APPARATUS AND METHOD FOR QUALIFYING LIGHT SOURCES FOR USE IN OPTICAL FIBER COMMUNICATIONS

Abstract

An infrared wave front phase analyzer that can be used for measuring the wave front phase of laser diode (LD) beam to provide a quality characterization specification for the LD chips that are intended for use in optical sub-assemblies (OSAs), which has applications in optical transceiver manufacturing and fiber optic communications. An optical system mimics the OSA and includes optical elements to collect and collimate the LD output beam and to focus the collimated beam through a focus, which could otherwise be into the end of an optical fiber, before re-collimating the laser beam for evaluation by a wave front sensor. This imaging process measures a wave front of the output beam, yielding a quality specification of the LD for screening out the out-of-spec LDs in advance of their assembly within TOSA/BOSA manufacturing to lower production costs.

Claims

1. An inspection device for a light source of an optical transmitter subassembly arranged for collecting and collimating light from the light source with a given numerical aperture (NA) and converging the collimated light into an optical transmission system, comprising: a wave front sensor comprising a detector; a first optic for collecting and collimating light from the light source with a NA mimicking the optical sub-assembly (OSA) system of a transmitter subassembly and focusing the collimated light through a point; and a second optic for resizing the focused light and to match the size of wave front sensor detector for measuring wave front aberrations for qualifying the light source for use in the optical transmitter subassembly, wherein the first and second optics together with the wave front sensor form an optical system having the exit pupil located at the detector of wave front sensor that is conjugated to the entrance pupil that is usually located at, or close to, the first collimation lens.

2. The inspection device of claim 1, wherein the second optic provides for re-collimating the collected light in advance of the wave front sensor.

3. The inspection device of claim 2, wherein the wave front sensor comprises an array of focusing optics for focusing different transverse portions of the re-collimated light onto the detector.

4. The inspection device of claim 3, wherein the wave front sensor is a Shack-Hartmann sensor.

5. The inspection device of claim 1, wherein the first optic provides for collimating the collected light from the light source in advance of focusing the collected light through the focused point for imitating the OSA system of the transmitter subassembly and the second optic provides for re-collimating the focused collected light in advance of the wave front sensor.

6. The inspection device of claim 3, further comprising a programmable processor for evaluating output of the wave front sensor for qualifying the light source for use in the optical transmitter subassembly.

7. The inspection device of claim 6, wherein the programmable processer provides for identifying quantifiable wave front aberrations or characteristics or metric and provides for comparing the quantifiable wave front aberrations or characteristics or metric against threshold values for qualifying the light source for use in the optical transmitter subassembly.

8. The inspection device of claim 7, wherein the quantifiable wave front aberrations comprise at least one of astigmatism and coma.

9. The inspection device of claim 1, wherein the light source comprises a light-emitting-diode (LED) or a laser diode (LD), the LD comprises a vertical-cavity surface-emitting laser (VCSEL) or an edge emitter laser, and the edge emitter laser comprises a distributed feedback laser (DFB), electro-absorption modulated laser (EML), or a Fabry-Perot laser.

10. The inspection system of claim 5, wherein the first optic comprises a collimating optic for collimating the collected light and a focusing optic for focusing the collimated light through the focused point.

11. The inspection device of claim 5, wherein the light source is a laser diode (LD) or light-emitting-diode (LED).

12. The inspection device of claim 11, wherein the laser diode is one of a plurality of laser diodes supported in a mechanical cartridge that is translatable for aligning outputs of the laser diodes with the first optic.

13. A method of qualifying a light source for use in the optical transmitter subassembly in which light from the light source is focused though a given optical sub-assembly (OSA) system into an optical transmission system comprising steps of: collecting light from the light source with a first optic comprising a numerical aperture that mimics that of the transmitter subassembly focusing the collected light through a focused point; collimating the focused light passing through the focused point with a second optic for resizing the light in advance of a wave front sensor; measuring wave front aberrations in the collected light with the wave front sensor; comparing the measured wave front aberrations or computed parameters or metric from the wave front errors or other optical characteristics against threshold values for qualifying the light source for use in the optical transmitter subassembly, wherein the method as applied to multiple light sources distinguishes qualified light sources from unqualified light sources; and segregating the qualified light sources from the unqualified light sources for advancing the qualified light sources for assembly within optical transmitter subassemblies.

14. The method of claim 13, wherein the step of collecting light comprises collimating the collected light from the light source in advance of focusing the collected light through the focused point for mimicking the OSA system of the transmitter subassembly.

15. The method of claim 13, wherein the step of collimating light comprises re-collimating the focused light passing through the focused point in advance of the wave front sensor.

16. The method of claim 13, wherein the step of measuring comprises focusing different transverse portions of the re-collimated beam onto an electro-optical sensor.

17. The method of claim 13, wherein the step of measuring further comprises identifying quantifiable wave front aberrations.

18. The method of claim 13, wherein the step of comparing comprises comparing the quantifiable wave front aberrations or characteristics or computed metrics against the threshold values for qualifying the light source for use in the optical transmitter subassembly.

19. The method of claim 13, wherein the light source comprises a light-emitting-diode (LED) or a laser diode (LD), the LD comprises a vertical-cavity surface-emitting laser (VCSEL) or an edge emitter laser, and the edge emitter laser comprises a distributed feedback laser (DFB), electro-absorption modulated laser (EML), or a Fabry-Perot laser.

