Method for design and development of a semiconductor laser device

A method for testing semiconductor laser devices is described. The method includes testing a monolithically integrated semiconductor laser device via electrical contact testing and/or far field testing. These tests will provide the total performance of the entire device. Further, the method includes accurate cleaving off of a portion of the laser device and re-testing to determine the relative performance of the remainder of the device. Through comparison of the test and re-test results, it is possible to reduce the design cycle for monolithically integrated semiconductor laser devices by detecting design flaws and imperfections or by ascertaining a more advantageous design.

FIELD OF THE INVENTION
 The present invention relates to a method for use in designing and
 developing a semiconductor laser device. More particularly, the present
 invention relates to a method of testing an overall laser device, then
 cleaving and testing subparts thereof to obtain information which can be
 used in the design and development process for the laser device.
 BACKGROUND OF THE INVENTION
 The performance requirements of semiconductor laser devices have increased
 over the last few years. As the requirements continue to increase,
 monolithically integrated devices are increasingly being used. The
 increased use of monolithically integrated devices require more complex
 methodologies for examining their performances and for improving the
 efficiency of the design cycles for such devices.
 Conventionally, for a complex device--for example, a monolithic integration
 of two devices which operate together--difficulty arises in attempting to
 measure the output performance parameters of the overall device.
 Uncertainty exists, using conventional methodology, as to which of the two
 devices are affecting the performance of the overall device. For example,
 in a monolithically integrated device combining a laser and an expander,
 if the light outputted from the expander is less than expected, it is
 difficult to determine if the problem is due to the laser or the expander.
 For example, in such a device, optical light is expected to be absorbed in
 the expander. Computer models for predicting the amount of light that
 should be absorbed are not accurate. Further, for a monolithically
 integrated device having an expander shaped to allow the beam of light to
 expand, conventional measuring techniques are incapable of discerning how
 the beam is transformed as it moves through the device.
 It is possible to include a less complex device, such as non-integrated
 devices including only a laser, to compare to the integrated device
 performance A deficiency with the present state of the art using a less
 complex semiconductor laser device as a test device for a more complex
 integrated device is that unintentional flaws between the device and the
 test device, such as, for example, bonding damage or process variation
 across a wafer, are indistinguishable from flaws in the device design.
 This deficiency is likely to increase with increasing complexity of
 semiconductor laser devices. Due to the inability to distinguish between
 flaws in the device design and unintentional process differences between
 the complex device and the simplified test device, it is difficult to
 ascribe performance imperfections to the design of the device or in
 subparts thereof. This lengthens the design cycle time. In addition, it
 may not be known at the time of device mask design what the optimal test
 device layout is.
 It is therefore necessary to have a design tool and methodology which is
 capable of accurately measuring the performance parameters of complex
 devices, thus shortening design cycle time and cutting design costs.
 Further, it is necessary to have a design tool and methodology which
 obviates the need for a separate testing device.
 SUMMARY OF THE INVENTION
 The present invention provides a method for determining performance
 characteristics of subcomponents of a monolithically integrated
 semiconductor laser device. The method includes testing performance
 characteristics of an entire semiconductor laser device, accurately
 cleaving off a portion or subpart of the laser device, re-testing a
 subpart of the laser device, and comparing results of the test and re-test
 to determine performance characteristics of the remainder subparts of the
 laser device relative to the performance characteristics of the entire
 semiconductor laser device.
 The present invention also provides a method of reducing design cycle time
 for semiconductor laser devices. The method includes testing performance
 characteristics of a semiconductor laser device, accurately cleaving off a
 portion of the laser device, re-testing a remainder of the laser device,
 comparing results of the test and re-test to determine performance
 characteristics of the remainder of the laser device relative to
 performance characteristics of the laser device, and ascertaining from the
 comparison whether the design of the laser device can be improved.
 These and other features and advantages of the invention will be more
 clearly understood from the following detailed description of the
 invention which is provided in connection with the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 The present invention utilizes destructive testing of a laser device to
 obtain information on the performance characteristics of various subparts
 of the laser device. The testing utilizes accurate cleaving technology to
 separate and test subparts of the device.
 With reference to FIGS. 1-3, where like numerals designate like elements,
 there is shown an expanded beam laser 10 having an expander region 12 and
 a laser region 20. Expanded beam lasers 10 alleviate the mismatch in
 optical mode dimensions between a semiconductor laser and an optical fiber
 to which the laser 10 is coupled. Expanded beam lasers 10 further find
 application in conjunction with planar waveguide devices, which also
 suffer from the optical mode mismatch problem.
 The expander region 12 includes an outlet facet 16 through which laser
 light 14, which is expanded in the expander region 12, exits the device
 10.
