1, wherein the examination light source (16) comprises a tunable QCL. [3] The system of claim 2, wherein the QCL emits MIR light. [4] The system of claim 1, wherein the optical system (31) focuses the output light signal onto a point (42) on the transparent surface (12) identified by a location, the point (42) being located at a point determined by the controller (20). [5] The system of claim 4, wherein the controller (20) causes the dot (42) to move across the transparent surface (12) while measuring an intensity of light exiting the amplifier chip (11) as a function of the position of the dot on the transparent surface (12) while the amplifier chip (11) is operating. [6] The system of claim 1, wherein the output light signal (17) comprises a collimated light beam and the optical system (31) comprises an optical element (13) that focuses the collimated light beam to a point (42) on the transparent surface (12). [7] The system of claim 6, wherein the optical element (13) moves along a path that is parallel to the transparent surface (12) and at a distance from the transparent surface (12) that remains constant while the optical element (13) moves along the path. [8] The system of claim 1, wherein the mounting table (19, 39) moves the amplifier chip (11) such that the amplifier chip (11) is parallel to the transparent surface. [9] The system of claim 1, wherein the controller (20) repeats the measurements for a plurality of different power levels in the amplifier chip (11). [10] The system of claim 4, further comprising a reflectance standard (44) having a known reflectance as a function of the wavelength of the light in the output light signal, the reflectance standard (44) being positioned so that the controller (20) can cause the optical system (31) to focus the output light signal (17) onto the reflectance standard (44) and measure an intensity of the light reflected from the reflectance standard (44). [11] The system of claim 1, wherein the controller (20) is configured to measure an intensity of the light reflected from the transparent surface (12) at locations that do not include the waveguide (43). [12] A method for measuring properties of an amplifier chip (11) having a waveguide (43) having a reflective surface (42) on a first surface of the amplifier chip (11) and a transparent surface (12) on a second surface of the amplifier chip (11), the method comprising: Generating an examination light signal characterized by an examination wavelength; Focusing the examination light signal into the waveguide (43); and Measuring the intensity of the light leaving the transparent surface (12) for a variety of different examination wavelengths, both with and without operation of the amplifier chip (11). [13] The method of claim 12, wherein the examination wavelength is an MIR wavelength. [14] The method of claim 12, further comprising focusing the probe light signal at a plurality of different points (42) on the transparent surface (12) while operating the amplifier chip (11), and measuring an intensity of the light exiting the transparent surface (12) at each of the plurality of different points (42) to determine a location for the waveguide (43). [15] The method of claim 12, wherein the amplifier chip (11) is electrically powered by an amplifier chip power supply (15) and the measured intensities are repeated for a plurality of different power levels from the amplifier chip power supply (15). [16] The method of claim 12, further comprising measuring an intensity of the examination light signal reflected from the transparent surface (12) at locations not including the waveguide (43). [17] The method of claim 16, further comprising measuring an intensity of the inspection light signal reflected from a reflection standard (44) to determine a reflectivity for the transparent surface (12).

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