Description

BREIF DESCRIPTION OF THE DRAWINGS

[0017] The accompanying drawings illustrate one or more embodiments of the present disclosure and, together with the written description, serve to explain the principles of the present disclosure, wherein:

[0018] FIG. 1 illustrates an exemplary intensity profile measured by an Ophir Goniometric Radiometer LD 8900 Far-Field Profiler, in accordance with an embodiment of the present disclosure;

[0019] FIG. 2 illustrates an exemplary chip-to-fiber coupling by a two-lens packaging system, in accordance with an embodiment of the present disclosure;

[0020] FIGS. 3A-3D illustrate an exemplary optical quality of a TOSA packaging system at 1550 nm, wherein FIG. 3A shows an exemplary optical layout, FIG. 3B shows an exemplary wave front phase map, FIG. 3C shows an exemplary Point Spread Function (PSF) with a Strehl ratio of 0.891, and FIG. 3D shows an exemplary graph of enclosed energy, in accordance with an embodiment of the present disclosure;

[0021] FIG. 4 illustrates an exemplary system to directly measure both wave front phase and PSF, in accordance with an embodiment of the present disclosure;

[0022] FIGS. 5A-5C illustrate an exemplary configuration A of a wave front phase metrology system for a small Numerical Aperture (NA) laser diode, wherein FIG. 5A shows a first example of Configuration A, FIG. 5B shows a second example of Configuration A, and FIG. 5C shows a third example of Configuration A, in accordance with an embodiment of the present disclosure;

[0023] FIGS. 6A-6C illustrate an exemplary configuration B of a wave front phase metrology system for an intermediate NA laser diode, wherein FIG. 6A shows a first example of Configuration B, FIG. 6B shows a second example of Configuration B, and FIG. 6C shows a third example of Configuration B, in accordance with an embodiment of the present disclosure;

[0024] FIGS. 7A-7B illustrates an exemplary configuration C of a wave front phase metrology system for a large NA Laser diode, wherein FIG. 7A shows a first example of Configuration C and FIG. 7B shows a second example of Configuration C, in accordance with an embodiment of the present disclosure;

[0025] FIG. 8 illustrates an exemplary prototype system for a wave front phase metrology of a laser chip, in accordance with an embodiment of the present disclosure;

[0026] FIG. 9 illustrates an exemplary 1577 EML laser diode chip on Cartridge slot for testing, in accordance with an embodiment of the present disclosure;

[0027] FIG. 10 illustrates an exemplary beam intensity measured at a wave front phase testing pupil, in accordance with an embodiment of the present disclosure;

[0028] FIG. 11 illustrates an exemplary beam wave front phase measured at a testing pupil, in accordance with an embodiment of the present disclosure;

[0029] FIG. 12 illustrates an exemplary point spread function computed from a measured beam wave front phase, which is an intensity distribution at a laser-fiber coupling focus point, in accordance with an embodiment of the present disclosure;

[0030] FIG. 13 illustrates an exemplary intensity axial distribution of a laser beam along a propagation axis at a beam waist with an M-square (M.sup.2) value of 4.78, in accordance with an embodiment of the present disclosure;

[0031] FIG. 14 illustrates exemplary coefficients of Zernike polynomials, in accordance with an embodiment of the present disclosure;

[0032] FIG. 15 illustrates an exemplary wave front defocus terms of Zernike polynomials, in accordance with an embodiment of the present disclosure;

[0033] FIG. 16 illustrates exemplary wave front astigmatism values of Zernike polynomials, including astigmatism 0 degree and astigmatism 45 degree, in accordance with an embodiment of the present disclosure;

[0034] FIG. 17 illustrates an exemplary wave front astigmatism at 0° of Zernike polynomials, in accordance with an embodiment of the present disclosure;

[0035] FIG. 18 illustrates an exemplary wave front astigmatism at 45° of Zernike polynomials, in accordance with an embodiment of the present disclosure;

[0036] FIG. 19 illustrates an exemplary wave front coma term of Zernike polynomials, which include coma 0 degree and coma 45 degree, in accordance with an embodiment of the present disclosure;

[0037] FIG. 20 illustrates an exemplary wave front coma at 0° of Zernike polynomials, in accordance with an embodiment of the present disclosure;

[0038] FIG. 21 illustrates an exemplary wave front coma at 90° of Zernike polynomials, in accordance with an embodiment of the present disclosure;

[0039] FIG. 22 illustrates exemplary wave front 3rd order spherical aberrations of Zernike polynomials, in accordance with an embodiment of the present disclosure;

[0040] FIG. 23 illustrates an exemplary residual wave front error after 1-12 terms of Zernike polynomials are removed, which include the zonal wave front residual error after modal estimation, in accordance with an embodiment of the present disclosure; and

[0041] FIG. 24 illustrates an exemplary zonal residual wave front error after modal estimation, which is a zonal residual error that cannot be modeled to Zernike coefficients, in accordance with an embodiment of the present disclosure.

[0042] The following is a list of the reference numbers used in the drawings and the detailed specification to identify components:

[0043] 010: Laser diodes light source, 020: First lens, 030: Mirror, 040: Beamsplitter, 050: Microscope Objective lens, 060: Video camera, 070: Video Monitor, 080: Wave front sensor, 090: Computer, 110: DC power supply, 120: Power connector, 130: Mechanical cartridge containing Laser diode chips, 140: Collecting Lens A, 170: Collimation Lens C, 180: NIR Wave front sensor, 190: Computer, 111: DC power supply, 121: Power connector, 131: Mechanical cartridge containing Laser diode chips, 141: Collecting Lens A, 171: Collimation Lens C, 181: NIR Wave front sensor, 191: Computer, 112: DC power supply, 122: Power connector, 132: Mechanical cartridge containing Laser diode chips, 142: Collecting Lens A, 162: Field lens D, 172: Collimation Lens C, 182: NIR Wave front sensor, 192: Computer, 210: DC power supply, 220: Power connector, 230: Mechanical cartridge containing Laser diode chips, 240: First Lens A, 250: Focusing lens B, 270: Collimation Lens C, 280: NIR Wave front sensor, 290: Computer, 211: DC power supply, 221: Power connector, 231: Mechanical cartridge containing Laser diode chips, 241: First Lens A, 251: Focusing lens B, 271: Collimation Lens C, 281: NIR Wave front sensor, 291: Computer, 212: DC power supply, 222: Power connector, 232: Mechanical cartridge containing Laser diode chips, 239: Adjustment stage for Laser diode, 242: First Lens A, 249: Optical system for laser diode chip inspection, 252: Focusing lens B, 262: Field lens, 272: Collimation Lens C, 282: NIR Wave front sensor, 289: Adjustment stage for optical metrology system, 292: Computer, 310: DC power supply, 320: Power connector, 330: Mechanical cartridge holding laser diodes, 340: First Lens A, 350: Focusing lens B, 370: Collimation Lens C, 380: NIR Wave front sensor, 390: Computer, 311: DC power supply, 321: Power connector, 331: Mechanical cartridge holding laser diodes, 341: First Lens A, 351: Focusing lens B, 371: Collimation Lens C, 381: NIR Wave front sensor, 391: Computer, 410: Mechanical cartridge slot, and 420: Laser diode chip.

DETAILED DESCRIPTION

[0044] The present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the present disclosure are shown. The present disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure is thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art. Like reference numerals refer to like elements throughout.

[0045] The terms used in this specification generally have their ordinary meanings in the art, within the context of the present disclosure, and in the specific context where each term is used. Certain terms that are used to describe the present disclosure are discussed below, or elsewhere in the specification, to provide additional guidance to the practitioner regarding the description of the present disclosure. For convenience, certain terms may be highlighted, for example using italics and/or quotation marks. The use of highlighting and/or capital letters has no influence on the scope and meaning of a term; the scope and meaning of a term are the same, in the same context, whether or not it is highlighted and/or in capital letters. It is appreciated that the same thing can be said in more than one way. Consequently, alternative language and synonyms may be used for any one or more of the terms discussed herein, nor is any special significance to be placed upon whether or not a term is elaborated or discussed herein. Synonyms for certain terms are provided. A recital of one or more synonyms does not exclude the use of other synonyms. The use of examples anywhere in this specification, including examples of any terms discussed herein, is illustrative only and in no way limits the scope and meaning of the present disclosure or of any exemplified term. Likewise, the present disclosure is not limited to various embodiments given in this specification.

[0046] It is understood that when an element is referred to as being “on” another element, it can be directly on the other element or intervening elements may be present therebetween. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

[0047] It is understood that, although the terms Firstly, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below can be termed a second element, component, region, layer or section without departing from the teachings of the present disclosure.

[0048] It is understood that when an element is referred to as being “on,” “attached” to, “connected” to, “coupled” with, “contacting,” etc., another element, it can be directly on, attached to, connected to, coupled with or contacting the other element or intervening elements may also be present. In contrast, when an element is referred to as being, for example, “directly on,” “directly attached” to, “directly connected” to, “directly coupled” with or “directly contacting” another element, there are no intervening elements present. It is also appreciated by those of skill in the art that references to a structure or feature that is disposed “adjacent” to another feature may have portions that overlap or underlie the adjacent feature.

[0049] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the multiple forms as well, unless the context clearly indicates otherwise. It is further understood that the terms “comprises” and/or “comprising,” or “includes” and/or “including” or “has” and/or “having” when used in this specification specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof.

[0050] Furthermore, relative terms, such as “lower” or “bottom” and “upper” or “top,” may be used herein to describe one element's relationship to another element as illustrated in the figures. It is understood that relative terms are intended to encompass different orientations of the device in addition to the orientation shown in the figures. For example, if the device in one of the figures is turned over, elements described as being on the “lower” side of other elements will then be oriented on the “upper” sides of the other elements. The exemplary term “lower” can, therefore, encompass both an orientation of lower and upper, depending on the particular orientation of the figure. Similarly, for the terms “horizontal”, “oblique” or “vertical”, in the absence of other clearly defined references, these terms are all relative to the ground. Similarly, if the device in one of the figures is turned over, elements described as “below” or “beneath” other elements will then be oriented “above” the other elements. The exemplary terms “below” or “beneath” can, therefore, encompass both an orientation of above and below.

[0051] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present disclosure belongs. It is further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0052] As used herein, “around,” “about,” “substantially,” “generally” or “approximately” shall generally mean within 20 percent, preferably within 10 percent, and more preferably within 5 percent of a given value or range. Numerical quantities given herein are approximate, meaning that the terms “around,” “about,” “substantially,” “generally” or “approximately” can be inferred if not expressly stated.

[0053] As used herein, the terms “comprise” or “comprising,” “include” or “including,” “carry” or “carrying,” “has/have” or “having,” “contain” or “containing,” “involve” or “involving” and the like are to be understood to be open-ended, i.e., to mean including but not limited to.

[0054] As used herein, the phrase “at least one of A, B, and C” should be construed to mean a logical (A or B or C), using a non-exclusive logical OR. It should be understood that one or more steps within a method may be executed in different order (or concurrently) without altering the principles of the present disclosure.

[0055] Embodiments of the present disclosure are illustrated in detail hereinafter with reference to accompanying drawings. It should be understood that specific embodiments described herein are merely intended to explain the present disclosure, but not intended to limit the present disclosure.

[0056] In order to further elaborate the technical means adopted by the present disclosure and its effect, the technical scheme of the present disclosure is further illustrated in connection with the drawings and through specific mode of execution, but the present disclosure is not limited to the scope of the implementation examples.

[0057] Conventional laser diodes generally comprise a laser chip, a bonding solder, a mounting substrate and a heat sink. The typical size of an LD is about a few 100s microns. Two tiny pads connected by wire bonding are generally used for an electrical cathode and anode. The laser chip can be mounted over the mounting substrate via a bonding solder, and a heat sink can be attached underneath for the mechanical base as well as for heat dissipation. For optical transceivers, the Chip-on-Carrier/Submount (CoC/CoS) bonding is typically done first. Then, the CoC/CoS is bonded onto a common baseplate for lens/mirror attachment before putting it into a package. The latest trend is for more components such as the lasers, capacitors, and thermistors to be attached onto a common carrier by either eutectic or epoxy die bonding.

[0058] As an edge emitter, the dimensions of active layer in the vertical and lateral directions are usually unequal, which causes the diode laser to emit beams having wave fronts with unequal curvatures in the two orthogonal directions normal to the direction of propagation. Thus, the laser beam generally has an inherent aberration of astigmatism. The far field beam divergence, defined by the full angle at the half maximum (FWHM) of the intensity peak, is typically at 25-30 degrees vertically by 15-20 degrees laterally. As shown in FIG. 1, for example, for a 10 G TEML laser diode, the far field angle is about 28 degrees in Horizontal direction and 19 degrees (FWHM) in vertical direction. Using traditional symmetrical lenses to collimate the emitting light, the collimated beam usually ends up being elliptical in shape. For the gain guided diode laser, the astigmatism difference could be up to about 30 μm. The elliptical aberrated output beam from the edge emitting LD can significantly degrade the coupling efficiency to the fiber.

[0059] However, in a refractive index-guided diode laser, such as the guiding in Buried-Hetero (BH) or the Etched-Mesa Buried-Hetero (EMBH) structure lasers diodes, the emission laser wave front tends to have smaller differences between the two angles in two perpendicular directions than the gain-guided laser diodes. Due to the design as well as processing errors, a certain amount of wave front astigmatism still exists in most of products.

[0060] On the other hand, while the far field beam divergence is up to 30 degrees or more, the far field angle of a single mode fiber is about 14 degrees for a NA of 0.122. Due to mismatch between the optical fields of a butt ended standard fiber and the edge-emitting laser chip, the coupling efficiency of simple single mode butt fiber to edge-emitting laser chip can be as low as 10% or lower. To gain a better coupling performance, an optical system is needed for the optical mode field transformation. From the perspective of geometric optics as shown in FIG. 2, this optical system is designed to collect the light emitted from the active layer of the laser chip and image it into fiber end for coupling. A simple two ball-lens system was first utilized to adapt the two mode fields, and the coupling efficiencies can be up to 30%. Also, the approaches of using graded index lenses or selfoc-lenses were also investigated.

[0061] FIG. 3A shows an example of a mature packaging technology presently on the market. It utilizes a micro-lens (006) with a large numerical aperture (NA) to collimate the light emitted from the laser chip (002) and use another micro-lens (008) to refocus the beam into the fiber end (005) for fiber coupling. The collimation lens (006) is designed as an aspherical lens to eliminate the spherical aberration, and a sapphire anti-reflection coated window (007) is employed for sealing the hermetic TO-can. The optical quality of the whole optical packaging system is generally excellent, and for this example shown in FIGS. 3A-3D, it is diffraction-limited at the wavelength of 1550 nm. With the optical packaging technologies available, the fiber coupling efficiency has been promoted to 70% or more. To well align the optical packaging system in relative to the chip (002) and the fiber (005), high precision optical alignment is required. Several actuator-driven precision kinematic systems that offer six degrees of freedom active alignment for each of the OSA element adjustment have been developed and commercially available with translation resolution of better than 0.02 μm and angular resolution of better than 1″ arc second.

[0062] The laser beam as presented for coupling has wave front phase errors from the LD and optical aberrations from optical packaging module and optical alignment. In order to ensure maximum coupling efficiency, beam quality at or close to the diffraction limit is generally required. With the implementation of automatic active aligning system for the chip-to-fiber alignment, the OSA system can be positioned in XYZ directions with sub-micron accuracy and fixed in place by laser welding, which indicates that the wave front aberrations due to optical misalignment of OSA system is negligible and can be ignored. Given that the delicate optical packaging modules generally are (or closely) diffraction-limited, thus the inherent wave front phase error from laser diode is believed to be the main cause for the degradation of the laser-fiber coupling performance in the fiber optic transceivers.

[0063] For high-performance laser chips, such as DFB and EML lasers, manufacturers have experienced low LD productivity as well as low laser chip-fiber coupling efficiency in optical packaging, and thereby low yields of optical transceiver modules. Laser chip-fiber coupling efficiency has generally been limited to about 70%, but more commonly the laser chip-fiber coupling efficiency tends to be in the range of about 30% to 60%. The low coupling efficiency has put more optical transceivers out of specification. If the coupling efficiency is increased, more low-power LDs will become in spec thereby alleviating heating issues in optic transceivers, because the low-power LDs generate less heat.

[0064] Laser chip-fiber coupling efficiency is believed to be more directly determined by wave front phase error of laser diode. However, at present, wave front phase is not a specification for charactering LD chips or OSA products in production of semiconductor laser and fiber optics industry. That is, present quality specifications for laser diode chips used, including electrical and optical, are not known to include specifications for wave front phase. The widely used specifications include operating current, threshold current, slope efficiency, forward voltage, LD reverse voltage, dark current, wavelength, spectral width, continuous wave (CW) output power, resonance frequency, side mode suppression ratio (SMSR), near field, far-field divergence angles, operating temperature and lifetime, etc. Table 1 shows a list of characteristics of a 1550 DFB diode laser purchased from market, which apparently does not include wave front phase parameters.

TABLE-US-00001 TABLE 1 Specifications table of a commercial 1550 nm DFB laser diode (LDI-1550-DFB-2.5G-20/70) ELECTRICAL-OPTICAL CHARACTERISTICS (TYPICAL @ T = 25° C. Wavelength 1550 nm CW, P 20 mW Spectral Width 0.09 nm Spectral Width <500 kHz Wavelength-Temperature Coefficient 0.1 nm/° C. Side-Mode Suppression Ratio 40 dB Threshold Current 8 mA Operating Current (CW 20 mW) 105 mA Operating Current 450 mA (Pulse, P = 70 mW) Operating Voltage 1.3 Volts Slope Efficiency 0.23 mW/mA Pulsed Rise, Fall 80 ps (pachage type Times (20-80%) dependent) Resonance Frequency 6 GHz Monitoring Output Current 1.5 mA Dark Current <100 nA

[0065] The intensity distribution of an ideal laser beam is Gaussian. At present, the dominant laser beam characterization method is to measure the second order moments of the Wigner density function to form a 4×4 beam matrix. The Wigner distribution function represents a signal in the space and frequency domains simultaneously. The zeroth-order moment describes the total power, the first-order moments provide the centroid and propagation direction, the second-order moments are related to the beam width and far-field divergence angles, and the third- and fourth-order moments are linked to the beam symmetry and sharpness of a laser beam. The ISO standard 11146 provides a general procedure for characterizing the propagation properties of an arbitrary beam by measuring beam irradiance with a CCD-camera in a single transverse plane.

[0066] M-square (M.sup.2), or laser beam quality factor, is calculated by evaluating the second moment width of the real beam intensity profile transverse to beam propagation direction. M-square has been widely used as a metric for characterizing laser beam quality. It defines how many times of “diffraction limited beam size” a real beam is in the transverse direction, which represents the degree of variation of the real beam from an ideal Gaussian beam. For a single mode TEM00 Gaussian beam, its M-Square is exactly one. However, the M-square is not a precise characterization of beam quality. It is generally true for a beam with small wave front error (less than ˜0.1λ RMS) that M-square parameter and wave front phase error are comparable. When wave front phase error increases, the M-Square parameter increases dramatically nonlinearly (>>1) and is less predictable, because the impacts of different wave front aberrations on M-Square are dramatically different. Furthermore, real laser beams are usually non-Gaussian, multi-mode or mixed-mode. There are many cases in which a beam looks very Gaussian yet its M-square value is far from unity; likewise, a beam intensity profile appears very “un-Gaussian” yet its M-square value is still close to unity. Generally, M-square increases as a laser's output power increases, so it is difficult to obtain excellent beam quality and high average power at the same time due to thermal lensing in the laser gain medium. In many cases, the formulae for calculating the beam matrix do not apply, and M-Square becomes a more qualitative other than a quantitative measure of beam quality. Many authors have provided methods to evaluate the beam quality factor from wave front phase error, but it is almost impossible to obtain the wave front phase information from an M-Square value. However, it is the wave front phase information rather than the intensity profile that is believed to be the dominant influence on beam propagation. Thus, to more precisely characterize the beam quality of a LD product, the laser wave front phase should be measured. However, so far, wave front phase measurement of laser diode is not widely implemented for quality inspection. A deeper reason behind is that how to define the phase of an incoherent or multimode beam is still not well understood.

[0067] Several wave front sensors are available that can be used for infrared wave front phase measurement. As a candidate for laser diode specification, wave front phase measurement should be implementable, accurate, repeatable and efficient.

[0068] Foucault knife-edge testing involves moving a knife-edge through the focus of a beam and observing the intensity pattern on a screen. Like interferometry, knife-edge testing allows high-spatial frequency aberrations to be observed, but it requires very accurate alignment of the knife-edge to the beam focus, and it is a qualitative visional test.

[0069] Curvature sensing is a technique used typically in adaptive optics to measure the Laplacian of the wave front by measuring the normalized differential intensity change along the optical axis, but it has not been widely made commercially available.

[0070] While ptychography allows reconstructing both the intensity profile and wave front phase of laser beam for complete beam matrix and derived quantities such as M-Square and wave front phase. However, its computational complexity is heavy and its efficiency is an issue because it requires a 2D sample scanning.

[0071] Interferometry is a technique to make the wave front interfere with itself or an ideal wave front. It is good for measuring aberrations of high spatial frequency and low amplitude aberrations but can be degraded by air motion and mechanical vibrations, especially for testing large optics. Its instrument volume is usually large and its optical layout is complicated. Sophisticated software is necessary for extracting meaningful and accurate information from interferograms. Commercial interferometers are typically very costly.

[0072] The Shack-Hartmann sensor is deemed herein as the most convenient wave front sensor for diode laser beam characterization. The Shack-Hartmann sensor measures the wave front phase by measuring wave front gradients in real time. It is independent of higher-order aberrations and intensity profile variation, and it has good photon efficiency over the anticipated wavelength bands. The commercial instrument usually provides software for real time image analysis and product characterization, which is ideal for production online testing. Unlike the Wigner density function approach, the Shack-Hartmann wave front sensor does not need a priori knowledge about the beam before the measurement is conducted.

[0073] Due to its large numerical aperture (NA) and nonsymmetrical radiation profiles of the diode laser in the two perpendicular directions, it is much more difficult to measure the wave front phase profiles than the intensity profiles. LD output beams tend to exhibit a very large diverging field due to the diffraction at the end facet of the laser cavity, which is inversely proportional to the emission aperture size, but the aperture size of OSA in optical packaging is very small, usually less than 1 mm (˜0.7 mm), with a about 0.2 mm working distance, which makes direct wave front phase measurement of laser diodes with the OSA attached very difficult. The wave front phase of a laser diode may be measured at the focus point of OSA system, and the metrology system needed has small NA, as small as 0.1, which is easy to achieve, but the wave front phase error measured in this way has the residual aberrations from the OSA system and the entire OSA packaging system must be discarded even if only the LD is out of spec.

[0074] For highly divergent wave fronts, the wave front phase measurement is also sensitive to the input NA and the location of the entrance pupil of the metrology system. To obtain the true wave front phase of laser diode, the location of entrance pupil and the size of input NA matter. In addition, the laser diode is a non-Lambertian source, and its emitting power is relatively low (generally a few 100s of mW). The volume intensity distribution profile of laser diode is inhomogeneous and not radially symmetrical, and it varies from chip to chip, which makes the irradiance-based wave front phase measurement very challenging.

[0075] As described below, an infrared Shack-Hartmann sensor may be used to measure the wave front aberrations of diode laser Chip on Carrier (CoC), providing an in-situ wave front phase measurement for the laser chips, yielding a new metric for characterizing the optical performance of laser diodes.

[0076] In this disclosure, the following philosophy of metrology system design is presented: The way a diode laser beam is measured should be the same as or similar to the way the diode laser beam is intended to be used and functioning in the OSA packaging system. The metrology system design should imitate the OSA packaging system design to ensure that the laser diode is measured under the conditions similar to that of its expected use.

[0077] With the proposed method implemented in production, the wave front phase parameters measured can be used to reject the out-of-spec chips before incorporating the chips in optical module packaging, which provides a new approach to screen out the out-of-spec laser diodes for BOSA/TOSA production. The metric used for screening laser chips can be the wave front phase error or aberrations from measurements or a new metric that is computed from the wave front phase aberrations and other parameters of optical property of laser diode. The optical property of diode laser chips can be inspected as early as possible in production to avoid the costs of assembling and scrapping out-of-spec OSA packaging systems.

[0078] To imitate the OSA packaging system, a large NA lens can be used to collimate the laser beam from a laser diode and a focusing lens can be used to focus the collimated beam into a focused point: The first two lenses can form a replica of OSA packaging system used in optical transceivers. The focused beam becomes divergent after the focused point and it can then be re-collimated to a desired size in advance of the wave front sensor. By dedicated optical design, the exit pupil of the metrology system can be matched to the detector of wave front sensor, and the detector is conjugated to the entrance pupil of the metrology system. With this arrangement, the true value of wave front phase of laser diode that is intended to be used in optical transceiver can be obtained, yielding an optical specification for laser diode, OSA system and optical module characterization.

[0079] This disclosure also describes direct wave front phase measurement of Optical Sub-Assembly (OSA) devices, which provides an improved specification for characterizing OSA quality and optical transceiver performance. In order to increase the laser-fiber optic coupling efficiency of optical transceiver, the mode match between laser beam and fiber end should be optimized to reduce the energy loss in optical packaging. The laser beam from the OSA devices to the fiber has wave front errors from both the optical packaging system and the laser diode. It is a prerequisite for mode matching that the wave front error from the laser diode and the optical packaging system should be at a possible minimum level. With the infrared wave front phase analyzer, we can examine the true value of wave front phase from laser diode and separate it from the wave front phase errors measured from OSA, yielding a quality diagnosis for OSA system.

[0080] Before explaining the disclosed embodiments of the present disclosure in detail, it is to be understood that the disclosure is not limited in its application to the details of the particular arrangements shown since the disclosure is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation.

[0081] Working Wavelength

[0082] Earlier optical links were designed for the 800-900 nm window of the optical fiber transmission spectrum, consistent for the wavelength of AlGaAs material system for semiconductor lasers and LEDs. The development of diode lasers and detectors in the 1.3-1.5 μm wavelength range and the further improvement in optical fiber loss over that range has directed most applications to either the 1.3 μm window for low dispersion or the 1.55 μm window for minimum loss. The design of dispersion-shifted single mode fiber along with the availability of erbium doped fiber amplifiers (EDFA) has solidified the 1.55 μm window as the choice for high-speed communications. Also, the 1.55 μm laser diodes are classified as “retina-safe’ for human and 1550 nm is the ideal wavelength of choice for LiDAR applications due to its eye safe nature at high power. For these reasons, the laser diodes that have 1550 nm wavelengths (including 1550 nm DFB and 1577 nm EML) are becoming more widely used in consumer and industrial applications.

[0083] In this disclosure, we feature a 1550 nm LD as the semiconductor light source to demonstrate an intended implementation, but the implementation description is also applicable to other light sources.

[0084] The considered laser diode source is monochromatic light source whose emitting wavelength is in near infrared band. For example, for industrial DWDM (Ch13-Ch61), its wavelength range is of 1528-1568 nm. TEML and DWDM EML are products with multiple working wavelengths. Semiconductor Optical Amplifier (SOA) assisted EML is with wavelengths of 1532 nm, 1577 mm and 1596 nm. In order to accommodate for more laser diode products, the metrology system is designed with a working wavelength range of 1528 nm-1577 nm, whereas the central working wavelength is set as 1552 nm (193.1 THz).

[0085] Optical Design

[0086] An objective for at least some of the embodiments envisioned herein is to provide direct and absolute measurement of wave front phase of a diode laser. According to Imagine Optic, the absolute measurement is aspirationally defined as “the ability to provide the most accurate, quantitative and objective data possible on the optical qualities of a light source without relying on a similar external reference in order to establish a proportional relationship.” If the wave front phase measurement is not of the true value of a laser diode, then it will not be much use for optical transceivers production. In order to create a quality specification for laser diodes or LED's, it is beneficial to have absolute phase measurement of the laser beam. However, as we described in the previous section, to achieve absolute measurement over a wide dynamic range is a challenge, because Shack-Hartmann sensor is irradiance-dependent wave front sensor while the diode laser beam intensity is inhomogeneous and there is significant intensity variation over the pupil.

[0087] To minimize inhomogeneous beam intensity to obtain a nearly perfect spherical wave front, a single mode fiber coupled to a laser diode or a laser diode put in distance can be utilized as a light source in a metrology system as shown in FIG. 4. However, the scenario that the measured beam is convergent or divergent to the sensor should generally be avoided, because of systematic errors that could prevent the envisioned sensor from measuring the true wave front values.

[0088] In view of the asymmetry and inhomogeneity of the emitting beam profile of laser diode, as shown in FIG. 10 and FIG. 11 in next section, the optical testing pupil at which wave front phase is measured is irregular. Special boundary treatment is required in wave front phase estimation algorithm, which introduces more complexity of the problem.

[0089] Also, the laser diode emitting beam is highly divergent, so the wave front phase measurement of laser diode is metrology NA dependent. The wave front phases measured at different NAs within the diode laser emitting profile are different.

[0090] In this disclosure, the following philosophy of metrology system design is presented: In order to measure the true wave front phase associated with an intended use of laser diode, the optical design of the metrology system should imitate the OSA packaging system design in which the laser diode is intended to be integrated and normally functioning. More reliable results are expected from metrology system designs those better approximate relevant details of the OSA packaging system design.

[0091] To implement this philosophy, the divergent beam wave front of diode laser is preferably substantially captured by an aspheric lens with the same NA as the collimation lens used in the OSA packaging system so as to capture the same portion of emitting light from the laser diode as it is normally collimated in the OSA system into a parallel beam, and a focusing lens can be used to re-focus the parallel beam into a focused point, which can be characterized as the Point Spread Function (PSF) in Fourier domain. Another collimation lens can be used to re-collimate the beam from the focused point to match the size of the detector of wave front sensor while still satisfying the metrology system pupil conjugation requirement. The collimating and focusing functions could also be performed by other known types of optics providing the desired optical powers.

[0092] The output beam of a laser diode is highly divergent. Given that the divergent angle of the diode laser beam is generally more than 22 degrees for most cases, therefore a collimation lens designed for collecting the divergent beam from laser diode preferably has a numerical aperture (NA) larger than 22 degrees. According to the NA size of the collecting system, the optical design for laser diode metrology can be classified into three configurations as described below:

[0093] Config A: Small NA Metrology System

[0094] As shown in configuration A of FIGS. 5A-5C, when the NA is small, the first collimation lens and the focusing lens are so close in position that they can be combined into one collecting lens (130, 141 or 142) while the optical quality of metrology system is still very good. In that case, the metrology system can be a two-lens system. The first lens is a collecting lens A (140, 141, 142) with NA larger than 22 degree that is used to substantially capture the divergent beam emitting from laser diode and image it into a point image, and the second lens is collimation lens C, which could be a singlet (170, 171) or doublet lens (172), that is used to collimate the laser beam to match the detector size of the wave front sensor while also satisfying the pupil and detector conjugation requirement. A field lens D (162) can be added to improve the imaging quality and radiometry performance. Examples of this two-lens system are shown as Examples A and B in FIGS. 5A-5B, respectively, and an example of three-lens system is shown as Example C in FIG. 5C. For Example A, Lens A (140) is an aspherical lens with focal length of 11 mm, and Lens C (170) is a spherical singlet with focal length of 25 mm, offering an objective NA of about 0.22 (25 degree) and a very good imaging quality. A singlet is used in Example C as field Lens D (162). While the exit pupil is at the location of the detector of wave front sensor, the entrance pupil is set as physical aperture of Lens A. The merit of this system is its compactness: The optical length is short and simple, but the objective aperture (NA) offered is relatively small.

[0095] Config B: Intermediate NA System

[0096] For a metrology system with intermediate NA, a three-lens system can be employed as shown in FIGS. 6A-6C. Config B design imitates the most commonly used OSA packaging system. The First lens A (240, 241 or 242) is used to collimate the divergent beam from laser diode and make it parallel, focusing Lens B (250, 251 or 252) then focuses the parallel beam to a point, and the collimation Lens C (270, 271 or 272) re-collimates the divergent laser beam from the focused point to match the detector size of wave front sensor (280, 281 or 282), while also satisfying the pupil conjugation requirement. The way the laser diode in Config B is being tested for wave front phase measurement is a mimic of the way it is being used in an OSA packaging system. As shown in FIG. 6A, Example A, a system with f=11 mm aspherical collimation lens A (240) plus an f=45 mm lens B (250) and a f=25 mm lens C (270) offers NA of 0.23-0.29, which equates NA incident angle of 27-33 degree. The first two lenses in FIG. 6A form a replica or a mimic of the OSA system as presented above with respect to FIGS. 2 and 3A. As shown in FIG. 6B Example B, the collimation Lens C (271) can be designed as a doublet instead of a singlet lens, and a field lens (262) can also be added to guarantee the pupil conjugation and high image quality as shown in FIG. 6C Example C. While the exit pupil is at the location of the detector of wave front sensor, the entrance pupil is set as the physical aperture of First Lens A in general.

[0097] Config C: Large NA System

[0098] For the laser diode with large NA emitting beam, the metrology system design is more challenging. A large NA aspherical single lens can be used to collect the emitting beam from the laser diode and make it parallel, and a focusing lens plus a collimation lens can be employed to reduce the laser beam to match the detector size of wave front sensor, while still satisfying the pupil conjugation with wave front error minimized. FIGS. 7A-7B illustrate a metrology system for large NA diode laser. A system with f=10 mm aspherical collimation Lens A (340) offers NA of 0.53-0.58, which is equivalent to incident angle of 65-71 degrees. A plano-convex spherical lens (350) plus a bi-convex lens (370) (as shown in FIG. 7A Example A) or a bi-convex lens (351) plus a plano-convex spherical lens (371) (as shown in FIG. 7B Example B) can be used for image relation and pupil conjugation. While generally the exit pupil is at the location of the detector of wave front sensor, the entrance pupil is set as the physical aperture of First Lens A. Both examples can yield excellent image quality.

[0099] The major wave front aberrations of laser diodes that affect the performance of laser-to-fiber coupling performance include spherical aberration, astigmatism, and coma. The most well-known wave front aberration of laser diodes is astigmatism. In order to obtain the maximum laser-fiber coupling efficiency, the wave front phase error of laser diode should be minimal, and the optical quality of the metrology system should be at or close to the diffraction limit. In other words, Rayleigh criterion should be satisfied or approximately satisfied.

[0100] As a metrology system, it should be calibrated periodically. As shown in the prototype system in the next section, there is a residual systematic error in the optical design. Also, there are manufacturing errors and optical design tolerance error in the system. We can periodically calibrate the system either with a golden standard sample of laser diode or with small pin hole illuminated by the infrared light source to replace standard laser diode sample. Based on the calibration measurements, a reference file can be generated and saved for implementation in real data measurement.

[0101] Prototype System of Wavefront Phase Analyzer for Laser Diode Chip Inspection

[0102] In the following example, a HASO4 NIR sensor by Imagine Optics SA of Orsay, France is used for purposes of prototyping. HASO4 NIR is a technology that can get wave front local slope cartography in the infrared band of 1500-1600 nm. It is bundled with WaveView metrology software, yielding wave front parameters, such as PSF, Strehl ratio, MTF and M.sup.2.

[0103] In this section, a prototype metrology system for intermediate NA (FIG. 6A) is described. Given that the far field beam divergence laser diode is typically less than 30 degrees, a collimation Lens A (240), with focal length of 11 mm plus a focusing Lens B and collimation Lens C can provide a metrology system with intermediate NA of 0.284 (far field angle 33 degree), which could accommodate most laser diode products on market. Lens A (240) is a plano convex lens with an aspherical surface for correcting the spherical aberrations. Lens B (250) has a focal length of 45 mm, and Lens C (270) has a focal length of 25 mm. The characteristics parameters of the prototype system are listed in Table 2, and its experimental system is shown in FIG. 8.

TABLE-US-00002 TABLE 2 Characteristics of prototype metrology system Entrance Pupil 6.68 mm Objective NA 0.284 (32.97 deg) Wave front RMS error 0.2567 μm Wave front PV error 0.891 μm Total length 140 mm Effective Focal Length EDL = −6.0 mm Back Focal Length 38.85 mm Image Space F/# 0.9 Paraxial Working F/# 74.34526 Working F/# 84.534 Image Space NA 0.006714 Object Space NA 0.2838 Stop Radius 3.34 mm Paraxial Image Height 0.433 mm Paraxial Magnification 44 Entrance Pupil Diameter 6.68 mm Entrance Pupil Position 1.097 mm Exit Pupil Diameter 3.60 mm Exit Pupil Position 0 mm Field Type Angle in degrees Maximum Radial Field 0.05 Primary Wavelength [μm] 1.55252 μm Angular Magnification −1.855

[0104] Results of Laser Diode Chip Inspection

[0105] With the prototype system developed for intermediate NA metrology, we performed a wave front phase measurement of a 1577 EML laser diode chip on carrier (CoC). FIG. 9 shows a 1577 EML laser diode chip on the slot that is ready for testing. FIG. 10 shows the beam intensity measured at the wave front testing pupil. We can see that the intensity has an approximate Gaussian distribution profile. FIG. 11 shows the laser diode wave front phase measured at the testing pupil. FIG. 12 is the point spread function (PSF) computed from wave front phase measurement of laser diode, which would otherwise be the intensity distribution at the end of an optical fiber for coupling in Optical Sub-Assembly (OSA). Given the PSF function, the Strehl ratio of 0.284 can be obtained. FIG. 13 shows the axial distribution of intensity of laser beam along the propagation axis at the position close to the beam waist, from which the M-Square value of 4.78 can be obtained. FIG. 14 shows the coefficients of Zernike polynomials and the orientations of major wave front aberration components of the diode laser beam, which include wave front alignment errors (tilts and defocus), spherical aberrations, astigmatisms, comas, etc. From this plot, it is apparent that the dominant wave front aberrations are astigmatism and coma. FIG. 15 shows the plot of defocus term, which is 0.214 micron RMS and 0.803 micron PV. FIG. 16 shows the astigmatism term, which is 0.214 micron RMS and 1.116 micron PV. The astigmatism term can be further decomposed into astigmatism at 0 degree (0.030 micron RMS, 0.223 micron PV) (FIG. 17) and astigmatism at 45° (0.212 micron RMS and 1.104 micron PV) (FIG. 18). FIG. 19 shows the plot of coma term (0.278 micron RMS,1.258 micron PV), which can be further decomposed into coma at 0 degree (0.236 micron rms, 1.012 micron PV, (FIG. 20) and coma at 90 degree (0.147 micron rms, 0.664 micron PV, FIG. 21). FIG. 22 shows the plot of 3rd order spherical aberrations, which is 0.022 micron in rms and 0.114 micron PV.

[0106] The wave front residual error after removing the 1-12 Zernike terms is showed in FIG. 23, which is 0.137 micron RMS and 1.02 micron PV. The residual error also includes the zonal residual error from modal estimation that cannot be fitted into Zernike coefficients, which is 0.088 micron RMS and 0.577 micron PV as showed in FIG. 24.

[0107] While the disclosure has been described, disclosed, illustrated and shown in various terms of certain embodiments or modifications which it has presumed in practice, the scope of the disclosure is not intended to be, nor should it be deemed to be, limited thereby and such other modifications or embodiments as may be suggested by the teachings herein are particularly reserved.

[0108] The foregoing description of the present disclosure, along with its associated embodiments, has been presented for purposes of illustration only. It is not exhaustive and does not limit the present disclosure to the precise form disclosed. Those skilled in the art will appreciate from the foregoing description that modifications and variations are possible considering the said teachings or may be acquired from practicing the disclosed embodiments.

[0109] Likewise, the steps described need not be performed in the same sequence discussed or with the same degree of separation. Various steps may be omitted, repeated, combined, or divided, as necessary to achieve the same or similar objectives or enhancements. Accordingly, the present disclosure is not limited to the said-described embodiments, but instead is defined by the appended claims considering their full scope of equivalents.