 The laser region 20 includes a metallization pad 24 which overlays a laser
 stripe 26. The laser stripe 26 is defined by mesa etching of the laser
 active region 34. A waveguide 33 is positioned between a spacer layer 32
 and a substrate 25. The laser light 14 which is expanded within the
 expander region 12 is propagated along the waveguide 33 and exits the
 device 10 at the facet 16.
 The active region 34 is the lasing region of the device. In the laser
 section 20, the laser mode is located in the active region 34, which is
 covered by a cladding region 35. As the mode propagates into the expander
 region 12, a large portion of the energy of the mode transfers to the
 underlying waveguide 33, where the mode is expanded before exiting the
 device at the facet 16.
 As illustrated, the expanded beam laser 10 is approximately 500 micrometers
 (.mu.m) in width, and 600 .mu.m in length. As a means for reducing the
 design cycle time of expanded beam lasers 10, the present invention
 includes cleaving the expander region 12 from the laser region 20. Such a
 cleaving would create a cleave line 30, as well as a cleave surface 18 on
 the expander region 12 and a cleave surface 22 on the laser region 20. As
 illustrated in FIG. 1, the cleave line 30 is approximately 300 .mu.m from
 the outlet surface 16, roughly the length of the expander region 12.
 The cleave line 30 is cleaved accurately. Specifically, the cleave line 30
 is cleaved along a line within a tolerance of plus or minus 2 microns from
 that line. One known cleaving apparatus which may be used to accurately
 cleave the cleave line 30 is the Dynatex III.
 With reference to FIG. 4, next will be described an electro-modulated laser
 device 40 which is shown therein. The electro-modulated laser device 40
 includes a modulated region 42 and a laser region 50. The modulated region
 42 includes a metallized pad 44. The laser region 50 includes a
 metallization pad 54 which overlays a laser stripe 56.
 The modulated region 42 is, as shown, approximately 200 .mu.m in length. In
 order to test the respective performances of the modulator region 42 and
 the laser region 50 as compared to the total performance of the
 electro-modulated laser device 40, the modulator region 42 is cleaved from
 the laser region 50 along a cleaved line 60. This creates a cleaved
 surface 46 on the modulator region 42 and an opposing cleaved surface 52
 on the laser region 50.
 A distributed Bragg reflector (DBR) laser, such as a DBR laser 70
 illustrated in FIG. 5 and described in detail below, is a monolithic laser
 structure which has, at the end of a semiconductive laser medium (active
 section), a passive optical waveguide in which a grating is formed that
 constitutes a Bragg reflector (Bragg section). The passive optical
 waveguide may have a portion, that does not include any grating,
 interposed between the amplifying medium and the reflector grating,
 thereby defining a phase control section in the DBR structure. This
 section makes it possible to avoid any mode jumping while tuning the DBR
 section.
 With reference to FIG. 5, the distributed Bragg reflector laser 70 includes
 a modulator region 72, a reflector region 80, and a laser region 90. The
 modulator region 72 includes a metallization pad 74. The reflector region
 80 includes a metallization pad 84. The laser region 90 includes a
 metallization pad 94 which overlays a laser stripe 96. The distributed
 Bragg reflector laser 70 may be cleaved in several places to determine the
 relative performances of each of the regions 72, 80, 90. Specifically, the
 modulator region 72 may be cleaved from the reflector region 80 at a
 cleave line 77. This creates a cleave surface 76 on the modulator region
 72 and an opposing cleave surface 82 on the reflector region 80. Further,
 the reflector region 80 may be cleaved from the laser region 90 at a
 cleave line 87. This creates a cleave surface 86 on the reflector region
 80 and an opposing cleave surface 92 on the laser region 90.
 Next will be described the method for utilizing accurate cleaving as a
 design tool. The theory underlined in the below-described methodology is
 that to properly understand the complete performance of an integrated
 laser device it is necessary to ascertain the total performance of the
 device and then accurately cleave the device to ascertain each of the
 cleaved portions' relative performances. Thus, with reference to FIG. 6,
 the method is started at step 100. At step 101, the entire device 10, 40,
 or 70 is tested. As illustrated in FIGS. 7-10, the device 40 is tested.
 The device 40 may be tested in one of two illustrated ways. The first
 test, taking pathway 122 to step 102, is an electrical contact test of the
 laser device 40. In the electrical contact test, an electrical probe 160
 contacts the metallization pad 54 within the laser region 50 (FIG. 7).
 Next, the laser light exiting the area 14 is optically coupled to an
 industry standard power meter 162 to obtain an LI curve (power
 output/current in). From the LI curve, slope efficiency and threshold
 current can be determined.
 The second type of test of the laser device 40, taking pathway 124 from
 step 101 to step 104, is a far field test. In the far field test, an
 electrical probe 160 is attached to the metallization pad 24 of the laser
 device 10 (FIG. 9). A movable photo-detector 170 is moved along an arc as
 illustrated in positions 170.sub.a -170.sub.n which are spaced apart on a
 swing arm 172 in front of the output surface 46. The expansion of the
 laser light exiting the area 14 is determined over a specified distance
 D.sub.a -D.sub.n (the distance between the output surface 46 and the
 photo-detector 170) to ascertain the laser beam divergence, or far field
 parameters. By ascertaining the laser light intensity at each of the
 positions the photo-detectors 170.sub.a -170.sub.n, a laser far field
 pattern may be mapped out.
 It is to be understood that the laser device 10, 40 or 70 may be tested at
 step 102 and then take pathway 125 to be tested at step 104, or tested at
 step 104 and then take pathway 125 to be tested at step 102, or may be
 tested at either step 102 or step 104.
 Once the laser device as a whole has been tested, a portion of the laser
 device is cleaved off at step 110, such as the expander region 12.
 Although the cleaved off portion, shown in FIGS. 1 and 3, is approximately
 200-300 .mu.m in length, with current technology the amount cleaved off
 may be as small as 50 .mu.m in length.
 Once a portion of the laser device 40 has been cleaved off at step 110, the
 laser device is re-tested at step 120. Thus, for example, if the modulated
 region 42 of the electro-modulated laser device 40 is cleaved off at
 cleave line 46, the laser region 50 is re-tested at step 120 (FIG. 8) by
 taking route 121 back to step 101 and subjecting the laser region 50 to
 the same testing, at step 102 and/or step 104, to which the whole laser
 device 40 was previously subjected. Thus, if the whole laser device 40 was
 subjected to the far field test at step 104, the laser region 50 is
 re-tested by taking branch 124 to step 104. Likewise, if the whole laser
 device 40 was also tested at step 102, branch 125 is taken and the laser
 region 50 is tested at step 102.
 Comparing measurements of device threshold current and slope efficiency
 before and after cleaving will indicate any optical loss from the cleaved
 off portion, here the expander region 12. Analysis of the far field
 pattern before and after cleaving can demonstrate the evolution of the
 expanded beam laser device 10 mode shape from the laser region 20 to the
 expander region 12, which helps in ascertaining unwanted structure within
 the laser device 10. By making such a comparison, it is possible to more
 quickly redesign the laser device 10 or to ascertain a difference in the
 performance characteristics of the various subparts of the laser device 10
 (or lasers 40, 70) and where there are imperfections or flaws within the
 laser device 10 which can be designed out. Further, through this process
 the relative performances of each of the portions making up the laser
 device 10 is ascertained. Additionally, since a separate test device is
 not used, all of the performance characteristics determined in the tests
 come from the laser device 10 itself or its subparts.
 Furthermore, by taking repeated electrical contact test measurements (step
 102) of cleaved sections, it is possible to determine the amount of light
 absorbed, or lost, per micron of the device. Such experimental
 determination of the loss/micron of light is more accurate than
 conventional modeling techniques.
 After re-testing the laser device at step 120, a decision is made at step
 130 whether another portion is to be cleaved from the laser device 40. If,
 for example, the electro-modulated beam laser device 40 is cleaved, and
 the cleaved portion is not the entire laser region 50, route 132 can be
 taken and the laser device 40 can be re-cleaved at step 110 and then
 re-tested at step 120 to ascertain that cleaved portion's, or the
 remainder of the laser device's 40, relative performance.
 Further, if expanded beam laser 10 is cleaved at step 110, and the cleaved
 portion is not the entire expander region 12, branch 132 can be taken and
 the laser device 10 can be re-cleaved at step 110 and then re-tested at
 step 120. The same is true for the distributed Bragg reflector laser 70 if
 less than all of the modulator region 72 was initially cleaved. Further,
 if the distributed Bragg reflector laser 70 was initially cleaved along
 cleave line 77, it can be cleaved again along the cleave line 87 at step
 110. If no further cleaving or testing is determined necessary at step
 130, branch 134 is taken and a computer (not shown) records the various
 measurements and performs an analysis at step 140 for any possible design
 changes. The method is stopped at step 150.
 While the invention has been described in detail in connection with the
 preferred embodiments known at the time, it should be readily understood
 that the invention is not limited to such disclosed embodiments. Rather,
 the invention can be modified to incorporate any number of variations,
 alterations, substitutions or equivalent arrangements not heretofore
 described, but which are commensurate with the spirit and scope of the
 invention. For example, while the electrical contact test at step 102 has
 been described in terms of contacting a probe to the metallization pads
 44, 54, obviously the metallization pads 24, 74, 84 and 94 may be
 contacted and/or other tests performed. Accordingly, the invention is not
 to be seen as limited by the foregoing description, but is only limited by
 the scope of the appended claims.
 What is claimed as new and desired to be protected by Letters Patent of the
 United States is: